J. Dairy Sci. 84(E. Suppl.):E79-E83 © The American Dairy Science Association, 2001.

Vol. 84, E. Suppl., 2001 E79

Seasonal Changes in the Chemical Composition of Commingled Goat Milk

Ming R.Guo*, Peter H. Dixon, Young W. Park1,

James A. Gilmore, and Paul S. Kindstedt

Department of Nutrition and Food Sciences The University of Vermont Burlington VT 05405 1Agricultural Research Station College of Agriculture, Home Economic, and Allied Programs Fort Valley State University Fort Valley, GA 31030

ABSTRACT

Production of goat milk cheese in North America has been growing rapidly during the past several years. However, information on chemical composition and its seasonal varia-tion of year-round bulk-collected goat milk is limited. The objective of this study was to analyze the chemical composi-tion of commercial goat milk shipments for an entire year to provide fundamental information for cheese making and milk cheese yielding potential and pricing. Samples were collected weekly from bulk milk shipments to a commercial cheese company over 12 mo, beginning in April, 1996, and analyzed for contents (%) of total solids (TS), fat (F), lactose, crude protein, casein, nonprotein nitrogen (NPN), ash, minerals, and specific gravity (G). Chemical composition of the goat milk varied widely during the year. The contents of fat and TS de-creased over the first 20 wk from 3.6 and 12.7% to 3.0 and 11.3%, respectively, and then increased to peak values of 13.4 and 4.4% in January. Crude protein and casein contents also decreased over the first 20 wk, from 3.5 and 2.7% to 3.2 and 2.3%, respectively, before increasing gradually to 3.8 and 2.9% in February. The concentration of lactose seemed to de-crease below mean levels during August and January. Ash content declined during the first 20 wk from 0.82 to 0.78%, and then increased sharply to 0.90% by wk 36 before decreas-ing sharply again toward the end of the study. Calcium content decreased steadily from about 0.16 to 0.14% by wk 20, before increasing to 0.16% by around wk 40. It was found that TS content could be estimated using the equation: TS = 0.13 G + 1.41 F + 4.28 (r2 = 0.94, P < 0.01). (Key words: goat milk, composition, seasonal) Abbreviation key: TS = total solids

INTRODUCTION

The production of Chevre, the classic fresh goat milk cheese, has increased steadily in North America in the past 20 yr. The market is served by many small companies making Chevre on individual farms and in industrial cheese plants. Some research has been done on goat cheese varieties in the

United States (6, 14, 19, 20) and on gross milk composition from small herds and different breeds of goats (4, 7, 12, 21, 25). However, little has been done to investigate the properties of commercial commingled bulk goat milk used in making cheese in North America.

While bulk cow milk supplies exhibit a small seasonal ef-fect with a flush in late spring, commingled goat milk is much less uniform. A peak to trough ratio of 4:1 (July: February) has been measured for bulk-collected goat milk delivered to a commercial cheese company in Vermont, and this figure may be even higher for individual farmstead cheese operations. This seasonal production pattern results from having most of the milking animals at the same stage of lactation, which can cause problems with procurement of adequate supplies for cheese production (8, 13). In addition, a seasonal pattern of milk production causes significant variation in levels of milk constituents throughout the year, which affects the properties of milk used for making cheese (4, 5).

Making saleable Chevre cheese may pose a problem with respect to composition because commingled goat milk is more variable in fat and protein contentS than cow milk (15). This variation in composition may create potential problems with the declaration of the correct nutritional information. While no federal standards for Chevre cheese composition exist, cheese companies as well as consumers are still concerned with mar-keting a product with uniformity in composition and func-tional properties.

Furthermore, milk coagulation properties and cheese yields are dependent on the levels and types of casein, the fat content, and the ratio of casein to fat in milk (2, 3, 5). There-fore, it benefits companies that produce Chevre cheese to have a general idea of the seasonal changes in bulk goat milk com-position so as to better understand the effects on cheese mak-ing and finished product composition. With this information, companies can design strategies to reduce variability in cheese composition such as designing breeding programs for milk producers, making seasonal alterations to the cheese making process, or using milk standardization.

