*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 allergenfree. 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

#### **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.*

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.

## **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 postextrusion.

#### **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.
