《乳品生物化学》（英文版） 6 Vitamins in milk and dairy products

6 Vitamins in milk and dairy products 6.1 Introduction Vitamins are organic chemicals required by the body in trace amounts but which cannot be synthesized by the body. The vitamins required for growth and maintenance of health differ between species; compounds regarded as vitamins for one species may be synthesized quate rates by other pecies. For example, only primates and the guinea pig require ascorbic acid (vitamin C; section 6.4) from their diet; other species possess the enzyme gluconolactone oxidase which is necessary for the synthesis of vitamin C from D-glucose or D-galactose. The chemical structures of the vitamins have no relationship with each other. The principal classification of vitamins is based on their solubility in water. Water-soluble vitamins are the b grour (thiamin, riboflavin, niacin, biotin, panthothenate, folate, pyridoxine(and elated substances, vitamin B6 )and cobalamin(and its derivatives, vitamin B12))and ascorbic acid(vitamin C)while the fat-soluble vitamins are retinol (vitamin A), calciferols(vitamin D), tocopherols(and related compounds vitamin E)and phylloquinone(and related compounds, vitamin K). The water-soluble vitamins and vitamin K function as co-enzymes while vitamin A is important in the vision process, vitamin D functions like a hormone and vitamin e is primarily an antioxidant Milk is the only source of nutrients for the neonatal mammal during rly stage of life viding macronut ents(protein, carbohydrate and lipid)and water, milk must also supply ufficient vitamins and minerals to support the growth of the neonate Human beings continue to consume milk into adulthood and thus milk and dairy products continue to be important sources of nutrients in the diet of many peoples worldwide. The concentrations of macronutrients and min erals in milk have been discussed in Chapters 1 and 5: vitamin levels in milk and dairy products will be considered here. Milk is normally processed a lesser or greater extent before consumption. Thus it is important to consider the influence of processing on the vitamin status of milk and dairy Recommended dietary allowances(RDA) for vitamins are recommended intake of various vitamin to ensure Ith of a high proportion of the human population. The rDa below refer to the United States population (Whitney and Rolfes, 1996). Reference nutrient intake

6 Vitamins in milk and dairy products 6.1 Introduction Vitamins are organic chemicals required by the body in trace amounts but which cannot be synthesized by the body. The vitamins required for growth and maintenance of health differ between species; compounds regarded as vitamins for one species may be synthesized at adequate rates by other species. For example, only primates and the guinea-pig require ascorbic acid (vitamin C; section 6.4) from their diet; other species possess the enzyme gluconolactone oxidase which is necessary for the synthesis of vitamin C from D-glucose or D-galactose. The chemical structures of the vitamins have no relationship with each other. The principal classification of vitamins is based on their solubility in water. Water-soluble vitamins are the B group (thiamin, riboflavin, niacin, biotin, panthothenate, folate, pyridoxine (and related substances, vitamin B6) and cobalamin (and its derivatives, vitamin BIZ)) and ascorbic acid (vitamin C) while the fat-soluble vitamins are retinol (vitamin A), calciferols (vitamin D), tocopherols (and related compounds, vitamin E) and phylloquinone (and related compounds, vitamin K). The water-soluble vitamins and vitamin K function as co-enzymes while vitamin A is important in the vision process, vitamin D functions like a hormone and vitamin E is primarily an antioxidant. Milk is the only source of nutrients for the neonatal mammal during the early stage of life until weaning. Thus, in addition to providing macronutri￾ents (protein, carbohydrate and lipid) and water, milk must also supply sufficient vitamins and minerals to support the growth of the neonate. Human beings continue to consume milk into adulthood and thus milk and dairy products continue to be important sources of nutrients in the diet of many peoples worldwide. The concentrations of macronutrients and min￾erals in milk have been discussed in Chapters 1 and 5; vitamin levels in milk and dairy products will be considered here. Milk is normally processed to a lesser or greater extent before consumption. Thus it is important to consider the influence of processing on the vitamin status of milk and dairy products. Recommended dietary allowances (RDA) for vitamins are recommended intake of various vitamin to ensure the good health of a high proportion of the human population. The RDA values quoted below refer to the United States population (Whitney and Rolfes, 1996). Reference nutrient intake