Currently, some goat cheese companies are paying for milk on a protein basis with higher payments for milk pro-duced during the low production period (24). Measuring ca-sein and fat levels and their relationship to cheese yield pro-vides information to companies for determining milk pricing formulas.

Received July 26, 2000. Accepted November 7, 2000. Corresponding author: M. R. Guo; e-mail: mguo@uvm.edu.

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The objective of this study was to analyze the chemical composition of commercial commingled goat milk for an en-tire year to provide fundamental and useful information to goat milk producers for breeding programs and milk proces-sors for cheese standardization, yield, and milk pricing.

MATERIALS AND METHODS

Collection of Milk Samples Bulk milk from the same group of 12 dairy farms located

in the northeast of the United States (New Hampshire and Vermont) was delivered by a tanker truck to a commercial cheese plant. The bulk milk was commingled from Saanen, Nubian, LaMancha, Alpine, and Toggenburg breeds. The milk production cycle was highly seasonal, with the majority of does freshening in March-April, reaching peak production in June-August, and drying off in November-December. Because of the off-season breeding practices on some of the farms, milk shipments continued throughout the entire year; the low-est production coming in December-February.

Samples of this commingled milk from six consecutive milkings were collected from the tanker on the same day of each week for 52 wk, from April 2, 1996, to March 25, 1997. The samples were stored at 4°C and transported to the Dairy Chemistry laboratory at University of Vermont in ice chest during the following day for analysis.

Compositional Analysis Total solids or moisture content of the samples were

measured by forced-draft oven method (17). The fat content in the milk was determined by the Babcock method (17). The concentrations of crude protein, casein, and NPN were ana-

lyzed according to the procedure of Lynch et al. (16). Specific gravity was determined following the method of Watson (26). Ash content was measured by gravimetric method (17). Lac-tose content was determined by difference.

Mineral Analysis Portions (2 g) of milk samples were dry ashed in a porce-

lain crucible, solubilized with 10 ml of 6 M HCl, quantita-tively transferred into 25-ml volumetric flasks, and diluted to volume with double-deionized water. Contents of calcium, phosphorus, sodium, and trace elements were measured using an Inductively Coupled Argon Plasma Emission Spectrometer (Jarrell-Ash Co., model number Atom-Comp-100) according to the procedures of Park (19) and Guo et al. (10).

Statistical Analysis Correlation and regression analysis and other tests were

performed according to statistical methods by Snedecor and Cochran (22). The data were analyzed using the general linear models procedures in SAS.

RESULTS

Sample collection began in early lactation for the majority of the goats producing milk for this study. From March 1996, milk production increased steadily each month, reaching a peak in July, declining slightly in August through September, and decreasing sharply during the fall until entering the lowest period of production in December 1996 to February 1997. Therefore, the milk production followed a typical 7-mo lacta-tion pattern, except for milk shipments in December, 1996, to February, 1997, which came from does freshening in the sum-mer.

Gross Composition The means, with standard deviations, of the various con-

stituents were as follows: CP, 3.47 ± .21%; fat, 3.61 ± 0.47%; total solids (TS), 12.38 ± 0.71%; lactose, 4.47 ± 0.15%; ash, 0.83 ± 0.04%. The chemical composition of the commingled goat milk varied widely during the year, with the levels of various constituents exhibiting wide ranges (Table 1) and

