Extrusion Processing of Biomass By-Products for Sustainable Food Production

Abstract

The sustainability of the food supply chain is gaining increasing attention in the quest to balance economic, environmental, and social dimensions. A key opportunity to enhance food system sustainability is by addressing food waste through upcycling strategies to generate higher value, functional foods. Extrusion is a food manufacturing technology that is emerging as a promising option for the incorporation of various types of biomass by-products, such as fruit and vegetable pomace, brewer’s spent grain, bagasse, and oil press cake. In this chapter, we present an overview of the latest research conducted on incorporating biomass by-products into extruded food products, with an emphasis on the challenges and opportunities associated with this approach. A meta-analysis study was conducted regarding a key challenge for product quality when incorporating by-products, which is the reduction in radial expansion index of expanded snack and breakfast cereal products. To highlight future opportunities, two case studies illustrate successful examples of by-product incorporation for commercial extruded food products, while emerging protein sources from waste-consuming insects were also explored. Overcoming these challenges and leveraging opportunities can contribute to a more sustainable food system through the integration of by-products into value-added extruded foods.

1. Introduction

1.1 Chapter overview: from biomass to sustainable food products

There are a myriad of specialty biobased products that can be generated from biomass across the broad subject areas of biofuel, biochemical, biomaterial, pharmaceutical, food, and feed production. While biobased products have been around for centuries, recent advancements in technology have allowed for more efficient and cost-effective production, improving their functionality and economic viability. The focus of this chapter will be on the production of sustainable food products using biomass by-products, specifically manufactured through extrusion technology. It is clear from Figure 1 that extrusion technology is linked to sustainability through the processing of food by-products into functional foods, in the effort to fight food waste. Biomass by-products that have been incorporated into extruded products include fruit and vegetable pomace, bagasse, oil press cake, brewer’s spent grain (BSG), crop by-products such as bran or husks, and general food waste that forms the feed material for insect-based protein. The chapter outlines the latest research into by-products incorporated into extruded foods, detailing the major challenges facing this topic and sharing a couple of case studies of commercial ventures incorporating by-products into extruded food. This chapter forms a resource for researchers to study when developing a research questions and hypotheses within this field, and for industrials to consult when investigating opportunities to up-cycle biomass by-products into extruded foods across a range of applications including snack foods, breakfast cereals, and plant-based meats.

Figure 1.

Scope of the book chapter.

1.2 What is food system sustainability?

Food system sustainability is a complex topic that spans aspects all along the supply chain, from on-farm growth of crops by agricultural producers, ingredient preparation and product processing by food manufacturers, the selling of food products by retailers, and the consumption of food by consumers and the food service industry [1]. Sustainability can be addressed from multiple aspects, including sustainable agricultural practises to optimise crop yield with minimal inputs, the substitution of existing food ingredients with sustainable alternatives, optimised efficiency of food manufacturing processes, reducing food waste, and promoting social sustainability through food security and equity considerations.

A key opportunity to enhance food system sustainability is through addressing food loss and food waste. Food loss refers to a reduction in food production due to pre-consumer losses that occur all along this supply chain from harvest, post-production, storage, transportation, primary processing and wholesale. Recent estimates from the Food and Agriculture Organisation of the United Nations (FAO) stating that around 14% of global food production is wasted before reaching the retail market [2]. On the other hand, food waste refers to the disposal of food that is fit for human consumption. The United Nations Environment Programme (UNEP) estimates that an additional 17% of global food production is wasted in retail and consumer settings [3]. Although a high level of investment is dedicated toward agricultural research, greater than 90% of this investment is targeted at optimising crop productivity, while approximately 5% of investment is aimed toward research to reduce food losses [4]. This is a significant misjudgement, as greenhouse gas (GHG) emissions embedded in the wasted food product increase as the product moves along the supply chain [2]. Addressing food waste can have significant environmental, social, and economic benefits. It can help to reduce GHG emissions, conserve natural resources, and increase food security. Additionally, reducing food waste can also help to save money for both businesses and consumers.

1.3 Food waste hierarchy

Various options have been proposed to fight food waste, ranging from most to least preferable, including waste prevention, recycling, recovery, and disposal. Waste prevention involves strategies to avoid food waste from being generated in the first place, such as reducing the overproduction of perishable food products, optimising food manufacturing processes, and improving retail inventory management and storage practises. It also involves reduction strategies to limit the loss of food waste streams to worthless end point such as landfill, achieved through food recovery and redistribution approaches. Examples of this strategy include donating excess food to food bank organisations, marketing and selling imperfect-looking produce, or diverting food waste to animal feed applications. Waste recycling strategies involve the repurposing of food waste as an input into a subsequent process to create a product of higher value than the original waste stream. Examples of this strategy include the use of anaerobic digestion to produce biogas and biosolids, commercial composting to produce bio-fertiliser, or other bioconversion processes to generate valuable ingredients, chemicals, and materials [5]. Lastly, waste recovery aims to extract the residual energy stored within the biomass. Figure 2 outlines the food waste hierarchy, which is presented to create an emphasis on moving food up the waste hierarchy.

Figure 2.

Food waste hierarchy developed by Stop Food Waste Australia.

1.4 Food by-product upcycling through extrusion

Based on the food waste hierarchy, by-product recycling into new food products is the second most preferable option behind waste prevention. A significant opportunity for waste upcycling is the incorporation of by-products into existing food manufacturing processes. There is a dual benefit for this approach, which includes: (1) Environmental sustainability benefits relating to reduced demand for existing food ingredients, which involves considerable resource use in terms of land, water, fertiliser, fuel, chemicals for pest and weed control, and fertile soil [6]; and (2) Health benefits by creating value-added food products that have a higher nutrient content, improved textural properties, and/or enhanced functionality.

