Ascorbic acid in milk

The concentration of ascorbic acid in milk (11.2-17.2mgl-1) is sufficient to influence its redox potential. In freshly drawn milk, all ascorbic acid is in the reduced form but can be oxidized reversibly to dehydroascorbate, which is present as a hydrated hemiketal in aqueous systems. Hydrolysis of the lactone ring of dehydroascorbate, which results in the formation of 2,3-diketogulonic acid, is irreversible (Figure 11.2). 

Figure 9-11 Effect of Exposure Time at Light Intensity of 200 Ft-C on the Loss of Ascorbic Acid in Milk. Packaging materials (1) clear plastic pouch, (2) laminated nontransparent pouch, (3) carton, (4) plastic 3-quart jug. Source From A. Sattar and J.M. deMan, Effect of Packaging Material on Light-Induced Quality Deterioration of Milk, Can. Inst. Food Sci. Technol. J., Vol. 6, pp. 170-174,1973.

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. 

Folate is a relatively unstable nutrient processing and storage conditions that promote oxidation are of particular concern since some of the forms of folate found in foods are easily oxidized. The reduced forms of folate (dihydro- and tetrahydrofolate) are oxidized to p-aminobenzoylglutamic acid and pterin-6-carboxylic acid, with a concomitant loss in vitamin activity. 5-Methyl-H4 folate can also be oxidized. Antioxidants (particularly ascorbic acid in the context of milk) can protect folate against destruction. The rate of the oxidative degradation of folate in foods depends on the derivative present and the food itself, particularly its pH, buffering capacity and concentration of catalytic trace elements and antioxidants. 

However, its presence is not the only determinant of whether or not oxidative deterioration occurs. Olson and Brown (1942) showed that washed cream (free of ascorbic acid) from susceptible milk did not develop an oxidized flavor when contaminated with copper and stored for three days. Subsequently, the addition of ascorbic acid to washed cream, even in the absence of added copper, was observed to promote the development of an oxidized flavor (Pont 1952). Krukovsky and Guthrie (1945) and Krukovsky (1961) reported that 0.1 ppm added copper did not promote oxidative flavors in milk or butter depleted of their Vitamin C content by quick and complete oxidation of ascorbic acid to dehydroascorbic acid. Krukovsky (1955) and Krukovsky and Guthrie (1945) further showed that the oxidative reaction in ascorbic acid-free milk could be initiated by the addition of ascorbic acid to such milk. Accordingly, these workers and others have concluded that ascorbic acid is an essential link in a chain of reactions resulting in the development of an oxidized flavor in fluid milk. 

The behavior of ascorbic acid in the oxidative reaction, however, is anomalous, as evidenced by the studies of several workers (Bell et al. 1962 Bell and Mucha 1949 Chilson 1935 Krukovsky and Guthrie 1946). Their results indicate that concentrations normal to milk (10 to 20 mg/liter) promote oxidative deterioration, while higher concentrations (50 to 200 mg/liter) inhibit the development of off-flavors. 

This is also true of the daily consumption of black tea (1 cup with 5 g tea in the morning and at lunchtime). Tea is deemed to be an iron-chelating agent (iron tan-nate), which significantly reduces the resorption of iron, particularly in a low-iron diet - with or without ascorbic acid or milk added. (424, 444) We have always made sure that these adjuvant measures are strictly adhered to. 

The practical use of added ascorbic acid has proved to be of benefit to the dairy industry 311,321). The amounts of ascorbic acid or sodium ascorbate used vary between twenty and several hundred milligrams per liter, 30-50 mg usually being suflScient for fresh fluid milk. Discrepancies in the results of some workers attempting to elucidate the value of ascorbic acid in the development of off-flavor may be due to their examination of an incomplete system of oxidative reactions. It has been dem-... 

Ingold (6) did not review ascorbic acid per se, but did present a chapter on metal catalysis including the metal content of vegetable oils and the eflFect of valence state of the metals on oxidation of fats and oils. He reports cobalt, manganese, copper, iron, and zinc at the higher valences acted catalytically to oxidize many substrates. The author discussed the antioxidant activity of ascorbic acid in radiation-induced free radicals, fats in emulsions, fluid milk, and frozen fish. He also discussed the quandary of metal reactions vs. valence state. 

Destruction of vitamins A, C, D and E induced by riboflavin-photosensitized oxidation has been reported. Vitamin A and its esters, along with carotenoids with pro-vitamin A activity, undergo ring opening upon sunlight exposure in the presence of riboflavin. Although an excellent antioxidant, ascorbic acid is rapidly photooxidized in the presence of riboflavin. Even a small decrease in riboflavin content in milk due to photosensitization can lead to virtual complete destruction of ascorbic acid. Fortunately, milk is not an important source of vitamin C in most diets. 