Table 1. Physio-chemical properties of commingled goat milk

n X ± SD Range Fat (%) 50 3.61 ± 0.47 3.00–4.40 Lactose (%) 50 4.47 ± 0.15 4.13–4.73 CP (%) 50 3.47 ± 0.21 3.19–3.86 Casein (%) 50 2.57 ± 0.15 2.34–2.86 NPN (% of CP) 49 5.04 ± 0.34 4.40–5.65 Total solids (%) 50 12.38 ± 0.71 11.17–13.44 Ash (%) 50 0.83 ± 0.04 0.79–0.89 Calcium (%) 50 0.15 ± 0.01 0.12–0.17 Phosphorus (%) 50 0.13 ± 0.02 0.10–0.16 Sodium (mg/kg) 49 672 ± 125 380–977 Magnesium (mg/kg) 49 160 ± 24 100–217 Zinc (mg/kg) 49 4.95 ± 1.93 1.30–9.50 Specific gravity 50 1.0235 ± 0.0007 1.0224–1.0262 Table 2. Pearson correlation coefficients between chemical com-ponents (n = 50).

Time Fat TS Casein Ca Lactose Ash NFS1

Time … Fat 00.83 - TS 00.72 00.96 … Casein 00.55 00.82 0.87 … Ca 00.57 00.88 0.92 00.81 … Lactose –0.26 –0.01 0.21 –0.02 0.23 … Ash 00.45 00.57 0.69 00.70 0.57 –0.14 … NFS 00.42 00.73 0.89 00.80 0.82 00.54 0.76 … 1Nonfat solids.

Figure 1. Changes in the contents of fat, casein, CP, and lactose of commingled goat milk during the 52 wk of this study.

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marked seasonal trends. The correlation coefficients between the chemical components are shown in Table 2.

Figure 1 shows that fat and CP contents were similar throughout the year except during two periods; from late May (wk 8) to late September (wk 21) the fat content was lower (3.0%) than CP content (3.2%) and from late November (wk 34) to late March (wk 52) the fat content was higher (ca. 4.2%) than CP content (ca. 3.6%).

Total solids were significantly (P < 0.01) correlated to fat (r = 0.96), casein (r = 0.87), and time of year (r = 0.72). Total solids reached a peak (13.44%) in January (wk 40) and a low point (11.17%) in late July (wk 17) (Figure 2). Ash content stayed in the range of 0.79 to 0.82% from April (wk 1) to Sep-tember (wk 24), reached peak levels (0.88 to 0.89%) from October (wk 28) to December (wk 38), and then declined steadily to a low point (0.79%) in March (wk 52); (Figure 3). Ash had a significant (P < 0.01) correlation to casein (r =

0.70).

Casein and NPN The average value of casein content in the commingled

milk was 2.57 ± 0.15% (Table 1). The seasonal variation in casein levels was very similar to CP content, but higher levels relative to the CP content during the first 23 wk indicated that the casein number was generally higher during the spring and summer and lower during the fall and winter (Figure 1). Ca-sein numbers ranged from 73 to 76% from the beginning of the study until late July, then decreases to 72% by mid of No-vember and stayed between 72 to 74%. The concentrations of NPN as percentage of CP also varied with season; the highest values (ca. 5.5%) occurred in the summer before a sharp de-crease to the lowest values (ca. 4.5%) in the winter (Figure 4).

Mineral Composition Table 1 shows that means and standard deviations of the

minerals studied were as follows: calcium, 0.15 ± 0.01%; and

Figure 2. Changes in total solids (TS) content of commingled goatmilk during the 52 wk of this study.

Figure 3. Changes in ash content of commingled goat milk duringthe 52 wk of this study.

Figure 4. Changes in NPN content of commingled goat milk during the 52 wk of this study.

Figure 5. Changes in the contents of calcium (Ca) and phosphorus (P) of commingled goat milk during the 52 wk of the study.

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phosphorus, 0.13 ± 0.02%; sodium, 672 ± 12 6mg/kg; magne-sium, 160 ± 24 mg/kg; zinc, 4.95 ± 1.93 mg/kg. Figure 5 shows that total calcium content declined steadily from 0.158% in April (wk 1) to 0.135% in mid September (week 24) before increasing to a peak of 0.165% on January 22 (wk 43). Changes in phosphorus over the period were shown a similar pattern to calcium (Figure 5).

