Antioxidants in milk

Carotenoids can act as scavengers of free radicals but whether or not they act as antioxidants in milk is controversial. 

It has been proposed that spontaneous milks have a high content (10 times normal) of xanthine oxidase (XO). Although addition of exogenous XO to non-susceptible milk induces oxidative rancidity, no correlation has been found between the level of indigenous XO and susceptibility to oxidative rancidity. The Cu-ascorbate system appears to be the principal pro-oxidant in susceptible milk. A balance between the principal antioxidant in milk, a-tocopherol (Chapter 6), and XO may determine the oxidative stability of milk. The level of superoxide dismutase (SOD) in milk might also be a factor but there is no correlation between the level of SOD and the propensity to oxidative rancidity. 

Ascorbic acid acts as an antioxidant in milk at normally low copper concentrations. However, during storage the concentration of ascorbic acid decreases continuously and is depleted by consuming dissolved oxygen. The concentrations of tocopherol and ascorbic acid in milk are thus affected by cow feeding and milk storage conditions. 

Beside having a direct anti-oxidative effect on deteriorative processes, low-molecular weight antioxidants in milk enter complex interactions (synergism) which are important in relation to the overall anti-oxidative balance in milk. 

Veitch NC. 2004. Horseradish peroxidase a modem view of a classic enzyme. Phytochemistry 65 249—259. Wegrzyn TF, Farr JM, Hunter DC, Au J, Wohlers MW, Skinner MA, Stanley RA and Waterhouse DS. 2008. Stability of antioxidants in an apple polyphenol—milk model system. Food Chem 109 310-318. 

Superoxide dismutase (SOD), an indigenous enzyme in milk, was discussed in section 8.2.10. A low level of exogenous SOD, coupled with catalase, was shown to be a very effective inhibitor of lipid oxidation in dairy products. It has been suggested that SOD may be particularly useful in preserving the flavour of long-life UHT milk which is prone to lipid oxidation. Obviously, the commercial feasibility of using SOD as an antioxidant depends on cost, particularly vis-d-vis chemical methods, if permitted. 

Sidhu, G. S., Brown, M. A. and Johnson, A. R. 1975. Autoxidation in milk rich in linoleic acid. I. An objective method for measuring autoxidation and evaluating antioxidants. J. Dairy Res. 42, 185-195. 

In cow s milk, nearly all of the vitamin E is a-tocopherol and the level can vary with the cow s feed and the season of the year (Lampert 1975). For example, summer milk can contain five times more vitamin E (1.1 mg a-tocopherol per quart) than winter milk (0.2 mg/quart) (Hertig and Drury 1969 McLaughlin and Weihrauch 1979). It is suggested that vitamin E, due to its antioxidant properties, may have some effect in retarding the development of oxidized flavor in milk (Lampert 1975). 

Commercial applications. Over the last decade, bovine LF has been commercialised in various applications, e.g., in milk-based infant formulas, health supplements, functional foods and drinks, cosmetics, oral care products, chewing gums and feed supplements (Steijns, 2001). Such products are targeted at optimal iron delivery, mimicking human breast milk or boosting natural defense systems against infections. Also, LF could be exploited as a natural antioxidant due to its strong ability to bind iron, which is an important catalyst for free radical formation inside the cells. 

Lindmark-Mansson, H. and Akesson, B. 2000. Antioxidative factors in milk. Br. J. Nutr. 84, S103-S110. 

Dairy phospholipids are important structurally, because they are able to stabilise emulsions and foams, and to form micelles and membranes (Jensen and Newburg, 1995). Phospholipids also have the potential to be pro-oxidants, because they contain mono-unsaturated and poly-unsaturated fatty acids and have the ability to attract metal ions. Phosphatidyl ethanolamine binds copper strongly and is believed to be important in copper-induced oxidation in milk (O Connor and O Brien 1995 Deeth, 1997). The polyunsaturated fatty acids and metal ions accelerate lipid oxidation, especially when heat is applied hence, phospholipids can be degraded during the processing of milk. However, in dairy products, the situation is complex and it appears that phospholipids are able to act as either pro-oxidants or antioxidants, depending on the pH, ratio of water and phospholipid species (Chen and Nawar, 1991). 

