Consumption of food products rich in antioxidants protects humans from oxidative stress and the resulting damage and minimizes the risk of cardiovascular diseases, cancer, and diabetes. Antioxidant compounds play a key role in strengthening the immune system and preventing brain dysfunction. Many studies have shown that the prevalence of neurological diseases such as cerebral ischemia, Parkinson’s, and Alzheimer’s is lower in groups that receive antioxidant supplements.
Milk and dairy products are a rich source of compounds such as sulfur-containing amino acids, whey protein (especially β-LG), and vitamins A, E, and C, which have antioxidant properties. In addition, the fermentation or ripening processes of cheese cause the release of bioactive peptides that have antioxidant potential.
What is oxidative stress?
Large amounts of free radicals are produced in the human body through natural physiological processes, the external environment, as well as inappropriate diet, and neutralized by the body’s antioxidant system under normal metabolic conditions. On the other hand, metabolic disorders disrupt the balance between free radicals and antioxidant reactions, leading to excessive accumulation of free radicals in cells, which is called oxidative stress. The excessive presence of these molecules in the cell leads to the damage of proteins, lipids, and nucleic acids, which can cause tumors, neurological and neoplastic diseases, and circulatory or nervous system disorders, and accelerate degenerative processes.
Defense mechanisms against oxidative stress
In order to defend against oxidative stress and subsequent damage, organisms have developed enzymatic and non-enzymatic mechanisms. The enzymatic antioxidant barrier consists of specialized enzymes such as catalase (CAT), and the non-enzymatic antioxidant barrier is supplied to the organism through food. Food antioxidants directly neutralize the free radicals produced during the oxidation process, repair the oxidized membranes, and help to increase the overall resistance of the organism against infections.
Antioxidant potential of milk and dairy products
The antioxidant potential of milk is directly related to the content of components with antioxidant properties. Many authors emphasize that animal milk contains enzymatic and non-enzymatic antioxidants (milk proteins, bioactive peptides with antioxidant properties, and vitamins) that are very important in preventing the production of reactive oxygen species (ROS) and help strengthening the immune system against OS.
The antioxidant capacity of milk and dairy products is the result of a complex balance between antioxidants and oxidants. Oxidation processes have a negative effect on the quality of milk, which means that they reduce the shelf life of the milk and cause an unpleasant taste. High concentrations of antioxidants can prolong the delayed phase of protein oxidation. Therefore, to improve the properties of milk, it is necessary to increase the level of bioactive substances with antioxidant properties.
Yogurt and kefir have the highest antioxidant activity compared to other fermented products. The antioxidant potential of fermented milk decreases during storage but increases 2.5 times during digestion due to the release of antioxidant peptides. Regarding kefir, it should be said that the type of milk (cow or sheep) and the culture used significantly differentiate its antioxidant activity. Also, fluctuations in the antioxidant activity of kefir during fermentation are probably due to the inhibition of microbial enzymes present in activated kefir grains in the early stages of fermentation.
Cheese is considered as the main source of bioactive peptides due to its high protein content, variety of proteolytic enzymes, and degree of proteolysis during cheese ripening. The antioxidant activity of cheese is significantly related to the effect of the season of raw milk collection, cheese ripening period, and vitamin A content.
Factors influencing the antioxidant potential of raw milk
- Animal species
According to studies, animal species have a significant effect on the antioxidant content of milk. Compared to cow’s milk, sheep, camel, and buffalo milk contain higher levels of these substances, mainly β-LG, LF, vitamins A and E, and polyunsaturated fatty acids. Therefore, the type of milk also differentiates the antioxidant activity of dairy products, for example, kefir drinks produced from sheep’s milk have more antioxidants than kefir produced from cow’s milk.
- Diet
The antioxidant content of milk can be changed through animal feeding. Feed additives used in animal nutrition have protective functions and contribute to an increase in the immunity of animals exposed to stress (weaning, changes in diet, or transportation), leading to effective absorption of essential nutrients.
According to the studies, using maize silage in the diet of cows, due to its low carotenoid content, is one of the main factors in the reduction of the amount of vitamins and antioxidants in milk. Adding pumpkin silage to the diet of dairy cows, as a valuable source of bioactive compounds, especially carotenoids and flavonoids, significantly improves the nutritional value of colostrum. It was shown that dried orange pulp (DOP) significantly improved the nutritional value of milk and increased the level of vitamin E, phenolic compounds, and antioxidant capacity of milk. Some plants used as natural additives in animal nutrition have a positive effect on dairy production and milk quality with their high content of bioactive compounds (such as flavonoids, saponins, and carotenoids). Rosemary, thyme, buckwheat, black pepper, cinnamon, garlic, fenugreek, savory, and mint are mostly used in animal feed due to their antioxidant properties. It was also shown that the addition of cow’s milk to green and black tea leads to an approximately 2.1-fold increase in their antioxidant potential.