Extrusion has emerged over the past decade as a processing option for food by-product valorisation, as demonstrated by a 2014 review article into current options for food waste valorisation only mentioning extrusion once [7]. Extrusion is a food manufacturing technology that is emerging as a promising option for the incorporation of various types of biomass by-products [8]. The extrusion process involves a rotating single or twin-screw conveyor enclosed within a heated barrel, which cooks the food material through the application of temperature, pressure, and mechanical shearing. The product is formed into the desired shape through the choice of the die size and shape. Extrusion is widely used to manufacture various food products, such as breakfast cereals, snacks, pasta, noodles, plant-based meat substitutes, and animal feeds. The key benefit of this technology is its functionality and versatility, which allows an extruder machine to perform multiple processing functions (i.e. mixing, heating, shearing, cooking, material expansion, and product shaping), all in a single, continuous process. The process can be fine-tuned by adjusting temperature, moisture content, feeding rate, screw configuration, and screw speed, which directly affect the functional, nutritional, textural and sensory attributes of the final product. The food ingredients involved in extruded food production can vary depending on the specific product type being manufactured. However, some common ingredients include starch-based flour (i.e. corn, rice, wheat) for expanded snacks, plant proteins (i.e. soy, pea, peanut) for plant-based meat analogues, and fibre-rich components (i.e. bran, hulls).

2. Biomass by-product sources incorporated in extrusion

In recent years, there has been growing interest in using alternative ingredients for extruded food production, such as food manufacturing by-products. These alternative ingredients are often rich in nutrients and have a lower starting cost compared to traditional ingredients. In fact, their utilisation may save costs in relation to waste management. Different types of biomass by-products have been incorporated into extruded foods, which include:

• Fruit and vegetable pomace [e.g., apple pomace (AP), carrot pomace, grape pomace]

• Industrial residues (e.g., BSG, sugarcane bagasse)

• Oil press cake (e.g., canola, rapeseed, hempseed)

• Emerging extrusion feedstocks (i.e. insects)

Each of these types of biomass by-products will be discussed in the following sections, with Table 1 summarising relevant journal articles that study the incorporation of these ingredients in extruded food products.