DAIRY CHEMISTRY AND BIOCHEMISTRY (RND) is the quantity of a nutrient sufficient to meet the needs of 97% of the population. Nutrient intakes equal to the rNi thus pose only a very small risk of deficiency. United Kingdom RNI values(Department of Health, 1991)are also quoted below. 6.2 Fat-soluble vitamins 6.2.1 Retinol ( vitamin A Vitamin A(retinol, 6.1)is the parent of a range of compounds known as retinoids, which possess the biological activity of vitamin A. In general, animal foods provide preformed vitamin A as retinyl esters( e.g. 6.5, which are easily hydrolysed in the gastrointestinal tract) while plant foods provide precursors of vitamin A, i. e carotenoids. Only carotenoids with a B-ionone ring (e. g. B-carotene)can serve as vitamin A precursors. B-Carotene(6.6) 6.1 6.2 Retinal 6.3 Retinoic acid 64 11-cis-retinal

266 DAIRY CHEMISTRY AND BIOCHEMISTRY (RNI) is the quantity of a nutrient sufficient to meet the needs of 97% of the population. Nutrient intakes equal to the RNI thus pose only a very small risk of deficiency. United Kingdom RNI values (Department of Health, 1991) are also quoted below. 6.2 Fat-soluble vitamins 6.2.1 Retinol (vitamin A) Vitamin A (retinol, 6.1) is the parent of a range of compounds known as retinoids, which possess the biological activity of vitamin A. In general, animal foods provide preformed vitamin A as retinyl esters (e.g. 6.5, which are easily hydrolysed in the gastrointestinal tract) while plant foods provide precursors of vitamin A, i.e. carotenoids. Only carotenoids with a /3-ionone ring (e.g. p-carotene) can serve as vitamin A precursors. p-Carotene (6.6) 6.1 3 6.2 6.3 6.4

O-C-C,sH, 65 Retinyl plamitate Cleavage at this point results 66

0

268 DAIRY CHEMISTRY AND BIOCHEMISTRY may be cleavaged at its centre by the enzyme, B-carotene- 15, 15'-oxygenase (present in the intestinal mucosa), to yield 2 mol retinol per mol. However, cleavage of other bonds results in the formation of only I molecule of retinol per molecule of B-carotene. In practice, 6 ug B-carotene will yield only 1 ug of retinol. Likewise, 12 ug other carotenes which are vitamin A precursors (i.e. which contain one B-ionone ring) are required to yield 1 ug of retinol Thus, I retinol equivalent(RE)is defined as 1 ug retinol, 6 ug B-carotene or 2 ug of other precursor carotenes Retinol can be oxidized to retinal (6. 2)and further to retinoic acid (6.3) Cis-trans isomerization can also occur, e.g. the conversion of all trans- retinal to 11-cis-retinal(6.4), which is important for vision Vitamin a has a number of roles in the body: it is involved in the vision process, in cell differentiation, in growth and bone remodelling and in the immune system. US RDAs for vitamin A are 1000 ug RE day"for men and 800 ug RE day- for women. UK RNI values for vitamin a are 700 and 600 ug Re day for adult men and women, respectively. The body will tolerate a wide range of vitamin A intakes(500-15 000 ug Re day" )but sufficient or excessive intakes result in illness. Vitamin A deficiency (15 000 ug RE day ) vitamin A is toxic. Symptoms of hypervitaminosis A include skin rashes, hair loss, haemorrhages, bone abnormalities and fractures, and in extreme cases. liver failure and death he major dietary sources of retinol are dairy products, eggs and liver, while important sources of B-carotene are spinach and other dark-green leafy vegetables, deep orange fruits (apricots, cantaloupe) and vegetables (squash, carrots, sweet potatoes, pumpkin). The richest natural sources of vitamin A are fish liver oils, particularly halibut and shark Vitamin A activity is present in milk as retinol, retinyl esters and as carotenes. Whole cows'milk contains an average of 52 ug retinol and 21 ug carotene per 100g. The concentration of retinol in raw sheeps and pas- teurized goats'milks is 83 and 44 ug per 100 g, respectively, although milks of these species are reported(Holland et al., 1991) to contain only trace amounts of carotenes. Human milk and colostrum contain an average of 58 and 155 ug retinol per 100 g, respectively. In addition to their role as provitamin A, the carotenoids in milk are reponsible for the colour of milk fat( Chapter 11) The concentration of vitamin a and carotenoids in milk is strongly influenced by the carotenoid content of the feed. Milk from animals fed on pasture contains higher levels of carotenes than that from animals fed on concentrate feeds. There is also a large seasonal variation in vitamin A concentration;summer milk contains an average of 62 ug retinol and 31 ug carotene per 100g while the values for winter milk are 41 and 11 ug per

