Detection of Nonfat Dried Milk in Whole Milk G. K. MURTHY and L. KAYLOR
U.S. Department of Health, Education, and Welfare, Food and Drug Administration, Cincinnati, Ohio 45226 Abstract
Proteins precipitated from milk by acetic acid at p H 4.7 were reacted with phosphomolybdic acid, and the blue color imparted to them was measured by reflectance spectrophotometry. The computed curves for unknown samples were: P e r cent nonf a t dried milk (fresh) = 789.2 × OD -129.8; per cent nonfat dried milk (old) -575.5 X OD -- 112.5, where OD is optical density (reflectance). The minimum amount of detectable nonfat dried milk was 10%, and the precision of the method was 0.014 OD. Sequential analyses of the fresh-raw, pasteurized, condensed, and dried skimmilk samples revealed significant change in the proteins during drying. Dialysis of milk or washing of the precipitated proteins prior to the reaction with phosphomolybdic acid considerably decreased the blue color imparted to the proteins. Similarly, increased f a t in milk progressively decreased the color reaction. Storage of fresh nonfat dried milk at 4 and 22 C had no effect on the test up to 8 and 4 weeks.
Several methods have been developed for the detection of nonfat dried milk ( N F D M ) in whole milk. These include the colorimetric method based on the presence of proteinreducing substances and their reaction with potassium ferricyanide (1, 2, 3, 5, 6) and resazurin (11), or determination of nitrogen due to whey protein denaturation and total calcium (4). These tests are relatively insensitive, detecting at best 10 to 20% of added NFDM. The uncertainty arises from the inherent variation of the interpretive base line criterion used for fluid milk and an even greater variability resulting from processing variables for N F D M . Recently, Mishra (8) developed a r a p i d qualitative visual color test, which is claimed to be sensitive enough to detect 1% N F D M in whole milk. While the method appears satisfactory for predicting the presence or absence
of N F D M in whole milk, it is not a quantitative procedure. Our investigation was undertaken to standardize and quantitate the procedure developed by Mishra (8) and to provide an explanation for the color reaction involving milk components and phosphomolybdic acid. Materials and Methods
Materials. Fresh samples of raw, pasteurized, condensed, and dried (low heat) skimmilk were obtained from Mid-America Dairymen Inc., Des Moines, Iowa. The samples, packed in insulated containers~ were kept at 0 to 4 C with freeze packs and shipped to Cincinnati by air express. Samples of pasteurized whole milk and N D F M were also obtained from the market in Cincinnati, Ohio. Apparatus. A Beckman Model DBG Spectrophotometer was used equipped with no. W136957 reflectance attachment. 1 Procedure. Transfer approximately 50 ml of milk to a 100-ml beaker. Precipitate proteins by adjusting the p H to 4.7 with 4% acetic acid with a p H meter. Hold mixture at room temperature for 20 rain with occasional stirring. Transfer sample to a 50-ml tube and centrifuge for 20 rain at 850 × g. Decant clear supernatant and weigh duplicate 2-g portions of the sedimented proteins into 16 X 120 mm test tubes. Add 5 ml of distilled water and 1.0 ml of 4% phosphomolybdic acid to the sample and mix contents with an electric stirrer. Place sample in a boiling water bath for 15 rain, then cool to 21 C in a cold water bath. Grind proteins thoroughly in the test tube with a Teflon tissue-grinding stirrer, and separate in a millipore type filter with 2.3-cm W h a t m a n no. 1 filter paper, continue suction 5 rain more to remove excess moisture and to obtain a uniform surface in all samples. Transfer sample with filter p a p e r to a 3.0- by 3.7 cm rectangular Teflon block containing a hole 2.5 em in diameter and 0.3 cm deep. Place a microscopic slide over the sample, held in position with cellophane tape, as block is placed vertically in the instrument. Measure reflectance at 460 nm with the MgO block in the reference 1 Mention of commercial products does not imply endorsement by the Food and Drug administration.