By-product type	Citation	Research question	Key findings
Apple pomace	[9]	How does varying the content of AP (from 0% to 20%) within corn-based extruded snacks affect the content of dietary fibre and other health-promoting compounds, such as polyphenols, flavonoids, phenolic acids, flavonols, flavon-3-ols, and dihydrochalcones?	Up to 20% incorporation of AP led to significant increases in health-promoting compounds, included chlorogenic acid (up to 36 times), cryptochlorogenic acid (up to 4 times), catechin (up to 6 times), procyanidin (up to 3 times), phloridzin (up to 25 times) and epicatechin (up to 8 times).
	[10]	Can hydrochloric acid (HCl) pre-treatment of AP improve the interaction with starch during extrusion processing and increase the expansion ratio of direct-expanded products?	HCl reduced the cellulose, hemicellulose, and lignin contents, and increased the sugar content of modified apple pomace (MAP). In general, AP extrudates has a higher expansion ratio, although the MAP extrudate at 5 g/100 g had the highest expansion ratio of 3.87. Fourier Transform-Infrared (FTIR) Spectroscopy highlighted the 1035 cm^−1 moiety, representing the C–O bonds between cellulose and lignin, as contributing to low expansion ratio of the extrudates.
	[11]	How does the addition of AP affect the total phenolic content, flavanols, total flavonoids, and antioxidant activity of yellow corn- based extruded ready-to-eat products?	The incorporation of AP increased the total phenolic content, flavonoids, and antioxidant activity of the extruded products. The maximum flavonoid content was generated at the die temperature of 160°C and feed moisture of 14 g/100 g.
	[12]	What is the effect of extrusion processing on the total phenolic content, antioxidant activity, textural, and functional properties of a blend of extruded products incorporating AP?	Extrusion processing at higher AP ratios and lower temperatures and screw speeds increased the total phenolic content and antioxidant activity. The extrudates had increased expansion ratio, brittleness, crispness, and water solubility index, but decreased hardness and water absorption index. The optimum conditions for the extrudates were 30% AP ratio, 25% moisture content, 132°C temperature, and 108 rpm screw speed.
	[13]	Can food industry co-products, such as fruit pomace and liquid whey, be converted into shelf-stable, nutrient-enriched extruded products using low-shear, low-temperature supercritical fluid extrusion?	Supercritical fluid extrusion generates extruded products that are rich in dietary fibre, phenolics, and antioxidants, while also exhibiting a low density of 0.21–0.35 g/cm^3. Replacing water with liquid whey did not impact the textural properties of extrudate products.
	[14]	Can AP be incorporated into extruded snacks to improve their nutritional value and antioxidant content?	The incorporation of up to 20% of AP in extruded snacks increased the fibre content, phenolic content, and antioxidant capacity when compared to the products with no AP added. Although loss of phenolic compounds occurred during extrusion, the generation of thermally induced degradative products of phenolic origin compensated for that loss in terms of radical scavenging activity.
	[15]	How do extrusion parameters (die temperature and screw speed) and the addition of AP affect the quality of a corn flour-based extruded snack?	Screw speed had the greatest impact on the quality of extruded snacks, with an increase in screw speed leading to increased bulk density and decreased expansion ratio, while AP addition led to decreased product expansion. The optimal formulation contained 7.7% AP and was processed at a die temperature of 150°C and a screw speed of 69 rpm. Insoluble fibre in AP lowered expansion and adhesion to bubble structures punctured cells, reducing cell extensibility, in addition to damaging aerated bubbles do to its hydrophilic nature.
	[16]	What are the effects of different process conditions on the response variables (i.e. system parameters) of AP-soy flour-corn grits blends during extrusion?	Moisture content, temperature, screw speed, and apple pomace level all had significant effects on system parameters such as viscosity, specific mechanical energy, mass flow rate, torque, die pressure, and dough temperature during extrusion. Increasing the temperature resulted in decreased viscosity, die pressure, and specific mechanical energy. Increasing the level of apple pomace in the blend resulted in decreased viscosity, SME, and torque, and increased mass flow rate.
	[17]	Can extrusion modify the functionality of apple pomace?	Extrusion of apple pomace increased breakdown of cell wall structure and water solubility and increased the antioxidant activity during in vitro digestion.
	[18]	Can corn extrudates fortified with rosehip or AP improve the physico-chemical, sensory, and nutritional properties of extruded products?	The addition of fruit pomace to corn extrudates positively influenced the total polyphenol content and antioxidant activity of the product, particularly in samples enriched with rosehip pomace. While there was a slight decrease in some quality features, such as taste, flavour, structure, and colour, the extruded products obtained using fruit by-products were still well received by panellists and can be an excellent source of bioactive compounds in the human diet.
Carrot pomace	[19]	What is the impact of carrot pomace inclusion and moisture content on the quality of corn starch-based extrudates?	The study showed that the inclusion level of carrot pomace significantly impacted the extrudate quality, with the best expansion observed in extrudates with 5 g/100 g of carrot pomace inclusion and 15 g/100 g feed moisture.
	[20]	What are the effects of extrusion temperature and storage period on the microstructure, texture, and functional attributes of extruded snacks?	The increase in extrusion temperature resulted in an increase in L-value, b-value, and hardness of the extrudates, but a decrease in a-value, crispiness, β-carotene, and vitamin C. Storage for 6 months at 30°C and 50% relative humidity resulted in a decrease in L-value, b-value, crispiness, β-carotene, and vitamin C, and an increase in a-value and hardness of the extrudates.
	[21]	Can a low-cost, high-protein, and high-fibre snack be produced from composite flour made of rice, defatted soybean flour, carrot pomace powder, and cauliflower trimmings powder using extrusion processing?	The optimum extrusion conditions were 164°C die temperature, 313 rpm screw speed, and 85 g/100 g rice flour with a desirability value of 76.0%. The blend proportions under the optimum extrusion conditions had improved nutritional quality with 10.25 g/100 g protein and 0.84 g/100 g of fibre content.
	[22]	How much vegetable (carrot or broccoli) solids can be incorporated into extruded snacks with wet or dried vegetables and what is their effect on the extruded snack expansion?	Only ∼3% (dry base) of wet vegetable (carrot or broccoli) can be incorporated into extrusion expanded snacks. Higher vegetable content (from 20% to 100%) in extruded snack is possible using dried vegetable powders. Increasing the amount of vegetable in the extruded snacks reduced the expansion.
	[23]	What is the effect of die temperature, feed rate, feed moisture, and carrot pomace incorporation on the quality of extruded products made from rice and gram flour?	Die temperature had the greatest effect on the quality of extruded products, followed by changes in feed moisture and carrot pomace level. The optimal incorporation level of carrot pomace powder was 5%, which resulted in extrudates with an overall acceptability score of 7.4. Feed rate was not significant for any of the response variables, indicating no impact on the quality of extruded products.
Berry pomace	[24]	Can hot air (HA) and microwave assisted hot air (MWHA) drying combined with extrusion be used to convert bilberry press cake into value-added extruded food products at different inclusion levels?	Different drying techniques to generate bilberry powders implied a difference in processing time (40% reduction with MWHA drying), but the retention of total phenolics and physical characteristics of extruded snacks were independent of drying techniques.
	[25]	Can nutritious products be generated by incorporating blackcurrant juice press residues into the extrusion process, while retaining advantageous textural and sensory properties?	Non-enzymatic press residue extruded with barley or oat flour exhibited superior properties in terms of extrudate expansion, lower hardness and density, and higher contents of fructose, glucose and fruit acids, which all contributed to positive sensory evaluation of texture, appearance, and flavour.
	[26]	How does the interplay between starch (waxy, regular) and pomace (blueberry, cranberry) impact the physicochemical characteristics of extruded products?	The addition of pomace to starch blends significantly decreased the expansion ratio (ER) of the extruded products as the level of pomace inclusion increased, with differences observed between different pomace types. Fourier-transform infrared (FTIR) and solid-state nuclear magnetic resonance (NMR) spectroscopy supported the conclusion that interactions occurred via hydrogen bonding, as opposed to covalent bonding.
	[27]	What is the effect of cherry pomace inclusion level and particle size on their expansion characteristics of corn starch extrudates?	The inclusion of 5% cherry pomace milled to the smallest particle size range (<125 μm) yielded extrudates with the highest expansion ratio, outperforming the control sample. The study found that particle size significantly affected the radial expansion ratio of extrudates, with the smaller particle size ranges yielding higher expansion ratio at all levels of pomace addition. Cherry pomace addition also significantly decreased the water absorption index and water solubility index of extrudates, while smaller particle ranges led to a higher water solubility index.
	[28]	How are the techno-functional and physical properties of ready-to-eat texturized cereal products affected by the incorporation of chokeberry pomace powder (CPP)?	Increasing the content of CPP decreased the cell pore size, expansion, and lightness of extrudates. The glucose release from extrudates during in vitro digestion decreased with higher levels of CPP inclusion. The dietary fibre content was unaffected by the addition of CPP. Anthocyanins were degraded by 70% in both CPP-starch extrudates, while phenolic acids and flavonols were fully retained.
	[29]	What is the effect of pomace chemical composition and physical properties, inclusion level, and extrusion processing parameters on the expansion of extrudates made from fruit (cranberry, blueberry, grape, and apple) pomace-corn starch blends?	Active interaction was exhibited between pomace and starch components through extrusion, such that the pomace type and inclusion level had a significant impact on the expansion of corn starch-based extrudates. The level of insoluble dietary fibre and material crystallinity correlates negatively with extrudate expansion, whereas soluble dietary fibre and sugar content presented a positive correlation with extrudate expansion at 30% pomace inclusion level.
	[30]	How does thermo-mechanical treatment change the chemical composition and physical properties of chokeberry pomace powder after extrusion processing at 90% loading?	Physicochemical transformation of the chokeberry pomace powder through extrusion led to a significant increase in water absorption index, anthocyanin and dietary fibre content, and glass transition temperature. However, non-significant change was observed in the water solubility index and sorption isotherms, with extrudates exhibiting a low expansion coefficient.
Tomato pomace	[31]	What is the effect of varied tomato pomace levels within extruded snack products on the total phenolic content and bioaccessibility, antioxidant capacity, and physicochemical properties?	Increasing levels of tomato pomace powder in extruded snacks resulted in increased content of individual phenolic compounds, as well as enhanced bioaccessibility of protocatechuic acid, chlorogenic acid, rutin, quercetin, and lycopene. However, snacks with higher levels of tomato pomace powder had lower expansion indices, water absorption index, and water solubility index, and higher hardness, redness, and yellowness values. Despite these outcomes, the extrudate with 10% tomato pomace inclusion had the highest overall acceptability.
	[32]	Can tomato pomace be incorporated into a corn and rice flour blend to create an acceptable and nutritious extruded snack food?	The fibre and protein content of tomato pomace incorporated extrudates were significantly higher, while sensory analysis showed the high desirability was the product with 25% tomato peel and 5% tomato seed, which was on par with the control sample.