Oxygen scavengers can be used to reduce the 02 content in packaged milk powder but this approach may not be cost-effective (Zimmerman et al., 1974). Another approach to reducing the 02 content in the package involves using H2 in the presence of a palladium or platinum catalyst. Although the use of several antioxidants is permitted in the manufacture of edible oils, fats and butter, their use in milk powder is prohibited. 

Findlay, I.D., Smith, J.A.B., 1945. Experiments on the use of antioxidants in spray-dried whole-milk powder. 1. Dairy Res. 14, 165-175. 

Clearly, more research is required to clarify the somewhat confused picture regarding the role of enzymes in the oxidation of milk lipids. However, the key factor affecting the susceptibility of milk to oxidation appears to be its relative content and distribution of pro-oxidants and antioxidants. Bruhn and Franke (1971) reported that spontaneous oxidation is directly proportional to the copper content and inversely proportional to the a-tocopherol content of milk. Charmley et al. (1991) showed that intramuscular injection of cows with a-tocopherol may overcome a spontaneous oxidized flavor problem caused by low levels of a-tocopherol in milk. In general, milk from pasture-fed cows is less susceptible to oxidation due to a higher content of tocopherols than milk from cows given dry feed (Bruhn and Franke 1971 Urbach, 1989, 1990). 

Wade et al. (1986) reported that BHA and BHT were effective in retarding oxidation of anhydrous milk fat but DL-a-tocopherol acted as a pro-oxidant. Natural antioxidants in betel and curry leaves have also been reported to retard oxidation of anhydrous milk fat (Sharma, 1981 Parmer and Sharma, 1986). Amr (1991) reported that turmeric and wheat grits were as effective as BHA and BHT in controlling oxidative rancidity in sheep s anhydrous milk fat for up to 4 months. However, rosemary, sage, rue and fennel exerted pro-oxidant effects. Quercetin and rutin are reported to be efficient antioxidants in butter (Eriksson, 1987). 

Sulphydryl oxidase, an indigenous milk enzyme, has been proposed for the oxidation of thiols in UHT milk to reduce cooked flavor and also thereby to serve as an antioxidant, in conjunction with lactoperoxidase (to destroy the resultant H2O2), by obviating pro-oxidants resulting from autoxidation of thiols (Swaisgood and Abraham, 1980). 

Calligaris et al. (2004) studied changes in antioxidant and pro-oxidant activity in milk subjected to different heat treatments. Their results indicated that short heat treatments can be potentially responsible for a depletion in the overall antioxidant properties of milk. Only the application of severe heat treatments, associated with the formation of brown melanoidins, allowed a recovery and even a possible increase in the antioxidant properties of milk. 

About one-third of the phospholipids in freshly drawn milk are located in the milk serum as small lipoprotein particles, sometimes referred to as milk microsomes. Their proportion in milk serum can be increased in processed milk as a result of disruption of the MFGM and release of membrane phospholipids into the aqueous phase (Mulder and Walstra, 1974 McPherson and Kitchen, 1983). Modification of the MFGM by processing treatments that may alter the distribution of pro-oxidants and antioxidants can markedly affect the stability of milk (McPherson and Kitchen, 1983). 

The composition of milkfat is somewhat complex. Although dominated by triglycerides, which constitute some 98% of milkfat (with small amounts of diglycerides, monoglycerides, and free fatty acids), various other lipid classes are also present in measurable amounts. It is estimated that about 500 separate fatty acids have been detected in milk lipids it is probable that additional fatty acids remain to be identified. Of these, about 20 are major components the remainder are minor and occur in small or trace quantities (4, 5). The other components include phospholipids, cerebrosides, and sterols (cholesterol and cholesterol esters). Small amounts of fat-soluble vitamins (mainly A, D, and E), antioxidants (tocopherol), pigments (carotene), and flavor components (lactones, aldehydes, and ketones) are also present. 

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