268 DAIRY CHEMISTRY AND BIOCHEMISTRY may be cleavaged at its centre by the enzyme, p-carotene-1 5,15'-oxygenase (present in the intestinal mucosa), to yield 2 mol retinol per mol. However, cleavage of other bonds results in the formation of only 1 molecule of retinol per molecule of p-carotene. In practice, 6 pg 8-carotene will yield only 1 pg of retinol. Likewise, 12 pg other carotenes which are vitamin A precursors (i.e. which contain one p-ionone ring) are required to yield 1 pg of retinol. Thus, 1 retinol equivalent (RE) is defined as 1 pg retinol, 6 pg p-carotene or 12 pg of other precursor carotenes. Retinol can be oxidized to retinal (6.2) and further to retinoic acid (6.3). Cis-trans isomerization can also occur, e.g. the conversion of all tvans￾retinal to 11-cis-retinal (6.4), which is important for vision. Vitamin A has a number of roles in the body: it is involved in the vision process, in cell differentiation, in growth and bone remodelling and in the immune system. US RDAs for vitamin A are 1000 pg RE day- for men and 800 pg RE day-' for women. UK RNI values for vitamin A are 700 and 600 pg RE day- ' for adult men and women, respectively. The body will tolerate a wide range of vitamin A intakes (500-15OOOpg REday-') but insufficient or excessive intakes result in illness. Vitamin A deficiency ( 15 000 pg REday-'), vitamin A is toxic. Symptoms of hypervitaminosis A include skin rashes, hair loss, haemorrhages, bone abnormalities and fractures, and in extreme cases, liver failure and death. The major dietary sources of retinol are dairy products, eggs and liver, while important sources of p-carotene are spinach and other dark-green leafy vegetables, deep orange fruits (apricots, cantaloupe) and vegetables (squash, carrots, sweet potatoes, pumpkin). The richest natural sources of vitamin A are fish liver oils, particularly halibut and shark. Vitamin A activity is present in milk as retinol, retinyl esters and as carotenes. Whole cows' milk contains an average of 52 pg retinol and 21 pg carotene per 1OOg. The concentration of retinol in raw sheep's and pas￾teurized goats' milks is 83 and 44 pg per 100 g, respectively, although milks of these species are reported (Holland et al., 1991) to contain only trace amounts of carotenes. Human milk and colostrum contain an average of 58 and 155pg retinol per lOOg, respectively. In addition to their role as provitamin A, the carotenoids in milk are reponsible for the colour of milk fat (Chapter 11). The concentration of vitamin A and carotenoids in milk is strongly influenced by the carotenoid content of the feed. Milk from animals fed on pasture contains higher levels of carotenes than that from animals fed on concentrate feeds. There is also a large seasonal variation in vitamin A concentration; summer milk contains an average of 62 pg retinol and 31 pg carotene per 100 g while the values for winter milk are 41 and 11 pg per

VITAMINS IN MILK AND DAIRY PRODUCTS 269 100 g, respectively. The breed of cow also has an influence on the concen tration of vitamin A in milk: milk from Channel Islands breeds typically contains 65 ug and 27 ug retinol per 100 g in summer and winter, respect ively, and 115 and 27 ug carotene per 100 g in summer and winter, respectively Other dairy products are also important sources of vitamin A(Append 6A). Whipping cream (39% fat) contains about 565 ug retinol and 265 ug carotene per 100 g. The level of vitamin A in cheese varies with the fat content(Appendix 6A). Camembert(23. 7%fat) contains 230 ug retinol and 315 ug carotene per 100 g, while Cheddar(34. 4%fat )contains 325 ug retinol and 225 ug carotene per 100 g. Whole-milk yogurt(3% fat; unflavoured) contains roughly 28 ug retinol and 21 ug carotene per 100g while the orresponding values for ice-cream(9.8% fat)are 115 and 195 ug per 100 g, Vitamin A is relatively stable to most dairy processing operations. In general, vitamin A activity is reduced by oxidation and exposure to light Heating below 100C(e.g. pasteurization) has little effect on the vitamin A content of milk, although some loss may occur at temperatures above 100C (e.g. when frying using butter). Losses of vitamin A can occur in UHT milk during its long shelf-life at ambient temperatures. Vitamin a is stable in pasteurized milk at refrigeration temperatures provided the milk is pro- tected from light, but substantial losses can occur in milk packaged in ranslucent bottles. Low-fat milks are often fortified with vitamin a for nutritional reasons. Added vitamin A is less stable to light than the indigenous vitamin. The composition of the lipid used as a carrier for the exogenous vitamin influences its stability. Protective compounds(e.gascor byl palmitate or B-carotene)will reduce the rate at which exogenous vitamin A is lost during exposure to light. Yogurts containing fruit often contain higher concentrations of vitamin a precursor carotenoids than natural yogurts. The manufacture of dairy products which involves concentration of the milk fat (e.g. cheese, butter) results in a pro rata increase in the concentration of vitamin A. The increased surface area of dried milk products accelerates the loss of vitamin A; supplementation of milk powders with vitamin A and storage at low temperatures minimizes these losses 6.2.2 Calciferols(vitamin D) Unlike other vitamins, cholecalciferol (vitamin D3)can be formed from a steroid precursor, 7-dehydrocholesterol (6.7), by the skin when exposed to sunlight; with sufficient exposure to the sun, no preformed vitamin D equired from the diet UV light(280-320 nm) causes the photoconversion of 7-dehydrochc terol to pre-vitamin D3. This pre-vitamin can undergo further photoconver sion to tachysterol and lumisterol or can undergo a temperature-dependent isomerization to cholecalciferol (vitamin D3, 6.8). At body temperature, this