	[33]	Can tomato pomace be incorporated into ready-to-eat extruded snacks to generate fibre and lycopene-rich products?	All four independent variables (pomace level, moisture content, screw speed and barrel temperature) had a significant impact on all the response variables investigated (specific mechanical energy, hardness, water solubility index, water absorption index, expansion index, bulk density, and colour). The optimum extrusion conditions were 10% tomato pomace addition, 14% moisture content, a screw speed of 300 rpm, and a barrel temperature of 170 C. The optimal lycopene content within the enriched extrudate was 2.73 ppm at the 10% tomato pomace addition level.
Other pomace types	[34]	Can grape pomace be effectively incorporated into extruded corn starch snack foods to improve their physicochemical quality and nutritional value?	Extrudates with the lowest feed moisture content (16%) had the highest expansion ratio (∼3.8) and the lowest density (∼0.11 g/cm^3). The extrudate samples with 5% grape pomace and 16% moisture content retained >74% of the total polyphenolic content when processed at all extrusion screw speeds.
	[35]	Can nutrient enriched extrudates be generated through the utilisation of cashew AP by-products?	Cashew AP can be used to biofortify, significantly increasing their protein and overall mineral content, in addition to improving their phenolic, flavonoid, and total antioxidant activities. However, incorporation of cashew AP had a negative impact on textural and physical properties of extrudates but had negligible impact on sensory evaluation. The extrudates made with 5% CAPP scored the highest for taste, crispness, odour, chewiness, colour, and surface character.
	[36]	How do different fractions of olive pomace, separated via centrifugation and freeze-dried into a powder, impact the physical properties of fibre and polyphenol enriched extruded products?	The addition of olive pomace fraction reduced the die pressure and specific mechanical energy during extrusion, resulting in lower radial expansion in the extruded product.
	[37]	How effectively can pineapple pomace be incorporated into extruded products as a source of dietary fibre?	Up to 10.5% of pineapple pomace can be added to extruded foods without affecting the hardness, yellowness, water absorption, and bulk density of the final product, but the addition of 21% of pomace negatively affected the characteristics of the extrudates.
Fruit / vege bagasse	[38]	What impact does processing temperature and moisture content on the colour and expansion ratio of extruded snacks formulated with cassava bagasse?	The moisture content and extrusion temperature significantly affected the physical and functional properties of snacks based on pre-gelatinized flours. Extruded snacks processed at 104°C and moisture of 16% exhibited a large expansion index, intermediate specific volume, and lighter yellow colour.
	[39]	What is the effect of extrusion temperature, moisture content, and naranjita bagasse content on the physical, sensory, and chemical properties of extruded snack foods?	The highest expansion index and the lowest penetration force were observed at high extrusion temperature and low moisture content, while the highest carotenoid content and colour difference (ΔE) were observed at high naranjita bagasse content. The optimal processing conditions were extrusion temperature of 125°C, moisture content of 23%, and naranjita bagasse content of 8.03%.
	[40]	How does orange bagasse, a low-cost, natural source of fibre, impact the physicochemical and structural properties of extruded snacks made with blue corn?	The incorporation of orange bagasse increased the crude fibre content and hardness of extruded snacks made from blue corn, in addition to decreasing the expansion index of the snacks, resulting in harder products with higher numbers of pores per area but of smaller sizes. The extruded starch lost its semicrystalline structure due to mechanical shearing and high temperature throughout the extrusion process.
Sugarcane bagasse	[41]	What impact does the addition of sugarcane bagasse fibre have on extrudate physicochemical characteristics?	The content of insoluble fibre decreased while soluble fibre increased during the extrusion process. High bagasse content and barrel temperature led to a decrease in expansion index values and an increase in water absorption and water solubility index values.
	[42]	How are the expansion characteristics of corn starch-based extruded products affected by the incorporation of sugarcane bagasse and AP by-products?	Inclusion of AP in corn starch-based extrudates produced higher initial and stable expansion indices, but also showed higher shrinkage than sugarcane bagasse extrudates, potentially due to the presence of sugars in the AP.
BSG	[43]	What is the effect of barrel temperature and BSG content on the physical properties of extruded snacks based on rice flour?	BSG reduced apparent density and volumetric expansion index of extruded snacks. SME input decreased significantly with an increase of BSG content, with the dilution of starch content leading to a reduction in melt viscosity. The frequency of structural ruptures was positively affected by barrel temperature, which is related to an increase in the number of internal pores of the extrudates and an increase in the crispness of extrudates.
	[44]	How does the incorporation of BSG impact the phenolic content, antioxidant capacity, arabinoxylan content and glycaemic index of extruded products?	The addition of BSG into extruded snacks increased the total phenolic content by 4 times, the DPPH radical scavenging activity by 19 times, and the antioxidant power by 5 times. The choice of base ingredients and extrusion conditions need to be optimised to produce low GI products more effectively with BSG incorporation.
	[45]	How do the extrusion processing parameters of moisture content, screw speed, and temperature profile impact the properties of barley based extrudates incorporating BSG?	The formulation water content and extrusion screw speed had the greatest impact on snack expansion. The addition of BSG led to an increase in the dietary fibre content of extruded snacks, while whey protein isolate addition led to extruded snacks with the highest protein content. Starch was required within the formulation to generate sufficient expansion and reduction in hardness for the extruded snacks.
	[46]	Can BSG be incorporated into extruded snack products to improve their nutritional and textural properties?	Increasing the level of BSG in the formulation led to a significant decrease in the sectional expansion index (SEI) of the extrudates, in addition to a decrease in the individual and total area of cells within the extrudate. BSG also created an increase in bulk density and phytic acid content.
	[47]	Can the addition of pectin resolve the expansion and textural problems of corn-based extruded snacks loaded with biomass by-products including BSG, sugar beet pulp, and AP?	While by-product addition lowered the expansion ratio and fracturability and increased the bulk density and hardness, the addition of pectin at 1% loading improved the expansion ratio of all extrudates incorporating by-products. However, no by-product incorporated extrudates reached the original expansion ratio of the corn-based control sample.
Oil press cake	[48]	What are the effects of defatted hemp cake addition on the physical properties of extruded snacks and process parameters optimise the quality of these products?	The level of defatted hemp cake addition and moisture content both decreased the expansion ratio and fracturability of the extruded snacks, while increasing their bulk density and hardness. In addition, the processing temperature significantly increased the hardness of extrudates.
	[49]	How does screw speed and semi-defatted sesame cake (SDSC) content affect the nutritional, textural, microstructural, and sensory properties of corn-based extrudates?	A 10% addition of SDSC resulted in a fourfold increase in protein content compared to the corn-based extrudates. Although, the addition of SDSC also reduced the sectional expansion and increased the puncture force of corn-based extruded snacks, which was hypothesised to be caused by the protein and fibre-rich sesame cake reinforcing the cell wall structure and increasing the resistance to deformation. It was also hypothesised that the introduction of fibre led to the premature rupture of gas cells during material expansion upon exiting the extrusion die Meanwhile, screw speed did not exhibit a significant impact on the puncture force and crispness work of extrudates. The product hardness was also increased due to the reduction porous cell size, which resulted in a less crispy extrudate.
	[50]	What are the optimal processing conditions for developing desirable extrudates in terms of physical, functional, textural, and sensory attributes by exploring the effect of virgin coconut oil (VCO) cake content, feed moisture and extrusion screw speed?	The optimal extrusion conditions were calculated to be 28.7% VCO cake addition, 14% feed moisture content, and a screw speed of 300 rpm, yielding a maximum desirability for extrudates of 0.88. A higher content of VCO cake generally led to a higher sensory acceptability and lower hardness, expansion ratio, peak viscosity, water absorption index, and water solubility index. A higher feed moisture content led to a lower sensory acceptability and water solubility index. A faster extrusion screw speed generally led to a higher expansion ratio, peak viscosity, and water absorption index.
	[51]	What is the impact of rapeseed press cake content, starch type, and extrusion temperature on the extruder response, rheological behaviour, expansion mechanisms, and technofunctional properties of extruded products?	The addition of rapeseed press cake from 0% to 70% increased the SEI of potato starch extrudates, although this relationship did not hold for waxy potato starch extrudates. It is hypothesised that the protein- and fibre-rich biopolymers within press cake by-products influences the rheological properties of the melted material, which impacts expansion behaviour upon exiting the extruder die through enhanced complex viscosity.
	[52]	What is the effect of varying concentrations of rapeseed press cake, moisture content and barrel temperature on the expansion behaviour of extruded starch blends and how do rheological properties influence this phenomenon?	For extrudates with 10% rapeseed press cake, the initial SEI of samples 10 seconds after exiting was in the very high range of 6–16, while the final SEI of extrudates dried at 40°C for 24 hours experienced severe shrinkage to reach ∼3. For extrudates with 40% rapeseed press cake, the initial SEI was very low at ∼1, but increased to ∼3 for the final SEI due to due to time-dependent swelling. It was hypothesised that the high initial SEI was due to the presence of soluble fibres in the melted extrudate exiting the die, which entailed high water binding and holding capacity and enabled higher swelling due to more nucleation sites for water vaporisation and higher melt elasticity.
	[53]	How can extrusion be optimised to produce high-quality, protein-enriched, nutrient-dense ready-to-eat extrudates with defatted sesame flour (DSF)?	The experimental factors that had the greatest impact on the tested variables, in descending order, were the DSF content, moisture content, die temperature, and screw speed. The optimised extrudate had a protein content of 19.21 g/100 g. The optimal extrusion conditions were identified as 22% DSF content, 15.6% moisture content, a die temperature of 130°C, and screw speed of 250 rpm, with an overall sensory desirability of 73%.
	[54]	How does extrusion affect the canola meal?	Extrusion at high barrel moisture (36%) favoured protein aggregation resulting in lower extractable protein compared to extrusion at the lowest barrel moisture (24%). At lower barrel moisture contents (24% and 30%), a longer kneading block length increased extractable protein but this was not the case at 36% barrel moisture. Canola protein digestibility was improved upon extrusion at 30% barrel moisture but there was no significant change at lower (24%) or higher (36%) barrel moisture.
	[55]	How does extrusion affect the hempseed oil cake?	Extrusion enhanced the production of free polyphenols, flavonoids, and phenyl propionamide content, and α-glucosidase and acetylcholinesterase inhibition activities of hempseed oil cake.
	[56]	How does extrusion affect hempseed oil cake flour?	Extrusion at 40% moisture content and 400 rpm screw speed produced optimal overall characteristics and has potential to manufacture texturized protein meat analogue.

