Scientifically Speaking …
By Matthew Wedzerai
Matthew Wedzerai holds an MSc in Animal Science (Specialisation: Animal nutrition) from Wageningen University, The Netherlands. He also holds a Diploma in Pig husbandry & Animal Feed, PTC+ College, The Netherlands. He has 7 years’ experience in the pig husbandry and animal feed industry and has been a writer of articles based on scientific research since 2014.
Common egg-yolk products Egg yolk is mainly sold in a dried form, and only a small part of the egg yolk – less than 1 percent – is sold in a refrigerated liquid yolk or frozen form (Huang and Dong, 2019). The yolk has excellent coagulation or gelation and emulsifying properties, and thus, its products can be used as a fundamental ingredient for emulsifying, colouring, flavouring, coagulating and nutrition supplementing agents (Miranda et al., 2015). The major uses of egg yolk products include mayonnaise, baking
THE VALUE OF EGG YOLK Due to negative perceptions of egg yolk, consumer demand for egg-white products such as frozen desserts, bakery mixes and batters is higher than that of egg yolk products. According to research, the negative perception comes from the wrong belief that egg yolk cholesterol increases serum cholesterol levels, thereby increasing the possibility of cardiovascular diseases. The value and use of egg yolk can be improved by fractionating egg yolk and separating functional proteins and lipids and developing them further into functional peptides and functional lipids. These can be used in several applications such as pharmaceutical products, functional foods and cosmetic health products. These approaches can diversify the use of egg yolk, triggering research into further improvements of egg yolk quality, ultimately increasing the consumption of the egg.
Egg and egg yolk facts Fresh egg yolk is an excellent source of proteins, lipids, and phospholipids. This is largely due to its composition; egg yolk is: 50.1 percent water, 30.6 percent lipids, 17 percent proteins, 0.6 percent carbohydrates and 1.7 percent minerals (Stadelman & Cotterill, 1995).
The price of dried egg white tends to be much greater than that
of dried yolk. This is due to high consumer demand for fat-free and cholesterol-free high-quality proteins from egg white compared to egg yolk. As described earlier, a negative perception on egg yolk results in relatively higher demand for egg white products, to avoid the ‘assumed’ serum cholesterol effects of egg yolk. However, in contrast, studies show that egg cholesterol does not increase the serum cholesterol in healthy people (Lemos et al., 2018). To increase overall egg consumption, diversifying the applications
for egg yolk is essential. Multiple products, which each have their uses in food processing, nutritional supplements, pharmaceuticals or cosmetics, can be separated from the egg yolk and used, either as they are or in a modified form.
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goods, noodles, creams, omelette, confectionery, sauces and salad dressing (Mannie, 2003). Egg yolk or whole-egg products produced from hens that have been immunised with specific antigens are also used to prevent or cure specific diseases in animal production and aquaculture – although in this respect their use is limited (Lu et al., 2008). It has also been suggested (Kiosseoglou, 2003) that due to the egg yolk’s complicated composition and structural characteristics, its functional properties have not been explored extensively.
Established and potential uses of egg-yolk components Egg yolks have several valuable properties that can be harnessed for diverse uses. It can be easily separated into two fractions: granules and plasma. The plasma fraction contains 78 percent of the yolk’s dry matter, and granules 22 percent. The plasma is mainly low-density lipoproteins (LDLs, 85 percent) and livetins (15 percent), while the granule part is mainly composed of proteins (60 percent) and lipids (34 percent) (Huang and Dong, 2019). Both fractions have high emulsifying properties. Figure 1 shows a possible separation method that might potentially produce value-added products. The following list of applications (harvested from several studies)
gives a sense of the range of the possible uses and products that can be developed from egg yolk: • as an ingredient for cosmetic cream formulations; • its granules fraction can be used as a low-cholesterol replacer for whole-egg yolk in the preparation of muffins; • its plasma fraction can be applied as a gelling and cryoprotecting agent due to its high LDL content; • the plasma fraction is a source of phospholipids due to its high lipid content; • the granule fraction’s functional proteins can be used as the starting material for immunoglobulin Y (IgY) extraction, and a source of functional peptides. The major applications for IgY are focused on the control of enteric infections such as Salmonella spp., enterotoxigenic E. coli, Campylobacter jejuni, porcine transmissible gastroenteritis virus, rota virus, porcine epidemic diarrhoea virus, influenza viruses and pathogen control in food products (Kumaran et al., 2018). For example, dietary administration of at least 0.2% egg yolk antibody powder for 3 weeks improved the intestinal health indices and immunological responses of broiler chicks orally challenged by
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