VITAMINS IN MILK AND DAIRY PRODUCTS 269 lOOg, respectively. The breed of cow also has an influence on the concen￾tration of vitamin A in milk: milk from Channel Islands breeds typically contains 65 pg and 27 pg retinol per 100 g in summer and winter, respect￾ively, and 115 and 27pg carotene per lOOg in summer and winter, respectively. Other dairy products are also important sources of vitamin A (Appendix 6A). Whipping cream (39% fat) contains about 565 pg retinol and 265 pg carotene per 1OOg. The level of vitamin A in cheese varies with the fat content (Appendix 6A). Camembert (23.7% fat) contains 230 pg retinol and 315 pg carotene per lOOg, while Cheddar (34.4% fat) contains 325 pg retinol and 225 pg carotene per 100 g. Whole-milk yogurt (3% fat; unflavoured) contains roughly 28pg retinol and 21 pg carotene per 1OOg while the corresponding values for ice-cream (9.8% fat) are 115 and 195 pg per 100 g, respectively. Vitamin A is relatively stable to most dairy processing operations. In general, vitamin A activity is reduced by oxidation and exposure to light. Heating below 100°C (e.g. pasteurization) has little effect on the vitamin A content of milk, although some loss may occur at temperatures above 100°C (e.g. when frying using butter). Losses of vitamin A can occur in UHT milk during its long shelf-life at ambient temperatures. Vitamin A is stable in pasteurized milk at refrigeration temperatures provided the milk is pro￾tected from light, but substantial losses can occur in milk packaged in translucent bottles. Low-fat milks are often fortified with vitamin A for nutritional reasons. Added vitamin A is less stable to light than the indigenous vitamin. The composition of the lipid used as a carrier for the exogenous vitamin influences its stability. Protective compounds (e.g. ascor￾by1 palmitate or p-carotene) will reduce the rate at which exogenous vitamin A is lost during exposure to light. Yogurts containing fruit often contain higher concentrations of vitamin A precursor carotenoids than natural yogurts. The manufacture of dairy products which involves concentration of the milk fat (e.g. cheese, butter) results in a pro rata increase in the concentration of vitamin A. The increased surface area of dried milk products accelerates the loss of vitamin A; supplementation of milk powders with vitamin A and storage at low temperatures minimizes these losses. 6.2.2 Calciferols (vitamin D) Unlike other vitamins, cholecalciferol (vitamin D,) can be formed from a steroid precursor, 7-dehydrocholesterol (6.7), by the skin when exposed to sunlight; with sufficient exposure to the sun, no preformed vitamin D is required from the diet. UV light (280-320 nm) causes the photoconversion of 7-dehydrocholes￾terol to pre-vitamin D,. This pre-vitamin can undergo further photoconver￾sion to tachysterol and lumisterol or can undergo a temperature-dependent isomerization to cholecalciferol (vitamin D,, 6.8). At body temperature, this

270 DAIRY CHEMISTRY AND BIOCHEMISTRY 6.7 68 Cholecalciferol. Vitamin D 69 Hydroxycholecalciferol conversion requires about 28 h to convert 50% of previtamin D, to vitamin D,. Thus, production of vitamin D, in the skin can take a number of days. Preformed vitamin D, is obtained from the diet. Vitamin D3 is stored in various fat deposits around the body. Regardless of the source of vitamin Da, it must undergo two hydroxylations to become fully active. Vitamin D3 is transported by a specific binding protein through the circulatory system to the liver where the enzyme, 25-hydroxylase, converts it to 25-hydroxy-

270 DAIRY CHEMISTRY AND BIOCHEMISTRY 6.7 HO 6.8 6.9 conversion requires about 28 h to convert 50% of previtamin D, to vitamin D,. Thus, production of vitamin D, in the skin can take a number of days. Preformed vitamin D, is obtained from the diet. Vitamin D, is stored in various fat deposits around the body. Regardless of the source of vitamin D,, it must undergo two hydroxylations to become fully active. Vitamin D, is transported by a specific binding protein through the circulatory system to the liver where the enzyme, 25-hydroxylase, converts it to 25-hydroxy-