Table 1.

Summary of publications investigating the impact of incorporating fruit and vegetable pomace by-products into extruded food products.

2.1 Fruit and vegetable pomace

2.1.1 Apple pomace

AP refers to the fibrous solid residue remaining after pressing for juice or cider production, consisting of skin, core, and the flesh of the fruit. In the past, it has been considered a waste product with few uses outside of low-value applications such as pectin production, animal feed, soil composting [9]. The remainder of this by-product stream is typically sent to landfill. However, there has been a recent increase in interest to incorporate AP into extruded snack food products.

AP has been highlighted as a by-product resource of potential value, as it is rich in dietary fibre and phytochemicals such as phenolic acids and flavonoids [14]. A summarised analysis of the chemical components within AP showed that it consists of total dietary fibres (36.8%), fructose (16.0%), starch (14.0%), sucrose (8.4%), glucose (7.5%), water (7.3%), protein (3.7%), ash (1.9%), amino acids (1.8%), triterpenoids (1.6%), macro/microelements (0.6%), polyphenolic compounds (0.4%), and malic acid (0.03%) [57]. Isolated extracts and components from within AP have pointed to promising antioxidative, anti-inflammatory, antibacterial, and antiviral activities within medical studies, which may help address health issues such as diabetes, cardiovascular disease, and high cholesterol [57]. In addition to the pro-health properties of AP, its incorporation into extruded snacks at low levels (<10%) can improve the functionality of the extrudate products, such as enhanced expansion. However, above this critical level, the incorporation of AP can form highly dense and low expanded products [10]. Extrusion of AP increased breakdown of the cell wall structure, water solubility and the antioxidant activity during in vitro digestion [17].

2.1.2 Carrot pomace

Carrot pomace is a byproduct generated during the processing of carrot juice and is produced in significant quantities by the juice industry. Despite containing a high amount of beneficial nutrients, including bioactive compounds with antioxidant properties, carrot pomace is traditionally used as animal feed, which is a relatively low-value application for the material [19]. Meanwhile, the potential use of carrot pomace in food products has been investigated due to its rich content of dietary fibre, crude protein, iron, calcium, β-carotene, and vitamins such as thiamine, riboflavin, vitamin B-complex [58]. Studies have shown that extrusion can modify the functional properties of food by-products, including those of carrot pomace, by increasing water solubility, water holding capacity, and viscosity [59]. Shelf-stable powder was obtained from carrot pomace or cosmetically degraded carrots, with incorporation of carrot powder (3–100%) in extruded snacks enhancing their nutritional value [22].

