It has been estimated that around 13% of global food is wasted before the food ever reaches the retail market. If some of this waste could be curtailed, it could increase the global food industry’s sustainability and provide both economic and environmental benefits.
Food waste streams
One challenge for food producers who want to reduce waste is to figure out how to upcycle food waste streams. This would involve finding ways to use ostensibly edible but often currently unwanted side stream products like pomace from fruit and vegetable processing, press cakes of soybean pulp formed in the aftermath of oil extraction, or spent grain from breweries.
Side waste stream products like these often contain many health-promoting compounds like vitamins or antioxidants. They may also exhibit certain desired functional properties, such as water absorption or solubility, that are relevant to many food applications. Unfortunately, many of the compounds that contain nutritive ingredients are not easily accessible within those side stream byproducts. Often, they are trapped in the cell walls of the byproducts, which means the components in food side streams need to be modified in order to valorize the nutritional content and optimize their potential utility in food development.
Extrusion technology in the food industry
Researchers have started to look at how can extrusion technology can be used to modify the functional properties of those byproducts and achieve that valorization. Various raw materials can be fed into an extruder where the applied thermal and mechanical stresses can break apart the cell walls and create composites with different functional properties. The process can also free up bioactive compounds, making them more accessible. Extrusion processing can lead to an increase in the amount of soluble fibers present in a product and thus improve its water holding capacity, which could lead to an uptick in digestibility and metabolic absorption.
There are two different ways to go about using an extruder for food side stream upcycling. One way could be to use a twin-screw extruder with a die plate mode. The other method is to use the extruder as a granulator where the die plate is removed, which allows the extruder to run with an open discharge. In both cases, an intermediate product is formed as the output of the extruder. The difference between the die-plate product and the granulated product is the physical form of the product. With the die plate in place, the output is an extruded strand which is usually further cut and milled and dried. Using the granulation process with open discharge, there is no compaction of the material at the end of the process and so the result is granules with improved flow properties because of their relatively lower density. The granulator in this case also eliminates the need for cutting or milling steps before further processing.
Upcycling food
A good example of this extruder application can be seen with apple pomace (pictured above), which is a byproduct of apple cider and juice processing. Apple pomace is known to have beneficial effects on gut health and can even help lower cholesterol, but dried apple pomace is generally insoluble: if raw flakes of apple pomace are placed into water, they will simply sit there and soak. Whereas if an extruder is used to process the material, the extruded apple pomace can be made into a suspension with water, and over time the suspension becomes a stereotypical paste. This means the extruder was used to process a generally unusable food product to make a new food ingredient that has better water holding capacity and can in turn develop a paste-like structure. This paste could then be used, for example, as a thickening agent in a dairy product.
Another sustainable product to which extrusion can be applied is plant-based meats. Meat substitutes are made with ingredients like wheat flour, pea protein, or soy protein like that found in the soy cakes mentioned earlier. For such materials, an extruder is a great tool to modify their properties in a way that achieves a fibrous meat-like structure more closely resembling the properties of muscle meat. If plant protein samples associated with this process are analyzed using a rheometer the extruder-processed products are found to exhibit lower structural strength, which means they are lower in hardness and chewiness. These rheological measurements also indicate the products would be perceived as more tender and more palatable, which is very important for a meat substitute.
As these examples demonstrate, an extruder can be used to modify food byproducts into viable food ingredients that can be reincorporated back into new products (such as a novel plant-based meat substitute) or existing foods (e.g., as a thickener in a dairy formula). Because extrusion is such a flexible process, it offers great potential for sustainable product innovation when it comes to upcycling of food side stream materials.
Read additional details, including extrusion trial materials, methods, schematics, and results in our application note Upcycling of food side streams: the benefits of twin-screw granulation of apple pomace.
Additional Resources
- Application Note: Upcycling of food side streams: the benefits of twin-screw granulation of apple pomace
- Application Compendium: Alternative protein formulations and valorization of food side-stream applications with twin-screw extrusion and rheology
- On-demand webinars: Development of sustainable food products