VITAMINS IN MILK AND DAIRY PRODUCTS 271 610 1, 25-Dihydroxycholecalciferol cholecalciferol (25(OH)D3; 6.9) which is converted to 1, 25-dihydroxy cholecalciferol (1, 25(OH)2 D3; 6.10) by the enzyme, 1-hydroxylase, in the 24, 25-dihydroxycholecalciferol(24, 25(OH)2D3). At least 37 on 24 to form kidney. Alternatively, 25(OH)D, can be hydroxylated at posit metabolites of vitamin D, have been identified, but only 3, 25(OH)2D3, 24, 25(OH)2 D3 and 1, 25(OH)2 D, have significant biological activity; 1, 25(OH)2D, is the most biologically active metabolite of vitamin D, Vitamin D,(ergocalciferol) is formed by the photoconversion of ergo- sterol, a sterol present in certin fungi and yeasts, and differs from cholecal ciferol in having an extra methyl group at carbon 24 and an extra double bond between C22 and C23. Ergocalciferol was widely used for many years as a therapeutic agent. The principal physiological role of vitamin D in the body is to maintain plasma calcium by stimulating its absorption from the gastrointestinal tract, its retention by the kidney and by promoting its transfer from bone to the blood. Vitamin d acts in association with other vitamins hormones and nutrients in the bone mineralization process. In addition, vitamin d has a wider physiological role in other tissues in the body, including the brain and nervous system, muscles and cartilage, pancreas, skin, reproductive organs and immune cells. The RDA for vitamin D is 10 and 5 ug day-I for persons aged 19- 24 years or over 25 years, respectively. RNI values for vitamin D are 10 ug day-I for persons over 65 years and for pregnant or lactating women With the exception of these and other at-risk groups, the RNI value for dietary vitamin D is O ug day. The classical syndrome of vitamin D deficency is rickets, in which bone is inadequately mineralized, resulting in growth retardation and skeletal abnormalities. Adult rickets or os- omalacia occurs most commonly in women who have low calcium intakes and little exposure to sunlight and have had repeated pregnancies or periods

VITAMINS IN MILK AND DAIRY PRODUCTS 271 6.10 cholecalciferol (25(OH)D,; 6.9) which is converted to 1,25-dihydroxy￾cholecalciferol ( 1,25(OH),D,; 6.10) by the enzyme, 1-hydroxylase, in the kidney. Alternatively, 25(OH)D, can be hydroxylated at position 24 to form 24,25-dihydroxycholecalciferol (24,25(OH),D3). At least 37 metabolites of vitamin D, have been identified, but only 3,25(OH),D,, 24,25(OH),D, and 1,25(OH),D, have significant biological activity; 1,25(OH),D, is the most biologically active metabolite of vitamin D,. Vitamin D, (ergocalciferol) is formed by the photoconversion of ergo￾sterol, a sterol present in certin fungi and yeasts, and differs from cholecal￾ciferol in having an extra methyl group at carbon 24 and an extra double bond between C,, and C23. Ergocalciferol was widely used for many years as a therapeutic agent. The principal physiological role of vitamin D in the body is to maintain plasma calcium by stimulating its absorption from the gastrointestinal tract, its retention by the kidney and by promoting its transfer from bone to the blood. Vitamin D acts in association with other vitamins, hormones and nutrients in the bone mineralization process. In addition, vitamin D has a wider physiological role in other tissues in the body, including the brain and nervous system, muscles and cartilage, pancreas, skin, reproductive organs and immune cells. The RDA for vitamin D is 10 and 5pgday-' for persons aged 19- 24years or over 25 years, respectively. RNI values for vitamin D are 10 pg day- ' for persons over 65 years and for pregnant or lactating women. With the exception of these and other at-risk groups, the RNI value for dietary vitamin D is Opgday-'. The classical syndrome of vitamin D deficency is rickets, in which bone is inadequately mineralized, resulting in growth retardation and skeletal abnormalities. Adult rickets or os￾teomalacia occurs most commonly in women who have low calcium intakes and little exposure to sunlight and have had repeated pregnancies or periods

