Meat And Milk composition

Milk is an important source of essential dietary nutrients. It provides a variety of digestible proteins that also impart functional properties important for the manufacture of various dairy products (cheese, ice cream, etc.). Milk is also an important source of calcium and other minerals and vitamins such as A, thiamine, riboflavin, pyridoxine, etc. Milk is a dietary source of lipids that provide flavor characteristics and functional properties for processed dairy products. The composition of milk is not constant during lactation but is influenced by various factors such as genetics, breed, stage of lactation, age, diet composition, nutritional status, environment, and season. For example, milk protein concentration can vary from 3% to 4%, and fat content from 3.5% to 6.0%, whereas lactose remains relatively constant around 5%.159 For example, in the first eight weeks of lactation, dairy cows are in negative energy balance, which means that the dairy cow is not taking in enough dietary nutrients to support milk production and therefore it must mobilize more lipids from body fat stores, leading to slightly higher milk fat content. Milk protein levels are reduced slightly. The cow subsequently adjusts its metabolism and feed intake to return to positive energy balance.

Because bST directs the flow of nutrients to the mammary gland to support lactation, the impact of bST supplementation on milk composition was evaluated. If changes in milk composition were observed, they would need to be compared with the normal fluctuation in milk composition that occurs across a lactation cycle to determine whether they fell within these limits. The nutritional composition of milk (e.g., fat, protein, lactose) has been monitored in numerous (more than 200) bST trials with dairy cows and no substantial alterations in nutrient composition have been reported.22,160 bST administration starts in the 9th to 10th week of lactation. If a dairy cow is in negative energy balance when supplemented with bST, there is an increase in fat percent in the milk. This has little practical impact on overall milk composition because individual dairy cows in a herd are at different stages of lactation, and the milk from all cows is combined in the bulk tank after milking.

The levels of milk components can vary considerably throughout lactation, as shown in Tables 7.7 and 7.8, and these variations are greater than any changes that have been observed between bST-treated and control dairy cows.161,162 Although levels of lactose are relatively constant throughout lactation, total protein and fat levels decrease considerably during the early weeks of lactation and gradually increase as lactation proceeds.162 Energy balance has a large impact on fatty acid composition as increases in the weight percent of C6 to C16 fatty acids are apparent during early lactation whereas the weight percent of C4 and C18:1 and C18:2 fatty acids

TABLE 7.7

Effect of bST on Milk composition — Full lactation period3

TABLE 7.7

Effect of bST on Milk composition — Full lactation period3

Range of Control

Values Across

Component

Control

bSTb

Lactation

Lactose

4.81c (0.02)d

4.85 (0.02)

4.61-4.87

Total protein

3.24 (0.02)

3.32 (0.02)*

2.85-3.55

Casein

2.53 (0.03)

2.56 (0.03)

2.2-2.7

True protein

3.08 (0.04)

3.13 (0.04)

2.7-3.3

Nonprotein nitrogen

0.172 (0.002)

0.179* (0.002)

0.167-0.196

Casein as % true

82.07 (0.30)

81.61 (0.30)

81.0-82.5

protein

Total fat

3.67 (0.06)

3.76 (0.06)

3.2-4.4

a Milk components measured Days 5 and 12 of each two-week bST injection cycle, starting Weeks 10

to Week 41 postpartum (one lactation cycle).161 b There were 39 control and 40 bST-treated Holstein dairy cows. c Least-square means adjusted for pretreatment values. d Standard error of the least-square means.

* Difference between control and bST group was significant, p < 0.05.

a Milk components measured Days 5 and 12 of each two-week bST injection cycle, starting Weeks 10

to Week 41 postpartum (one lactation cycle).161 b There were 39 control and 40 bST-treated Holstein dairy cows. c Least-square means adjusted for pretreatment values. d Standard error of the least-square means.

* Difference between control and bST group was significant, p < 0.05.

TABLE 7.8

Effect of bST on Milk Fatty Acid Composition for a Full Lactation Perioda

TABLE 7.8

Effect of bST on Milk Fatty Acid Composition for a Full Lactation Perioda

Component

Controlb (wt. %)

bSTb (wt. %)

Range of Control Values (wt. %) across Lactation

C4

2.9

2.8

2.5-4.3

C6

2.2

2.2

2.0-2.4

C8

1.2

1.1

1.0-1.25

C10

3.0

2.9

2.2-3.2

C12

3.9

3.8

2.0-4.0

C14

12.4

12.1

9.2-13.0

C14:1

3.1

3.2

1.1-3.5

C16

32.7

33.2

25-35

C16:1

4.3

4.3

3.25-5.1

18

8.5

7.9

1-14

18:1

23.0

23.7

21-34

C18.2

2.8

2.8

1.8-4.1

Cholesterol

0.388

0.405

0.21-0.45

Phospholipid

0.743

0.733

0.515-0.90

a Milk components measured Days 5 and 12 of each two-week bST injection cycle, starting Weeks 10

to Week 41 postpartum (one lactation cycle).162 b There were nine control and nine bST-treated Holstein dairy cows/group. There were no statistically significant differences (p < 0.05).