Specific Gravity There was a sharp decrease in the specific gravity of goat

milk from 1.0262 on April 2 (wk 1) to 1.0235 on May 21, 1996. From then on the specific gravity stayed in the approxi-mate range of 1.0230 to 1.0235; the lowest values occurring in September around the midpoint of the study (Figure 6). The content of TS can be estimated by fat content (F) and specific gravity (G, reading from lactometer) in the following equa-tion: TS = 0.13G + 1.41F + 4.28 (r2 = 0.94, P < 0.01) which was determined by generalized least square analysis.

DISCUSSION

Goat milk composition is known to depend on factors such as breeding, stage of lactation, feeding, individual animal differences, locality, and climate (4, 11, 18, 21, 25). The re-sults from this study clearly show that the commingled milk TS, fat, CP, casein, ash, and calcium contents and specific gravity were affected mainly by lactation stage since the milk was produced by the herds in the same region with the same climate and similar feeding programs. Therefore, these results may be different from those of studies in other regions.

Total Solids It has been shown that TS and composition are influenced

by many factors, including stage of lactation, diet, breed, age, health, season, and environment (23). In a 1985 Norwegian study of a seasonal herd of 70 lactating goats, Brendehaug and Abrahamsen (4) found that TS content was highly correlated with lactation stage (r = 0.88), with the peak coming in the last month of lactation. There was a regular decrease in TS from

the start of lactation in February until the beginning of the pasture period (early June) when levels increased again before decreasing to a minimum in late September at the end of pas-turing. The TS content of commingled milk also decreased in early lactation, reaching a minimum by the pasture period, but did not decrease at the end of the pasture period. This differ-ence may be attributed to feeding practices, as many of the herds in this study were feeding a constant diet in the barn and supplementing pasture feeding rather than having a distinct pasture grazing period to meet nutritional needs, which seems to affect the chemical composition of goat milk (4).

The mean TS content was very close to the mean TS con-tent of goat milk in Northern Ireland 12.27% (7). Voutsinas et al. (25) studied a herd of 42 lactating Alpine goats over a 42-wk lactation in 1988 on a farm in Ioannina, Greece. The goats were fed a constant ration for the entire lactation. The results showed that all milk constituents were significantly dependent on stage of lactation. The variation in TS content was similar to this study, while the mean (11.76%) was lower as were the means of many of the other constituents. Inconsistencies with our results were found by Mba et al. (18), who observed that the TS content of milks from Saanen, Red Sokoto, and West African dwarf goats was higher in mid- than in early lactation and lowest in late lactation, while researchers in Iraq (1) found that TS content of milk from a single herd wasn’t significantly dependent on lactation stage.

Fat and Crude Protein Wide variation in fat and protein contents is due to having

most of the milk-producing animals at the same stage of lacta-tion and also because midlactation, when fat and protein are expected to be low, usually comes in midsummer when cli-matic conditions favor the production of low fat, low solids milk (15). The above statement explains the trend in fluctua-tions of fat and protein contents in this study and in the results of others. Grappin found that the fat and protein contents of goat milk (mainly Alpine Chamoise and Saanen Chamoise breeds) showed marked seasonal variations, with low contents during the period from May to July (9). The changes in fat content of commingled goat milk observed in this study were consistent with Brendehaug and Abrahamsen (4), who found a sharp decrease from 5.5 to 3.0% by the beginning of pasturing, followed by an increase to 4.2% in late lactation. A similar trend in fat content variation (3.34–2.73–4.58%) was reported by Voutsinas et al. (25). In this study, fat content was signifi-cantly dependent on lactation stage (r = 0.85), which was also the case in both studies mentioned above and in a study by Ali and Hassan (1). The fat content had a mean of 3.61% and var-ied from 3.00 to 4.40%, which is consistent with the mean (3.63%) but not the range (2.55 to 5.35%) of fat levels in goat milk from Northern Ireland (7).