Received for publication November 27, 1970. 826
compartment and a slit width of 2.0 mm as recommended by the instrument manufacturer. Variation of the slit width does not affect the optical density. Determine reflectance maxima (460 nm) by measuring spectra of the colored protein sample. All optical densities (OD) should be made within 30 rain of sample preparation, after filtration, to avoid unnecessary drying of samples which decreases observed readings. Standard curve. To quantitate results, prepare standard curves for both freshly p r e p a r e d (less than one week old), and market samples (old, age unknown) of NFDM. Reeozlstitute nonfat dried milk with distilled water to 9% solids which should dissolve instantly. Reconstitute freshly pasteurized milk and N F D M to yield N F D M of 0, 1, 3, 5, 10, 20, 40, 60, 80, and 100% (v/v). Process samples as described under Procedure. Correct observed results for 3.5% fat and construct standard curves relating percent N F D M to reflectance (Fig. 1). Calculated regression equations are: % N F D M (fresh) = 789.2 × 0 D -- 129.8 % N F D M (old) = 575.5 × 0 D -- 112.5 Results of portions of the various reconstituted samples stored at 4 C for 2 to 6 days were within the expected experimental error of 5%. This indicated that the test is not affected by age of the milk and, therefore, is applicable to market milk samples. F i g u r e i shows (based on the precision of the method, OD = 0.014) that with fresh N F D M the minimum amount of N F D M detectable in fresh fluid milk was about 20%~ whereas with old N F D M it was about 5%. 5~4
NONFAT ORIEO MILK,
:FzC,-. 1. l~elationship between reflectance (0D) and percentage nonfat dried milk in whole milk. (A), per cent NFDM ~- 789.2 X OD - - ]29.8 (B), per cent NFDM ~ 575.5 X OD -- 112.5
Since age of N F D M in suspected market milk cannot be determined, the percentage of IqFDM may be expected to lie between the two curves in Figure 1. Although our standard curves are satisfactory, where the composition of milk varies greatly with season or because of geographical location, it is a p p r o p r i a t e to determine the standard curves experimentally. Results and Discussion
Preliminary Because expe~me~ts. the published method (8) was qualitative, various steps in the sample preparation were quantitated as follows: Milk proteins were precipitated with acetic acid by adjusting the p H to 4.7 with a p i t meter, the proteins were separated by centrifugation, and the amount of phosphomolybdic acid added was measured with a pipette. The precision and accuracy of the procedure were tested by determining the reproducibility of analysis. Six replicates each of two reconstituted lqFDM milk samples (9% TS) were processed as described before. The data showed reflectances of 0.277 ~- 0.009 and 0.341 - - 0.013 with a coefficient of variation of 3.3 and 3.9%, respectively. I n addition, duplicate analyses of a market sample of N F D M (age unknown) stored at 4 C for 8 weeks gave a reflectance of 0.353 ± 0.018 (dr = 8) with a coefficient of variation of 5.0%. Analyses of market milk. To determine the variation in the observed reflectance, market samples of pasteurized whole milk and skimmilk were obtained during this study. Analyses o£ samples showed reflectances as follows : whole milk, 0.191 ± 0.008 (dr -= 33); and skimmilk 0.244 ± 0.014 (dr ---- 8). Effect of fat co~te~t. Since data from market samples o£ whole milk and skimmilk showed marked differences in reflectances, the effect of f a t content on the color test was determined. Pasteurized skimmilk and 30% cream were mixed to yield 0.05, 2.0, 3.5, 4.5, 6.0, and 8.0% fat. Samples were analyzed in duplicate, and results were expressed, by the equation: reflectance (OD) = 0.227 -- 0.0041 × % fat. Reflectance progressively decreased as the fat content of milk increased. Presumably, the observed results are related to decreased protein of samples as the fat increased. Effect of processing mi~k. To determine the step or steps in the manufacture of N F D M that produced substances capable of reducing phosphomolybdic acid, fresh samples of raw, pasteurized, condensed (diluted 1:2 with water), and dried skimmilk (reconstituted to 9% solids) were analyzed. Averaged data of 3 milk samJOURI~AL
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8 weeks, respectively. Based on the precision of the method (OD =- 0.014), samples stored at 4 C did not change significantly, whereas those stored at 22 C did not change up to 4 weeks but thereafter changed significantly. Various explanations may be suggested for the changes in N F D M , which gives positive results with phosphomolybdic acid. Since analysis of freshly boiled milk had a reflectance of 0.204 -+- 0.012 compared to 0.191 -----0.008 for the control sample, the effect of sulphydryl compounds on the test has been ruled out (8). However, freshly p r e p a r e d N F D M gave a positive test and, therefore, mechanisms for the formation of reducing substances in the product, a p a r t from being accentuated by heat applied to milk during various steps in the manufacture of N F D M , may involve other processes. These include formation of reducing substances depending upon the moisture content of the product and relative humidity of the environment in which the product was stored (7), oxidation of lipid, and oxidation of leucine, methionine, and phenylalanine (9, ]0). Protein-reducing substances have been correlated with flavor defects (9). That some of the milk constituents responsible for reducing phosphomolybdic acid are dialyzable and become bound to proteins in the drying process suggests that nonproteinaceous materials also act as reducing agents. Whether these include lactose and oxidation products of amino acids needs further study and clarification.
