Soy remains the most widely used and studied plant-based protein source thanks to its high protein content and quality, relative to other plant-based ingredients.
But during the production of soy-based products, the ingredient generally undergoes several processing methods such as soaking, grinding, and heating to create soy milk, or dehulling and fat extraction to create soybean flour, and then removing soluble carbohydrates from this to create soy protein concentrate.
So although soy is well known for its high protein quality, little is known about the effect of processing methods on protein nutritional quality. Processing may influence the amino acid pattern and digestibility of soy protein, leading to different protein quality scores for different soy protein products. What’s more, consumers have expressed concern that processed soy products found in meat-free alternatives are made less nutritious by these processes.
To gain a better understanding of the effect of processing on the protein quality of soy, Unilever and Wageningen University researchers assessed the indispensable amino acid (IAA) composition and digestibility of varying soy products, to obtain both digestibility indispensable amino acids scores (DIAAS) and protein digestibility corrected amino acid scores (PDCAAS).
DIAAS is the current standard of evaluating protein quality and was developed by the UN's Food and Agriculture Organization about a decade ago in order to address certain limitations of the older PDCAAS score.
The major difference between these two protein quality scores, for example, is that the DIAAS relates the amount of ingested protein with the levels present at the end of the small intestine (‘ileal digestibility’). PDCAAS uses the levels of protein remaining in the faeces (‘faecal digestibility’).
However, as there is limited data on amino acid digestibility at the ileal level, the use of faecal digestibility is still a widely accepted method to evaluate protein quality and compare protein quality between sources. The researchers therefore used both methods to gain insight into the effect of processing on the protein quality of soy.
The significance of post processing
The results, based on a quantitative review of in-vivo and in-vitro studies, showed different protein quality scores between soy product groups. Soymilk had the highest DIAAS followed by soy protein concentrate, soybean meal, soybean, soy protein isolate, soy flour, soy hulls, soy flakes and tofu. For all soy products combined, mean DIAAS was 84.5 ± 11.4 and mean PDCAAS was 85.6 ± 18.2.
In addition, the researchers observed broad variations in protein quality scores within soy product groups, indicating that differences and variations in protein quality scores may also be attributed to various forms of post-processing (such as additional heat-treatment or moisture conditions). For example, the figures show that the majority of studies on soy resulted in DIAAS and PDCAAS values above 75, which classifies as high-quality protein. However, for some product groups, values below 75 were found, especially for post-processed soy products such as post-processed soybean, soy protein flour and soy protein isolate.
After excluding post-processed data points, for all soy products combined, mean DIAAS was 86.0 ± 10.8 and mean PDCAAS was 92.4 ± 11.9.
“This study confirms that the majority of soy products have high protein quality scores and we demonstrated that processing and post-processing conditions can increase or decrease protein quality,” the study’s authors wrote. They added: “Additional experimental studies are needed to quantify to which extent processing and post-processing impact protein quality of plant-based protein-rich products relevant for human consumption.”
Unilever called the research ‘a very exciting development’ which ‘proves that meat-free alternatives can fulfil our bodies’ protein requirements’.
Amelia Jarman, Unilever Future Health & Wellness Science and Technology Director, said: “The findings of this research put to bed concerns that processed soy-based foods are an inadequate source of protein. Our Unilever food brand, The Vegetarian Butcher, offers plant-based meats that provide a good source of protein, and can be used to create delicious and nutritious meals. Plant-based foods are better for the health of people and the planet, and this research demonstrates that protein quality is not compromised.”
Protein quality of soy and the effect of processing: A quantitative review
Frontiers in Nutrition