- Lactation Phase
There is a direct relationship between the lactation phase and the level of enzymatic and non-enzymatic antioxidants in cow’s milk. According to studies, there are higher amounts of antioxidants in colostrum and milk in the early stages of lactation (5-15 days), but its level was found to decrease gradually on the subsequent days. Higher levels of antioxidants in colostrum may protect calf health against oxidative stress.
Factors influencing the antioxidant potential of dairy products during processing
- Milk fat content
It has been shown that the fat content in milk affects its antioxidant potential. Full-fat raw milk has a higher antioxidant value than low-fat milk due to the lower content of lipophilic antioxidants in fat-soluble vitamins. Cow’s milk containing 3% fat has a much higher antioxidant capacity than milk with a lower fat content (1.5-0.5%) or fat-free milk.
- Thermally treated milk
Various reactions occur between milk components due to heating (for example, denaturation and aggregation of whey and formation of new complexes). Many studies show that β-LG is the most thermally unstable protein and easily undergoes thermally denaturation. Research has confirmed this theory, as milk without β-LG has approximately 50% less antioxidant activity than skim milk. UHT milk contains lower levels of β-LG, α-LA, and LF than ESL and sterile or VHT milk. Pasteurization does not affect the antioxidant capacity of milk, while sterilization increases this parameter. During heat treatment of milk at a temperature above 100°C, the antioxidant capacity increases due to protein unfolding and exposure of thiol groups.
- Probiotic strains
Milk fermentation with lactic acid bacteria contributes to the supply of a huge number of bioactive peptides and free amino acids that are capable of scavenging reactive oxygen species. Various researches show that probiotic strains exhibit significant antioxidant properties. The higher antioxidant capacity of probiotic yogurts than conventional dairy products is due to the release of antioxidant peptides by probiotic strains during fermentation. In addition, the type of probiotic strains used is very important in this regard.
According to the studies, the highest antioxidant activity of probiotic fermented milk was related to Lactobacillus acidophilus strain and the lowest activity was related to the symbiotic culture of Streptococcus thermophilus and Lactobacillus delbruecki. In another research, the antioxidant activity of fermented milk with 25 strains of lactic acid bacteria (LAB) was studied, the highest radical scavenging activity was related to Leuconostoc mesenteroides subsp. Cremoris, Lactobacillus acidophilus, and Lactobacillus genseni.
- Enrichment
In addition to bacterial cultures, enrichment of milk and dairy products with natural plant additives increases antioxidant potential.
Extensive research has focused on legumes, including soybeans, due to the presence of bioactive compounds that are added to dairy products to increase their antioxidant potential. Studies show that the antioxidant activity of soy-enriched yogurt (from both cow’s and camel’s milk) is higher than plain yogurt.
Studies on various types of fruits, including berries, have shown that their presence in the diet may act as an antioxidant barrier against cancer or DNA mutations. In addition, fruit additives are also an excellent prebiotic due to the presence of dietary fiber, which supports the proper function of the gastrointestinal tract. Dried grape pomace (GP), which is added to yogurt as an alternative source of antioxidant dietary fiber, can delay lipid oxidation during cold storage.
In addition, citrus peel, which is considered a waste product, protects cells from free radical damage by acting as an antioxidant and can be used as a functional food additive (e.g., dairy products). It has also been shown that the use of dried grape pomace (GP), rosemary leaves, pomegranate peel extract (Punica granatum), and pine needle extract (Cedrus deodara Roxb.) as preservatives in cheese significantly improves the oxidative stability of cheese, and contributes to the sensory attractiveness of the product.
In summary, various investigations indicate that adding ingredients containing bioactive compounds such as honey, chia seeds, flax, elderberry, cloves, lemongrass, sweet potato pulp, tea, red ginseng (Panax ginseng) extract, sea algae (spirulina), mangosteen fruits, and neem leaf (Azadirachta indica) extract to dairy products increase the antioxidant content.
Conclusion
In summary, it should be stated that the antioxidant capacity of milk and dairy products is mainly related to the content of antioxidant compounds such as proteins. Biopeptides produced during cheese fermentation or maturation also show antioxidant activity. Research shows the difference in the total antioxidant capacity of milk among animal species, is caused by the difference in the chemical composition of their milk. Sheep and buffalo milk have the highest antioxidant capacity. The content of antioxidant components of milk and its antioxidant potential can be changed through animal feeding or during milk processing. Adding plants or fruit by-products to animal diets helps to improve the nutritional value of milk by increasing the content of bioactive compounds and antioxidant potential. The antioxidant potential of dairy products is related not only to the quality of raw materials but also to the type of heat treatment, starter cultures, and natural plant additives used in the processing stage. Fermented products, especially probiotics, have the highest antioxidant potential. The antioxidant activity of products is also increased by using plant materials rich in phenolic compounds and carotenoids.
Reference:
Stobiecka M, Król J, Brodziak A. Antioxidant Activity of Milk and Dairy Products. Animals. 2022; 12(3):245. DOI: 10.3390/ani12030245