ples showed reflectances of 0.264 ----- 0.010, 0.264 --+ 0.010, 0.276 - - 0.015, and 0.342 - 0.011, respectively, indicating that a small but definite change in protein occurs during condensing, and a major change occurs during drying of milk. This was also seen by color comparisons of the test samples. Effect of dialysis of milk. To determine the nature of substances responsible for the reduction of phosphomolybdic acid, fresh samples of raw, pasteurized, condensed, and dried skimmilk, reconstituted as before where necessary, were dialyzed for 24 hr against running distilled water at 4 C. Portions of the original and dialyzed samples were analyzed as described under Procedure. Table 1 shows that dialysis of milk reduces the amount of blue color imparted to the proteins. The dialysis effect was less with the NFDM, indicating that parts of the dialyzable components are bound to the protein during drying. Washing the proteins (after precipitation with acetic acid) three times with water also gave results comparable to those obtained with dialysis. I t has also been shown (5) that the compounds responsible for the reduction of acid ferricyanide in raw skimmilk are dialyzable and anionic in nature. Effect of storage. To determine the effect of age of the N F D M on the test, freshly prepared N F D M was divided into several portions and stored in air-tight, screw-cap plastic bottles (20 ml) at 4 and 22 C. Samples were analyzed at intervals. The averaged refleetances for three samples were: at 4 C, 0.285, 0.295, 0.288, 0.290, and 0.300; at 22 C, 0.285, 0.296, 0.312, 0.316, and 0.329 OD, at 0, 2, 4, 6, and
Acknowledgment The authors are grateful to Dr. J. E. Campbell for his interest in the problem, and to R. E. Johonson of Mid-America Dairymen Inc., Des
TABLE ]. Effect of dialysis of milk and washing of protein on reflectance, a Milk Product
Dialyzed Pasteurized skimmilk None
--0.040 Condensed skimmilk
--0.038 Nonfat dried milk
a Average of two samples. JOURNAL
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I)R:ED MILK I)ETEOTION Moines, Iowa, for supplying fresh milk products without which this study would not have been possible.
References (1) Association of Official Agricultural Chemists. 1965. Official Methods of Analysis. 10th ed., Washington, D.C. (2) Belle, G., and P. Caspar. 1959. Colorimetric method of indicating the presence of reconstituted dried milk. Lait, 39: 241. (3) Cardwell, J. T., and F. H. tIerzer. 1958. A study of factors which eause variations in the protein-reducing value of fluid milk. J. Dairy Sci., 41" 702. (4) Chang, S. S., J. ¥ . Lin, tI. M. Mei, and C. N. Shih. 1966. Testing of raw milk adulteration. XVI][. Int. Dairy Congr. B. 239. (5) Hobbs, W. E., and S. T. Coulter. 1962. Characterization of the substances in raw and heated milk that are responsible for
reducing acid ferricyanide. J. Dairy Sci., 45 : 661. Junker, G. M. 1960. Method for detection of reconstituted milk in fluid market milk. J. Ass. Offi. Agr. Chemists, 43: 407. Labuza, T. P., S. R. Tennenbaum, and M. Karel. 1970. Water content and stability of low-moisture and intermediatemoisture foods. Food Tech., 24: 543. Mishra, M. 1966. A simple and rapid test for detection of skim milk powder in whole milk. Indian Vet. J., 43: 160. Ramshaw, E. tI., and E. A. Dunstone. 1969. The flavor of milk protein. J. Dairy Res., 36 : 203. Ramshaw, E. 1~., and E. A. Dunstone. 1969. Volatile compounds associated with the offflavor in stored casein. J. Dairy Res., 36 • 215. Toubal, V. 1960. Colour reaction of resazurin in fresh milk containing added reconstituted milk. Lait, 40: 18.
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