2.1.3 Berry pomace

Various types of berries are utilised in industrial processing to extract juice, including bilberries, blueberries, raspberries, among others. In the juice industry, up to 30% of the original fresh berry weight may left over from the extraction process in the form of a press cake [60]. Currently, a large degree of press cake biomass is discarded as waste or used for low-value applications such as composting, despite containing valuable compounds such as polyphenolic antioxidants, vitamins, pectin, and lipids. The incorporation of dried berry press cake powders into extruded snack and breakfast cereal foods presents a significant opportunity to improve the flavour and nutritional value of novel products over existing products within the market [24]. However, challenges still need to be overcome relating to the loss of desirable physical properties of extruded products after the incorporation of berry pomaces, including reduced expansion index and increased density and hardness, in addition to maximising flavour and micronutrient retention [61].

2.1.4 Tomato pomace

Tomatoes are the second most consumed vegetable in the US behind the potato, including both raw and processed products such as a wholefood, paste, sauce, puree, juice, or soup [62]. Low value by-products of industrial processing include tomato seeds and peels, although they retain relatively high amounts of beneficial components such as lycopene, phenolic compounds, β-carotene, vitamin C, dietary fibre, and protein [63]. The utilisation of tomato processing waste is a crucial aspect of fighting food waste in the food manufacturing industry. Waste utilisation is an important activity to recover valuable bioactive components and prevent squandering of embedded resources [63]. In addition, spoilage of tomato waste creates an anaerobic environment that leads to the production of GHGs such as methane, creating a double negative to not utilising these waste resources.

2.2 Bagasse

Bagasse refers to the fibrous residue that is left over after pressing out the target compounds from crop products, which typically include sugars, starch, or other soluble bioactive compounds. Bagasse is typically discussed in the context of pressed sugarcane after sugar extraction but can also be related to starchy root vegetables such as cassava, and the fibre rich rind of some fruits. It is predominantly composed of cellulose, hemi-cellulose, and lignin, and has in the past been incorporated as a functional additive into food products such as bread, meat products, dehydrated gravies, puddings, and feed, and extruded snacks [41]. Cassava bagasse has been highlighted as difficult to further utilise due to its high moisture content, which promotes microbial contamination and requires energy intensive drying to prepare into a suitable feedstock condition. For the drying process to be economically viable, high value applications for the incorporation of this by-product resource must be demonstrated [38]. Bagasse is also discussed in the context of citrus fruit peel removed prior to juice extraction. The chemical composition varies between different species, but sits in the range of 50% cellulose, 10% pectin, 18% hemicellulose, and 18% lignin [64].

2.3 Brewer’s spent grain

BSG is an abundant by-product generated during the wort filtration step of the brewing process. BSG has been shown to be a rich source of nutrients that remain after wort extraction, such as insoluble protein, cell wall fibre residue, and minerals [65]. The dietary fibre content present within the BSG may vary due to the type of malting process employed [45]. However, conventional applications have been limited to animal feed, with emerging applications in human nutrition products. The valorisation of BSG in extruded food not only reduces the environmental impact of the brewing industry but also offers a valuable source of dietary fibre and protein for human consumption.

2.4 Oil press cakes

Oil press cake is a byproduct of oil extraction process from seeds or nuts. Oilseed species most commonly cultivated for oil extraction are rapeseed and sunflower, while cultivated for proteins is soybean, and cultivated for fibre is cotton [66]. The press cake has a high residual protein content and has been commonly used for animal feed applications. Further processing of the oil press cake can extract residual oil through solvent extraction or expeller pressing to form a defatted cake. Alternatively, press cake can either be used as a feed for food applications that require high protein contents. Oil press cakes are considered a cheap source of valuable components since modern protein extraction technologies enable recovery of target compounds and upcycling of this material within the food system as functional additives [48]. Extrusion has been found to enhance the protein digestibility of the canola seeds [54] and enhanced the production of free polyphenols, flavonoids, and phenyl propionamide content, and α-glucosidase and acetylcholinesterase inhibition activities of hempseed oil cake [55]. Extrusion of hempseed cake flour showed the potential to convert it into a meat-like textured product [56].

2.5 Research into extruded food incorporating by-products

Table 1 outlines the studies that investigate the incorporation of fruit and vegetable pomace into extruded food products, outlining the main research question investigated and the key findings presented within the study.

3. Challenges for by-product incorporation into extruded foods

There is a growing desire for organisations across the entire food supply chain to enhance the sustainability of their operations, alongside existing priorities for generating economic prosperity and social value. Depending on where each organisation sits along the food supply chain and their business operations, different opportunities and challenges will be present for by-product valorisation through extrusion processing.

3.1 Logistics and transport

For agricultural producers, opportunities exist to extract more value from an existing crop yield through collecting and distributing certain types of crop residues that still contain valuable components, such as stems, leaves, and products out of specification. In addition, opportunities exist for industrial food and biochemical manufacturers to turn existing ‘waste’ streams into additional value streams. However, challenges centre around logistical hurdles for sufficient by-product volume to satisfy downstream manufacturing requirements. Furthermore, large countries such as Australia suffer from the ‘tyranny of distance’ that significantly increases the transportation cost to aggregate these low value by-product resources [67]. These challenges must be addressed to ensure economic viability to justify a change in standard operating procedures for by-product producers. An idea proposed to address these challenges is investment into manufacturing hubs located at regional hotspots of waste generation, which presents the opportunity to develop new food manufacturing industry for a range of different product types and drive the transition to a circular economic model [68].

Additional processing steps (i.e. drying, grinding) are also required to remove water and minimise microbial activity, which can reduce subsequent transport costs, minimise food spoilage, and enhance the manufacturing viability of downstream products. Effective drying methods for different by-product types are required to be investigated to ensure to viability of energy and cost consumption [69]. Due to the distributed nature of by-product production, it is not feasible to perform these additional processing steps at the by-product generation site. Centralised collection and stabilisation points need to be investigated to address these challenges.