Crude protein content was higher than fat content from April 30 to August 20, 1996 (Figure 1). Similar results were obtained by Voutsinas et al. (25) during midlactation (wk 20 to 34); fat content ranged from 2.93 to 3.62% and protein con-tent from 2.95 to 3.65%. The results of Espie and Mullan (7) showed a similar mean (3.39% compared with 3.47%) but a larger range (2.58 to 4.81% compared with 3.19 to 3.86%) in CP content. Changes in Norwegian goat milk %CP during lactation, paralleled the results in this study; the milk con-tained 3.80% CP in early lactation (February 24), decreased to a minimum of 2.85% on June 9, and increased steadily to

Figure 6. Changes in specific gravity of commingled goat milk dur-ing the 52 wk of this study.

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Vol. 84, E. Suppl., 2001 E83

3.65% at the end of lactation on November 11 (4). However, fat content was never lower than %CP during lactation, which was also the case with the milk from individual goats in dif-ferent stages of lactation in Nigeria (18).

Casein and NPN Changes in the percentage of casein N as percentage of

total N were not consistent with those found in Norwegian goat milk, which had low levels (70 to 71%) in early lactation, and the highest levels (73 to 76%) in midlactation, before de-creasing to 72% in late lactation (4). The variation pattern of casein content was also different from that found by Voutsinas et al. (25), who reported a steady increase from a minimum (2.11%) to a maximum (3.15%) over the entire lactation. The mean casein content was the same (2.57%) as that found by Espie and Mullan (7) in Northern Irish goat milk, but the stan-dard deviation and range was much smaller, 0.15 and 2.34 to 2.86, compared with 0.46 and 1.91to 3.86%, respectively.

The behavior of NPN as percentage of CP was consistent with the results of Grappin’s study of milk from 43 French goatherds during 1 yr (9) and from Norwegian goats with higher levels in summer than in fall (4). However, the actual NPN/CP values were higher in French and Norwegian goat milk, 9.3 and 7.1%, compared with 5.5 and 4.5% in summer and fall of this study, respectively.

Specific Gravity The major change in the specific gravity of goat milk oc-

curred during the first 10 wk of the study (from 1.0262 on April 2 to 1.0234 on June 14, 1996). After that period the spe-cific gravity stayed in a close range (1.0224 to 1.0240), which is similar to the observations of Ali and Hassan (1), who found no significant changes during lactation. Voutsinas et al. (25) found that Alpine goat milk had a significantly lower specific gravity in late lactation (>35 wk) than in early and mid-lactation. The commingled milk had consistently lower spe-cific gravity than the Alpine goat milk where the range was 1.0224 to 1.0262 compared with 1.027-1.031. Total solids content of goat milk could be estimated based on the content of fat and specific gravity by using the equation developed by this study.

CONCLUSIONS

The results indicated that summer milk had the highest yield potential per kilogram of protein due to a higher propor-tion of casein in CP. The milk with the highest CP content and lowest casein number was produced in late lactation by does that freshened in the summer. Nonprotein nitrogen as a per-centage of CP followed the same trend as casein number dur-ing the year, indicating that the noncasein nitrogen as a per-centage of total N was much higher in the winter milk (~12%) than in milk produced during the summer months (~9%).

The ratio of CP to fat content was >1 during the summer months. These results suggest that there will be a large varia-tion in cheese composition, i.e., fat, protein, and TS, during the year. Milk standardization, especially in February, will enable cheese producers to make products with more uniform composition and functionality. Varying the moisture content of cheeses during the year can reduce the variation in fat-in-dry matter, e.g., increasing moisture content when fat content

is low. This method will be successful as long as cheese func-tionality is not impaired by excessive moisture in the cheese.

ACKNOWLEDGMENTS

Funding for this project was provided by USDA Hatch Project VT-PS-00575 and Vermont Butter and Cheese Com-pany, Inc.

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