272 DAIRY CHEMISTRY AND BIOCHEMISTRY of lactation. Hypervitaminosis D(excess intake of vitamin D)is character ized by enhanced absorption of calcium and transfer of calcium from bone to the blood. These cause excessively high concentrations of serum calcium which can precipitate at various locations in the body, causing kidney stones or calcification of the arteries. Vitamin d can exert these toxic effects if consumed continuously at only relatively small amounts in excess of the RDA Relatively few foods contain significant amounts of vitamin D. In addition to conversion in situ by the body, the principal sources of vitamin D are foods derived from animal sources, including egg yolk, fatty fish and liver. Unfortified cows milk is not an important source of vitamin D The major form of vitamin D in both cows and human milk 25(OH)D3. This compound is reported to be responsible for most of the vitamin D in the blood serum of exclusively breast-fed infants. Whole cows milk contains only about 0.03 ug vitamin d per 100 g and 1 litre of milk per day will supply only 10-20% of the RDA. Therefore, milk is often fortified (at the level of c. 1-10 ug I")with vitamin D. Fortified milk, dairy products or margarine are important dietary sources of vitamin D. The concentration of vitamin D in unfortified dairy products is usually quite low. Vitamin D levels in milk vary with exposure to sunlight As with other fat-soluble vitamins the concentration of vitamin d in dairy products is increased pro rata by concentration of the fat (e.g. in the production of butter or cheese). Vitamin D is relatively stable during storage nd to most dairy processing operations. Studies on the degradation of vitamin D in fortified milk have shown that the vitamin may be degraded by exposure to light. However, the conditions necessary to cause significant losses are unlikely to be encountered in practice. Extended exposure to light and oxygen are needed to cause significant losses of vitamin d 6.2.3 Tocopherols and related compounds(vitamin E, Eight compounds have vitamin E activity, four of which are derivatives of tocopherol (6.11)and four of tocotrienol(6. 12);all are derivatives of 6-chromanol. Tocotrienols differ from tocopherols in having three carbon carbon double bonds in their hydrocarbon side chain. a, B-, y-or 8- tocopherols and tocotrienols differ with respect to number and position of methyl groups on the chromanol ring. The biological activity of the diferent forms of the tocopherols and tocotrienols varies with their structure. D-and L-enantiomers of vitamin E also occur; the biological activity of the D-form is higher than that of the L-isomer. Vitamin e activity can be expressed as tocopherol equivalents(TE), where 1 TE is equivalent to the vitamin E activity of I mg a-D-tocopherol. The biological activity of B.and7- tocopherols and a-tocotrienol is 50, 10 and 33% of the activity of a-D

272 DAIRY CHEMISTRY AND BIOCHEMISTRY of lactation. Hypervitaminosis D (excess intake of vitamin D) is character￾ized by enhanced absorption of calcium and transfer of calcium from bone to the blood. These cause excessively high concentrations of serum calcium which can precipitate at various locations in the body, causing kidney stones or calcification of the arteries. Vitamin D can exert these toxic effects if consumed continuously at only relatively small amounts in excess of the RDA. Relatively few foods contain significant amounts of vitamin D. In addition to conversion in situ by the body, the principal sources of vitamin D are foods derived from animal sources, including egg yolk, fatty fish and liver. Unfortified cows’ milk is not an important source of vitamin D. The major form of vitamin D in both cows’ and human milk is 25(OH)D,. This compound is reported to be responsible for most of the vitamin D in the blood serum of exclusively breast-fed infants. Whole cows’ milk contains only about 0.03 pg vitamin D per 100 g and 1 litre of milk per day will supply only 10-20% of the RDA. Therefore, milk is often fortified (at the level of c. 1-10 pg 1-’) with vitamin D. Fortified milk, dairy products or margarine are important dietary sources of vitamin D. The concentration of vitamin D in unfortified dairy products is usually quite low. Vitamin D levels in milk vary with exposure to sunlight. As with other fat-soluble vitamins, the concentration of vitamin D in dairy products is increased pro rata by concentration of the fat (e.g. in the production of butter or cheese). Vitamin D is relatively stable during storage and to most dairy processing operations. Studies on the degradation of vitamin D in fortified milk have shown that the vitamin may be degraded by exposure to light. However, the conditions necessary to cause significant losses are unlikely to be encountered in practice. Extended exposure to light and oxygen are needed to cause significant losses of vitamin D. 6.2.3 Eight compounds have vitamin E activity, four of which are derivatives of tocopherol (6.11) and four of tocotrienol (6.12); all are derivatives of 6-chromanol. Tocotrienols differ from tocopherols in having three carbon￾carbon double bonds in their hydrocarbon side chain. a-, p-, y- or 6- tocopherols and tocotrienols differ with respect to number and position of methyl groups on the chromanol ring. The biological activity of the different forms of the tocopherols and tocotrienols varies with their structure. D- and L-enantiomers of vitamin E also occur; the biological activity of the D-form is higher than that of the L-isomer. Vitamin E activity can be expressed as tocopherol equivalents (TE), where 1 TE is equivalent to the vitamin E activity of 1 mg u-D-tocopherol. The biological activity of p- and y￾tocopherols and u-tocotrienol is 50, 10 and 33% of the activity of a-D￾tocopherol, respectively. Tocopherols and related compounds (vitamin E)

