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MGP food scientists answer your questions about applications issues. Here, Senior Scientist Rachel Mitacek, PhD., discusses the impact of processing on the nutritional value of plant proteins.

Question: Why does the nutritional value of plant proteins change in my ready-to-eat and extruded products?

It can be tricky to develop protein products such as bars, snacks, or dairy and meat analogues to hit a certain nutritional claim. Typically, we formulate foods for consumers based on the protein amount needed to obtain an adequate amount of essential amino acids per serving or based on the protein digestibility corrected amino acids score (PDCAAS) of our protein ingredients. The PDCAAS, or nutritional quality of proteins, depends on their amino acid composition as well as digestibility, or bioavailability.

Unfortunately, most plant proteins are deficient in at least one essential amino acid, deeming them “incomplete.” The amino acid score of plant proteins depends on the type of protein classes present (globulin, prolamin, albumin, glutelin, etc.), their relative ratio of abundance in the protein, and structure. Furthermore, protein from a single source (ex: yellow peas) can have variance in their amino acid composition based on several factors, such as the weather and soil conditions where they were grown, the time of harvest, crop variety, and many more. In addition to inherent characteristics and crop conditions, the isolation and processing of proteins can influence the amino acid composition, as well as digestibility.

Protein ingredient production uses several methods and processing techniques. Most techniques utilize alkaline, acidic, solvent, or salt solutions to isolate proteins from flour/meal. The type of solution used will have an affinity for certain classes of proteins, altering the protein profile, thus influencing the amino acid composition. Additionally, processing conditions may alter protein structure and digestibility.

Processing conditions such as pH (alkaline or acidic), heat, shear, homogenization, etc. are often utilized to increase yield, food safety, or inactivate anti-nutritional compounds (ex. Trypsin inhibitor). The inactivation of anti-nutritional compounds has been shown to improve protein digestibility. However, these processing conditions can also negatively impact digestibility.

Through processing, plant proteins may unfold or denature, exposing previously tucked away amino acids. These newly exposed amino acids tend to interact with each other, as well as other food components. Excessive or prolonged processing conditions tend to cause protein crosslinking and aggregation, or hydrolysis into smaller peptides. When amino acids are linked to one another or to other compounds, they require more energy for your body to metabolize or may become indigestible, hence reducing nutritional quality. On the other hand, hydrolysis of proteins into smaller peptides may enhance digestibility.

So why does the nutritional quality of ingredients change when they are used in our consumer- ready food products? Protein amino acid composition and digestibility are altered in ways similar to the changes that occur during ingredient manufacturing. Cooking, boiling, baking, homogenization, shearing, pH adjustments, and extrusion will lead to similar protein structural changes that influence digestibility.

A commonly used method to correct for lower protein nutritional quality is combining proteins from various sources that have opposite limiting essential amino acids, such as corn-soy or wheat-pea protein blends. Other options to consider are the method of processing and preparation utilized when developing your products. Nevertheless, it is important to correctly analyze protein nutritional quality of your finished products.

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