The stability of ascorbic acid in plant products is very important in the food industry. Oxidation in milk is accelerated by copper and sunlight. Low-temperature storage of foods (below 42 F.) is helpful in preventing loss. 

Ascorbyl radical can be measured in a steady-state concentration in fresh milk. Oxidation of ascorbate by lactoperoxidase has been proposed to be the source of this radical, based on the increase in ESR signal upon an increase in the concenfra-tion of HjOjand the decrease in signal upon addition of azide (a lactoperoxidase inhibitor) (8). However, the radical may also stem from autoxidation of ascorbic acid in die presence of transition metals (9). 

The pro-oxidative activity of ascoibic acid in milk has mainly, as mentioned above, been linked to transition metals due to its ability to reduce metal ions and thereby make redox-cycling of transition metal ions possible (9). Nielsen and coworkers (23) have recently shown tiiat iron does not show pro-oxidative activity against ascorbic acid as opposed to copper ion within concentrations found in milk. It is supposed that iron is chelated by lactofeirin, citrate and proteins in the milk. 

Recent data show that ascorbic acid in fact seems able to recycle uric acid during light exposure of milk (36). Due to the above-mentioned data regarding the potential anti- and pro-oxidative activity of urate in miUc, fiirdier studies of the possible consequences of urate radical formation and its interaction with biological anti-oxidmts other than ascorbate (such as diiols) are warranted to predict its antioxidative activity in milk and odier foods. 

Figure 3. Regeneration of ascorbic acid in raw milk by thiol compounds. Ascorbic acid was oxidized by activation oflactoperoxidase through addition of 100 pM hydrogen peroxide. Regeneration of ascorbic acid was measured 30 minutes after addition of glutathione or cysteine to the hydrogen peroxide added sample. Control equals concentration of ascorbic add in raw milk prior to the addition of hydrogen peroxide.

Losses of ascorbic acid during storage of raw milk are considerable. Cold storage causes about 50% loss of vitamin C, which increases with increased temperature. Heat treatment of milk decreases the content of vitamin C by 20 50%, depending on temperature and time of heating. The UHT treatment of milk causes about 10 30% loss. Ascorbic acid in dried, vitamin-enriched mflk, packaged in an inert atmosphere, is relatively stable. 

Colorant containing annatto and Ca caseinate as carrier mixed with water to be added directly to cheese milk yielding uniform colored cheese mass Water-dispersible beadlet of p-carotene is mixed with oil to attein composition that remains stable even in presence of polyphosphates and with antioxidant action even in absence of ascorbic acid Blending carotenoid pigment and soybean fiber (wifii tomato juice) as effective ingredient for dispersion stability... 

Alpha hydroxy acids (AHAs) are water-soluble substances and thereby penetrate the outermost epidermal skin layers. In contrast, beta hydroxy acids (BHAs) are lipid (fat) soluble and are capable of penetrating to the underlying layers of skin (the dermis) located 1-5 mm below the surface of the skinJ2 Most AHAs are derived from plant materials and marine sources. Commonly used AHAs include malic acid (found in apples), ascorbic acid (a common ingredient in numerous fruits), glycolic acid (a constituent of sugar cane), lactic acid (a component of milk), citric acid (naturally abundant in citrus fruits), and tartatic acid (found in red wine). A common BHA is salicylic acid (an ingredient in aspirin). 

In view of the lack of satisfactory data, the evidence for substantial inter-individual differences in milk composition is limited, and satisfactory conclusions must await further study of both inter-individual and intra-individual differences. The available data suggest that interindividual differences may be substantial and important in the case of ascorbic acid, folic acid, and vitamin B12. 

Lactation Ascorbic acid is excreted in breast milk. 

Deficiency may occur in infants if no fruits or vegetables are added to their milk formulas. In alcoholics, and in elderly subjects who consume inadequate diets vitamin C deficiencies are frequent. Severe ascorbic acid deficiency is characterized by the syndrome known as scurvy. Its manifestations are generally based on a loss of collagen. Symptoms include hemorrhages, loosening of teeth. In children cellular changes in the long bones occur. 

Riboflavin absorbs light maximally at about 450nm and in doing so can be excited to a triplet state. This excited form of riboflavin can interact with triplet 02 to form a superoxide anion OJ (or H202 at low pH). Excited riboflavin can also oxidize ascorbate, a number of amino acids and proteins and orotic acid. Riboflavin-catalysed photo-oxidation results in the production of a number of compounds, most notably methional (11.1) which is the principal compound responsible for the off-flavour in milk exposed to light. 

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