3.2 Feedstock variability

For food manufacturers looking to incorporate by-products into new products, opportunities are available to improve the nutritional profile, functionality, material costs, and consumer acceptance of their products. However, significant challenges still exist regarding the seasonal availability of by-product resources [70], the reliance on upstream by-product suppliers to provide relatively consistent material over time and into the future [71], and the impact of by-product incorporation on the functional properties of products. Further details of the challenges that are expected when approaching the opportunity of incorporating biomass by-products into extruded food products are discussed below.

3.3 Product safety

To enhance consumer acceptance of novel extruded food products that incorporate by-products, the presence of detrimental compounds in the by-product material must be recognised and tested for in the final product. A small fraction of compounds within food products have allergenic potential, such as glycinin and β-conglycinin, which are the main proteins that cause allergenicity in peanut and soybean-based products [72]. In addition, antinutritional compounds within the by-product sources (i.e. oil press cake) must be quantified both before and after processing. Suitable processing techniques are required to obtain novel products that are safe and allergen-free. Toxicity studies implemented within the development of novel food will be essential to ensure product safety and consumer acceptance [69].

Biological and enzymatic degradation is another important aspect that needs to be considered during transformation of biomass by-products intended for incorporation into food-grade products. Biological degradation refers to the breakdown of organic matter by microorganisms, with enzymatic activity enabling the breakdown of complex carbohydrates, proteins, and fats present in the biomass. By-products derived from agricultural crops or food processing often naturally contain enzymes and microorganisms that can lead to spoilage or undesirable changes in the final product, such as causing off-flavours, odours, or reducing the product shelf life. Managing biological and enzymatic degradation during by-product transportation and transformation is essential to ensure the production of safe and high-quality food products.

3.4 Product quality

When incorporating by-products into extruded food products, one of the challenges is ensuring consistent and desirable product quality. The extrudate porosity is an important property impacting the quality of products such as expanded snacks, breakfast cereals, and texturised vegetable protein. Porosity is a ratio of the volume occupied by air within the external circumference of the extrudate piece. This is tightly linked to the extrudate expansion index, which refers to the increase in area (or puffing) of the extruded product as it exits the extruder die, relative to the area of the die orifice. Expansion ratio is an important parameter relating to product quality, as it impacts the texture (crispness), mouthfeel, and overall consumer acceptance of the product.

To quantify the relationship between by-product inclusion and expansion ratio in the literature, we performed a meta-analysis study to collect data on the impact of including pomace by-products on the radial expansion index (REI) of expanded snack products. A subset of the studies presented in Table 1 were selected for quantitative analysis, based on their data availability and the quality of the experimental design. Multiple regression analysis was performed on the results of each study to visualise the relationship between pomace addition and REI. This meta-analysis across the published literature demonstrates that almost all the relevant studies present an inverse relationship between pomace level and REI. As such, pomace addition across different fruit and vegetable types decreases the expansion index of extruded products. Figure 3 provides a summary of the relevant literature that studied the effect of AP incorporation into extruded snacks and the impact on extrudate physical properties.

Figure 3.

The relationship between pomace inclusion and expansion ratio across various studies, with the citation for each study located in the title box above each panel.

4. Case studies for by-product incorporation into extruded food

4.1 Case study 1: Nutri V

Nutri V is a food manufacturing company that is aiming to promote healthy eating while also addressing on farm food waste during vegetable production. This start-up has partnered with Fresh Select, a major grower-packer supplying a range of vegetables to Coles supermarkets across Australia, to commercialise the production of vegetable powders and vegetable-based snacks (Nutri V ‘goodies’). The vegetable powders can either be bought as is and consumed in a range of different product formats such as smoothies, bread, and pasta, or act as a feed ingredient for the extrusion process to generate the Nutri V ‘goodies’. This technology was developed through collaboration with the Commonwealth Scientific and Industrial Research Organisation (CSIRO), Australia’s national science agency and innovation catalyst, with the technology enabling retention of the natural colour and nutrients of the vegetable by-products. The vegetable by-products consist of out-of-specification products, which have been estimated at 20–30% of the overall crop yield for different vegetable types, in addition to vegetable stems and leaves that are not typically sold to consumers but are edible nonetheless. The vision of this venture is to provide greater benefit to agricultural producers (i.e. farmers) through shifting crop yield from the traditional range of 65–80%, to whole crop yield of 100% by collecting all the green biomass previously left on-farm. The complimentary value proposition of this venture is to encourage healthy eating for the Australian population by providing additional serves of vegetables in products that would not typically have nutritional value, such as traditional starch based ready-to-eat snacks.

The process to generate the vegetable fortified snacks begins with identify relevant crop by-products that do not meet aesthetic standard for wholesale, pre-processing of the fresh biomass through drying and grinding to yield a vegetable powder and stabilise the ingredients against microbial degradation, incorporation of vegetable powder into the extrusion process at approximately 10–15% loading alongside binding components (i.e. corn flour, potato starch), and finally flavour addition post-extrusion.

4.2 Case study 2: planetarians

Planetarians is a food technology company established in 2013 that aims to create food this is both healthy for people and good for the planet [73]. After initially launching a range of nutritionally complete drink products containing protein from oilseed cake by-products, which sold over one million servings, they pivoted into producing fibre and protein rich chip snacks from up-cycled sunflower oilcake in 2015. In 2017, they partnered with the pasta manufacturing company Barilla Group to produce high protein ‘black pasta’, once again incorporating oilseed cake by-products.

Finally, in 2021, they once again pivoted to enter the vegan meat space, utilising plant-based protein to produce ‘whole cut’ meat analogues through a high moisture extrusion process. A key point of difference within their production process is the incorporation of brewer’s spent yeast, a by-product of the fermentation process to generate beer. Planetarians state that the incorporation of this cheap, readily available by-product resource provides a range of benefits including masking the beany aftertaste of soy, adding umami flavour and meat-like colour, and increasing the protein content, quality, and functionality of the final product. In addition, yeast products have a Food and Drug Administration (FDA) status as Generally Recognised as Safe (GRAS), which facilitates convenient approval as a food-grade product. Recent capital raising has generated a $6.7 million seed II investment round to fund commercial production of its patented, plant-based protein product [74]. With verification of its technology at an industrial scale already complete, the company plans to use the capital to build a pilot facility and continue to increase production.