VITAMINS IN MILK AND DAIRY PRODUCTS 6.11 6.12 Tocotrienols R=量=RE Vitamin E is a very effective antioxidant. It can easily donate a hydrogen from the phenolic -oh group on the chromanol ring to free radicals. The sulting vitamin E radical is quite unreactive as it is stabilized by delocal ization of its unpaired electron into the aromatic ring. Vitamin E thus protects the lipids(particularly polyunsaturated fatty acids ) and membranes in the body against damage caused by free radicals. the role of vitamin E is of particular importance in the lungs where exposure of cells to oxygen is greatest. Vitamin E also exerts a protective effect on red and white blood cells. It has been suggested that the body has a system to regenerate active vitamin E(perhaps involving vitamin C)once it has acted as an antioxidant. Vitamin e deficiency is normally associated with diseases of fat mal- absorption and is rare in humans. Deficiency is characterized by erythrocyte haemolysis and prolonged deficiency can cause neuromuscular dysfunction Hypervitaminosis E is not common, despite an increased intake of vitamin E supplements. Extremely high doses of the vitamin may interfere with the blood clotting process The RDAs for vitamin e are 10 mg and 8 mg a-tE day for men and women, respectively. UK RNI values have not been established for vitamin E since its requirement is largely dependent on the content of polyunsatu rated lipids in the diet. However, the Department of Health(1991)suggested that 4 and 3 mg a-TE day-l are adequate for men and women, respectively The major food sources of vitamin E are polyunsaturated vegetable oils and products derived therefrom(e.g. maragrine, salad dressings), green and leafy

VITAMINS IN MILK AND DAIRY PRODUCTS 273 6.11 6.12 R, I Tocotrienols I R3 Vitamin E is a very effective antioxidant. It can easily donate a hydrogen from the phenolic -OH group on the chromanol ring to free radicals. The resulting vitamin E radical is quite unreactive as it is stabilized by delocal￾ization of its unpaired electron into the aromatic ring. Vitamin E thus protects the lipids (particularly polyunsaturated fatty acids) and membranes in the body against damage caused by free radicals. The role of vitamin E is of particular importance in the lungs where exposure of cells to oxygen is greatest. Vitamin E also exerts a protective effect on red and white blood cells. It has been suggested that the body has a system to regenerate active vitamin E (perhaps involving vitamin C) once it has acted as an antioxidant. Vitamin E deficiency is normally associated with diseases of fat mal￾absorption and is rare in humans. Deficiency is characterized by erythrocyte haemolysis and prolonged deficiency can cause neuromuscular dysfunction. Hypervitaminosis E is not common, despite an increased intake of vitamin E supplements. Extremely high doses of the vitamin may interfere with the blood clotting process. The RDAs for vitamin E are 10 mg and 8 mg c(-TE day- for men and women, respectively. UK RNI values have not been established for vitamin E since its requirement is largely dependent on the content of polyunsatu￾rated lipids in the diet. However, the Department of Health (1991) suggested that 4 and 3 mg a-TE day- are adequate for men and women, respectively. The major food sources of vitamin E are polyunsaturated vegetable oils and products derived therefrom (e.g. maragrine, salad dressings), green and leafy

