Currently, many animal by-products are primarily used for the most economically viable purposes, such as biogas production. As a result, much of their nutritional potential for human food use remains untapped. 

With the aim of increasing protein utilisation from selected by-products, a pilot study was conducted at the Danish Technological Institute and funded by Svineafgiftsfonden (Pig Levy Fund). The study investigated whether proteins from these by-products could be extracted through enzymatic hydrolysis and assessed their potential as future protein ingredients for human consumption.

Pig Blood

Blood consists of plasma and hemoglobin. After the plasma has been separated by centrifugation, the remaining fraction contains the red blood cells (hemoglobin). Hemoglobin is not, in itself, considered a high-value product as it requires further treatment before it can be used in food application. This is because the heme iron content gives it a red colour and a metallic taste. Hemoglobin can be treated through enzymatic hydrolysis using protein-degrading enzymes such as proteases, peptidases, and glutaminases.

Figure 1 outlines how the enzymatic hydrolysis process was carried out in experiments involving the processing of several side streams.

Figure 1. Process flow diagram of enzymatic hydrolysis of side streams.

The pilot studies demonstrated that it is possible to produce a colourless protein hydrolysate powder with a sufficiently high protein yield (75%) and an acceptable amino acid composition for use in food products. In addition, satisfactory decolourisation was achieved (see also Figure 2).

A small-scale application trial was conducted in which blood protein was added to a meatball mixture as a substitute for caseinate. The results showed that the acceptable addition range was between 1.8% and 10%. Higher concentrations significantly affected the taste of the fried meatballs, resulting in a more bitter and sharper flavour. 

Figure 2. Top: Protein hydrolysate (porcine hemoglobin) produced through enzymatic hydrolysis. Bottom: Three batches of meatballs, where 1 is the control with no added blood protein, 2 contains 1.8% added blood protein, and 3 contains 10% added blood protein.

Other protein-rich by-products

When processing by-products, it is economically advantageous to treat a range of different by-products together. This helps to ensure a sufficient supply of raw materials while also simplifying handling and processing. For this reason, several animal by-products - kidneys, spleen, lungs, and hemoglobin - were processed simultaneously. These materials contain valuable proteins and other essential nutritional components with potential for use in the production of new food ingredients and/or animal feed. The results from the combined enzymatic hydrolysis of kidneys, spleen, and lungs showed that acceptable protein yields (76%) can be achieved through enzymatic hydrolysis. Initial tests also indicated that hemoglobin must be processed separately in order to achieve a sufficient yield.

Perspective

Based on the pilot study, it has been demonstrated that enzymatic hydrolysis can be integrated into a process for producing dried animal protein powder using selected side streams as the raw material base.

Depending on the intended end use (feed or food) and the availability of raw materials, the input raw material mix can be adjusted. The enzymatic process, including the enzyme cocktail, would need to be optimised accordingly to maximise yield, and where necessary, improve the flavour profile.