a Milk components measured Days 5 and 12 of each two-week bST injection cycle, starting Weeks 10

to Week 41 postpartum (one lactation cycle).162 b There were nine control and nine bST-treated Holstein dairy cows/group. There were no statistically significant differences (p < 0.05).

are decreased.161 Percentages of most fatty acids were relatively constant at mid-lactation, and decreases in C16:1 and C18:2 fatty acids and increased C18:1 fatty acids occurred in mid to late lactation.161 The impact of the stage of lactation on fatty acid composition of milk fat was attributed to changes in the relative contributions of body fat mobilization and de novo synthesis of milk fat constituents in response to changes in energy balance.161

Milk components such as fatty acids, cholesterol, casein and whey proteins, P-lactoglobulin, a-lactalbumin, and minerals (calcium, phosphorous, etc.) from bST-supplemented cows are comparable to those of control cows and are well within the normal range of values that occur across lactation, as shown in Tables 1.1 and 1.8 and in the published literature.161 When milk composition was monitored following administration of bST across four lactations (Table 1.9), milk protein and lactose levels were not changed although milk fat percentages were slightly lower during the second through fourth lactations.163 Since the milk yields were higher in bST-treated cows, yields of total milk fat for bST-treated cows were not different from controls despite a slight decrease in milk fat percentages.

Since the manufacturing properties of milk are important to dairy product manufacturers, a variety of milk characteristics from bST-supplemented cows have been studied (e.g., freezing point, pH, alcohol stability, thermal properties, proteases, lipases,

TABLE 7.9

Milk compositiona,b for Holstein Dairy cows administered bst across Four lactation cycles all component lactation 1 lactation 2 lactation 3 lactation 4 lactations

TABLE 7.9

Milk compositiona,b for Holstein Dairy cows administered bst across Four lactation cycles all component lactation 1 lactation 2 lactation 3 lactation 4 lactations

Milk fat, % Control

3.35

3.62

3.61

3.47

3.51

bST

3.43

3.23*

3.28*

3.24*

3.29*

Milk protein, % Control

3.1G

3.15

3.2G

3.G5

3.13

bST

3.G9

3.G7

3.G8

3.G5

3.G7

Milk lactose, % Control

4.86

4.68

4.61

4.54

4.67

bST

4.89

4.62

4.61

4.38

4.63

a Least-squares means, covariate-adjusted for the pretreatment period.

b There were 39 control and 39 bST treated cows in first lactation; 12 controls and 14 bST-treated cows in second lactation; seven control and nine bST-treated cows in third lactation; and six control and six bST-treated cows in fourth lactation.163 * Statistically significantly different, p < 0.05.

a Least-squares means, covariate-adjusted for the pretreatment period.

b There were 39 control and 39 bST treated cows in first lactation; 12 controls and 14 bST-treated cows in second lactation; seven control and nine bST-treated cows in third lactation; and six control and six bST-treated cows in fourth lactation.163 * Statistically significantly different, p < 0.05.

susceptibility to oxidation, sensory characteristics including flavor, cheese-making properties, starter culture growth, coagulation, syneresis) and reviewed.164,165 Milk from bST-supplemented cows was shown to have manufacturing properties within the normal range of biological variation and comparable to milk from control dairy cows.

Because the sensory qualities of milk and derived dairy foods are important to consumers, the sensory characteristics and flavor stability of milk from bST-supplemented cows have been investigated.166,167 No meaningful differences in flavor and flavor stability of dairy foods were observed in milk from bST-supplemented dairy cows. Other factors inherent to milk production, such as bacterial count, highspeed pumping of milk at improper temperatures, adsorption of off-flavors from the air, transmission of off-flavors from feed, etc., are considered to have the most significant impact on milk flavor, independent of whether dairy cows received bST

supplementation.166,167

The results of all the milk composition and processing studies were subsequently reviewed in the aforementioned U.S. government report,34 as summarized below:

• There is slight variation in milk fat and milk protein content immediately after bST treatment, which is common after any feed or metabolic adjustment.

• Milk fat, protein, lactose, total solids, and solids-not-fat percentages are unaffected over a full lactation period and are not different from milk from nontreated cows.

• Milk ash or mineral content, specifically phosphorous and calcium content, are not altered by bST treatment.

• A slight shift in Kjeldahl nitrogen fractions (casein, whey protein, and nonprotein nitrogen) has been observed in some experiments (this does not affect milk quality but may affect cheese yield from milk).

• There are no effects on the relative proportions of short-, medium-, and long-chain fatty acids and no changes in free fatty-acid content have been noted; therefore, no influence on off-flavor "rancidity" is anticipated, nor is vulnerability to oxidized flavor development.

• Meat derived from bST-treated cows tend to have lower fat content but is otherwise identical to that from untreated cows.

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