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 macronutrients (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 minerals 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 (<500 ug RE day- )results in night blindness, xerophthalmia(progressive blindness caused by drying of the cornea of the eye), keratinization(accu mulation of keratin in digestive, respiratory and urogenital tract tissues)and finally exhaustion and death. At excessive intake levels(>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 tvansretinal 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 ( < 500 pg RE day- ') results in night blindness, xerophthalmia (progressive blindness caused by drying of the cornea of the eye), keratinization (accumulation of keratin in digestive, respiratory and urogenital tract tissues) and finally exhaustion and death. At excessive intake levels (> 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 pasteurized 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 concentration 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, respectively, 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 protected 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. ascorby1 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-dehydrocholesterol to pre-vitamin D,. This pre-vitamin can undergo further photoconversion to tachysterol and lumisterol or can undergo a temperature-dependent isomerization to cholecalciferol (vitamin D,, 6.8). At body temperature, this