Extrusion process: low and high moisture and their effect on digestibility

Extrusion is a technology used in various sectors – both manufacturing and food – that changes the structure of the source material used. In the food industry, this process is used for the production of pasta but also of more innovative foods such as meat and fish substitutes. The texturization process allows the elongation of protein fibres, resulting in products that are suitable for replicating different textures – spongy or fibrous – typical of meat.

Meat alternatives have been appearing in supermarkets for many years now. Pioneering companies in the sector include Beyond Meat, which launched its vegetable burger ‘Beyond Burger’ in 2012, followed by Impossible Foods, which launched the ‘Impossible Burger’ in 2016. Since then, the meat substitutes sector has expanded to include numerous companies, both large players and start-ups. However, according to data published in recent years by the Good Food Institute, after initial years of growth, sales of vegetable meat remain stable. Consumers do not seem to be convinced to buy – or repurchase – these products because of the taste, texture, and price. There is a need to drive more innovation in meat substitutes to improve their taste and texture, which are key factors for consumers, by optimising vegetable protein extrusion processes and experimenting with new flavourings.

Extrusion and texturization processes: the current situation

Extrusion processes are used to produce meat alternatives, be it burgers, mincemeat, patties or fillets. The most common are known as low and high moisture extrudates. Dry-extruded products are the most common product on the market, while wet-extruded products have only started to appear in recent years.

Low moisture extrudates (LME) – TVPs, textured vegetable proteins – are produced by means of an extrusion process in which the proteins are stretched in such a way to replicate a fibrous structure similar to that of meat. With this process, it is possible to obtain products of different shapes and sizes: from bite-sized chunks, strips, flakes or granules, making them as versatile as possible for a variety of uses. Low moisture extrudates have a higher water-retention capacity and, once cooked, can assume a consistency similar to that of meat or fish, depending on the flavouring and colourant used.

On the other hand, with the HME (High Moisture Extrusion) process, it is possible to obtain products with the fibrous consistency and humidity found in real meat. The main difference between dry and wet texturization lies in the level of humidity in the process. This is because the humidity content in HHMAs is around 50-70%, compared to TVPs produced using dry extrusion technology, with a humidity of around 20-40%, according to this article.

These products are gaining in popularity among consumers as they have a texture and chewiness very similar to meat. They do not have to be rehydrated, as in the case of dried TVPs, and can be consumed directly after the texturization process. However, the production process of wet extrusions requires further optimisation, as it involves a long cooling process, a refrigeration phase. What’s more, due to their water content, they have a shorter shelf life which means that to be fit for sale, a cold chain must be available.

The quality of the final extrudate depends not only on the properties of the raw materials, but also on process parameters such as humidity and temperature. According to the above-mentioned study, there are several theories on the effect of moisture in the extrusion process, especially in the creation of the fibrous structure and the rearrangement of protein molecules during the extrusion process. Furthermore, it appears that the increase in temperature combined with the decrease in moisture leads to the denaturation and aggregation of proteins during extrusion, improving the texture of the final product. Nevertheless, there is no perfect ‘recipe’ to obtain a good product, but the extrusion settings must be optimised with regard to the initial raw material and its properties.

Do extrusion processes improve protein digestibility?

Many food processing techniques can improve the nutritional quality of plant proteins, possibly eliminating anti-nutritional factors that interfere with protein digestion and reduce mineral and nutrient absorption. The extrusion process can improve the digestibility of proteins as it leads to their denaturation. Factors such as shear pressure during the process also contribute to the breakdown of protein bodies, leading to physical, chemical, and nutritional changes, improving protein digestibility. There are also several treatments to reduce anti-nutritional components and increase the digestibility of plant proteins, including technological processing procedures such as soaking, grinding, cooking, sprouting, and fermentation.