274 DAIRY CHEMISTRY AND BIOCHEMISTRY vegetables, wheat germ, whole-grain cereal products, liver, egg yolk, nuts The concentration of vitamin E in cows'milk is quite low (0.09 mg per 100 g) and is higher in summer than in winter milks. Human milk and colostrum contain somewhat higher concentrations(0.3 and 1.3 mg pe 100 g, respectively). Most dairy products contain low levels of vitamin E (Appendix 6A)and thus are not important sources of this nutrient. How ever, levels are higher in dairy products supplemented with vegetable fat(e. g some ice-creams, imitation creams, fat-filled dried skim milk). Like other fat-soluble vitamins, the concentration of vitamin E in dairy products increased ta with fat content. Vitamin E is relatively stable below .00C but is destroyed at higher temperatures(e. g. deep-fat frying). The vitamin may also be lost through oxidation during processing. Oxidative losses are increased by exposure to light, heat or alkaline ph, and are promoted by the presence of pro-oxidants, lipoxygenase or catalytic trace elements(e.g. Fe,cut). Pro-oxidants increase the production of free radicals and thus accelerate the oxidation of vitamin E. Exogenous vitamin E in milk powders supplemented with this nutrient appears to be stable for long storage periods if the powders are held at or below room temperaure The potential of feed supplemented with vitamin e to increase the oxidative stability of milk has been investigated, as has the potential use of exogenous tocopherols added directly to the milk fat 6.2.4 Phylloquinone and related compounds( vitamin K) The structure of vitamin K is characterized by methylnaphthoquinone rings with a side chain at position 3. It exists naturally in two forms: phyllo- quinone(vitamin Ki; 6.13)occurs only in plants, while menaquinones (vitamin K2; 6.14)are a family of compounds with a side chain consisting of between 1 and 14 isoprene units. Menaquinones are synthesized only by bacteria(which inhabit the human gastrointestinal tract and thus provide some of the vitamin K required by the body) Menadione(vitamin K3; 6.15) is a synthetic compound with vitamin K activity. Unlike K, and K menadione is water soluble and is not active until it is alkylated in vivo The physiological role of vitamin K is in blood clotting and is essential for the synthesis of at least four of the proteins(including prothrombin) involved in this process. Vitamin K also plays a role in the synthesis of a rotein(osteocalcin) in bone. Vitamin k deficiency is rare but can result from impaired absorption of fat. Vitamin K levels in the body are also reduced if the intestinal flora is killed(e.g. by antibiotics). Vitamin K toxicity is rare but can be caused by excessive intake of vitamin K supplements Symptoms include erythrocyte haemolysis, jaundice, brain damage and reduced effectiveness of anticoagulants The RDAs for vitamin K for people aged 19-24 years are 70 ug and 60 ug day-I for men and women, respectively. Corresponding values for

274 DAIRY CHEMISTRY AND BIOCHEMISTRY vegetables, wheat germ, whole-grain cereal products, liver, egg yolk, nuts and seeds. The concentration of vitamin E in cows' milk is quite low (0.09mg per lOOg) and is higher in summer than in winter milks. Human milk and colostrum contain somewhat higher concentrations (-0.3 and - 1.3 mg per 100 g, respectively). Most dairy products contain low levels of vitamin E (Appendix 6A) and thus are not important sources of this nutrient. How￾ever, levels are higher in dairy products supplemented with vegetable fat (e.g. some ice-creams, imitation creams, fat-filled dried skim milk). Like other fat-soluble vitamins, the concentration of vitamin E in dairy products is increased pro rafa with fat: content. Vitamin E is relatively stable below 100°C but is destroyed at higher temperatures (e.g. deep-fat frying). The vitamin may also be lost through oxidation during processing. Oxidative losses are increased by exposure to light, heat or alkaline pH, and are promoted by the presence of pro-oxidants, lipoxygenase or catalytic trace elements (e.g. Fe3+, Cu2+). Pro-oxidants increase the production of free radicals and thus accelerate the oxidation of vitamin E. Exogenous vitamin E in milk powders supplemented with this nutrient appears to be stable for long storage periods if the powders are held at or below room temperaure. The potential of feed supplemented with vitamin E to increase the oxidative stability of milk has been investigated, as has the potential use of exogenous tocopherols added directly to the milk fat. 6.2.4 Phylloquinone and related compounds (vitamin K) The structure of vitamin K is characterized by methylnaphthoquinone rings with a side chain at position 3. It exists naturally in two forms: phyllo￾quinone (vitamin K,; 6.13) occurs only in plants, while menaquinones (vitamin K,; 6.14) are a family of compounds with a side chain consisting of between 1 and 14 isoprene units. Menaquinones are synthesized only by bacteria (which inhabit the human gastrointestinal tract and thus provide some of the vitamin K required by the body). Menadione (vitamin K,; 6.15) is a synthetic compound with vitamin K activity. Unlike K, and K,, menadione is water soluble and is not active until it is alkylated in uiuo. The physiological role of vitamin K is in blood clotting and is essential for the synthesis of at least four of the proteins (including prothrombin) involved in this process. Vitamin K also plays a role in the synthesis of a protein (osteocalcin) in bone. Vitamin K deficiency is rare but can result from impaired absorption of fat. Vitamin K levels in the body are also reduced if the intestinal flora is killed (e.g. by antibiotics). Vitamin K toxicity is rare but can be caused by excessive intake of vitamin K supplements. Symptoms include erythrocyte haemolysis, jaundice, brain damage and reduced effectiveness of anticoagulants. The RDAs for vitamin K for people aged 19-24 years are 70pg and 60 pg day- for men and women, respectively. Corresponding values for