AB InBev, the largest beer manufacturer in the world, has committed to partnering with Planetarians for this venture, with the company stating that they are “excited to support new ways to bring circular solutions to the center of the plate though upcycled ingredients and alternative protein innovation” (Bernardo Novick, AB InBev). The vision for this partnership is to place its production line, which can be as compact as 3000 square feet, across AB InBev breweries all around the world.

5. Emerging opportunities in insect protein

Over the past decade, there has been growing interest in insect farming (e.g., raising and breeding insects as livestock) [75]. This attention is mostly attributed to their ability to efficiently convert low-value organic waste streams into high-quality products (e.g., protein, fat, chitin, and fertiliser) [76]. Compared with conventional livestock (e.g., pigs and cattle), research has also shown that insects produce lower GHG emissions and can be farmed with less land and water [77, 78]. Insect farming therefore provides a promising opportunity for both waste valorisation and sustainable food production.

5.1 Waste valorisation: insect-based bioconversion of agri-food waste and by-products into food

Insects can be farmed for food or feed. Currently, mealworms (Tenebrio molitor) and crickets (Acheta domesticus) are the main species farmed for human consumption. The nutritional composition of each insect varies due to factors such as rearing conditions, diet, and age. However, values typically sit between 15% and 25% protein, 4% and 13% fat and 2% and 4% fibre (made up mostly of chitin) (per 100 g edible portion) [79]. Both species also provide a source several vitamins and minerals, including iron, zinc, vitamin B₂ and vitamin B₁₂ [80]. While commercial mealworm and cricket farming remains mostly reliant on mixed grain and soy-based feeds [81, 82], several agri-food waste and by-products have been explored as sole or supplementary feeding substrates substrates [76] (Figure 4). This includes:

• fruit and vegetable waste (e.g. peel, pomace, and leaves)

• BSG

• spirit distillers’ grain

• beer yeast

• bakery remains (e.g. bread and cookies)

• pre-treated crop residues (e.g. wheat, rice and corn straw, rice bran and rice husk)

Figure 4.

Waste valorisation via insect-based conversion of agri-food waste and by-products into food.

Despite clear potential for agri-food waste and by-products as feed for mealworms and crickets, several challenges remain. Firstly, while recent findings suggest that insects can be reared on organic waste streams, growth performance and survival rates vary. Mixed feeds (composed of more than one substrate) are therefore recommended to better meet physical and nutritional diet requirements [76, 83]. Local and year-round substrate availability should also be considered, alongside regulatory restrictions. For example, in regions such as Europe and Australia, insects are regarded as farmed animals and therefore subject to animal feed regulation. Potential feed sources such as catering waste are consequently prohibited from use [76, 84].

5.2 Incorporation of insects in extruded foods

Edible insects such as mealworms and crickets can be used in food to improve nutritional composition, substitute conventional protein sources, or contribute sensory attributes (e.g., flavour and texture). To prepare these species for human consumption, they typically undergo a series of processing steps including (1) inactivation/killing (generally by freezing, blanching, or steaming); (2) washing; (3) blanching; and (4) drying (generally by oven-drying or freeze-drying). Following drying, mealworms and crickets can be used whole, ground, or further processed to extract protein, lipids, and chitin.

Poor consumer acceptance remains a key barrier to the commercialisation of insect foods in Western food cultures, mainly driven by a lack of familiarity with insects in the context of food. However, research suggests that the use of non-recognisable insect ingredients (e.g. powder, flour, insect protein concentrate, texturized insect protein) could help to change perceptions [85]. Familiar foods such as bread, baked goods, pasta, and corn chips incorporating insect powder and flour are well-established among commercially available insect-based foods (e.g., Grillon Le Pain ‘Crickbread’, Chirps ‘Chocolate Chirp Cookie mix’, Sens ‘Cricket Protein Pasta’, Circle Harvest ‘Cricket Corn Chips’). However, extrusion processing presents an emerging pathway for the expansion of non-recognisable insect ingredients in several product categories, including breakfast cereals, snack products and meat analogues. Several studies have already investigated applications for insect-ingredients in extruded food products, including protein enrichment in expanded snacks as well as soy substitution in meat analogues. Expanded snacks demonstrated acceptable sensory properties at 6–15% substitution [86, 87, 88, 89] while 30–40% substitution was shown to provide meat-like properties and enhance flavour in meat analogues [90, 91]. Due to poor expansion properties, the high lipid and protein content of cricket and mealworm powders remains a key barrier to further enrichment of expanded snacks [86, 87]. However, some early products are already commercially available (e.g., Actually Foods ‘Food puffs’).

While the potential for insect-based extruded food products is promising, the nascent nature of the edible insect industry presents some challenges. Crucially, regulatory issues (related to approval of novel foods and a lack of clear industry standards) remain a key constraint to the farming and use of insects in food. Further research is also required to ensure safe farming, processing, storage and transport of insects and insect-based foods, as well as for management of allergenic risks (e.g., cross-reactive allergies and potential novel insect allergens) [92].

6. Summary and future perspectives

As awareness grows around the sustainability impacts of food waste, investigation into technologies that increase the value of by-product resources becomes more salient. Extrusion processing is one such technology that has been proposed to upcycle biomass by-products into higher value food across a range of different product types. This chapter explores the challenges and opportunities associated with the incorporation of by-products into extruded food products, in addition to case studies that share success stories of commercialised products that incorporate biomass by-products. Lastly, the emerging industry of insect protein is introduced as an opportunity for waste valorisation and sustainable food production.

Looking ahead, there are several important areas for future exploration in the field of extruded food development incorporating by-products. Firstly, addressing the logistical challenges related to the collection and transport of by-products is crucial. Investing in manufacturing hubs located strategically at regional waste generation hotspots could facilitate the transformation of low-value by-products into valuable resources, promoting a transition to a circular economic model. In addition, more focus needs to be place on strategies to overcome the issue of low radial expansion when incorporating by-products. Currently, the level of by-product addition is limited by this issue, which creates uncertainty around the value proposition of by-product incorporated snacks if their functional properties degrade above 5% addition.

Overall, addressing these challenges and taking advantage of emerging opportunities in this area will require enhanced collaboration and knowledge exchange between stakeholders across the food supply chain, including primary producers, post-production, primary food processing industry, waste management sector, consumer markets, and research institutions covering this topic. Extruded food manufacturers will have to integrate into existing activities in this space to overcome challenges include by-product transportation and logistics, food safety, and product acceptance.