**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 byproduct 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:


• 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.











#### **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**
