free fatty acids in homogenized milk are bound or entrapped.

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1 Modified Copper Soap Solvent Extraction Method for Measuring Free Fatty Acids in Milk W. F. SHIPE, G. F. SENYK, and K. B. FOUNTAIN t Department of Food Science Cornell University Ithaca, NY ABSTRACT A modified copper soap method was developed to provide a sensitive, rapid method for determining the free fatty acids in milk and estimating the intensity of lipolyzed flavor. The modification is simpler and more rapid than the original copper soap method. Only a small sample (.5 ml) of milk is needed for this colorimetric test, so it would be feasible to take aliquots from a universal sample. The method can be semiautomatic with an automatic sampler and spectrophotometer with a flow-through cell to test up to 200 samples per hour. It would take approximately 2 days to run the equivalent number by the Bureau of Dairy Industry method. The coefficients of correlation between copper soap measures and flavor for nonhomogenized and homogenized milk were.83 and.82. The corresponding coefficients between Bureau of Dairy Industry measures and flavor were.67 and.78. Correlation between copper soap and Bureau of Dairy Industry milliequivalents per liter for 124 nonhomogenized milk samples was.90. The corresponding correlation for 109 homogenized samples was.88. A comparison of nonhomogenized and homogenized milk revealed that the availability of free fatty acids is less in homogenized milk. Fewer free fatty acids are recovered from homogenized milk by "solvent extraction, and they have a lower sensory impact. Presumably some of the Received June 11, PFW, Inc., 33 Sprague Avenue, Middletown, NY free fatty acids in homogenized milk are bound or entrapped. INTRODUCTION Numerous methods have been developed for measuring free fatty acids in milk to estimate the intensity of lipolyzed flavor. The most frequently used method in this country is the Bureau of Dairy Industry (BDI) method developed in 1955 by Thomas et al. (6). Although this method is relatively simple, it is not as rapid or sensitive as desired. Furthermore, it does not always correlate well with sensory results. Several methods have been developed to overcome these limitations. An extensive review of the literature on this subject is in (2, 3). In 1977, three modified methods were developed in Europe. Scientists at the National Institute for Research in Dairying in England (5) developed an extraction-titration procedure that gave higher butyric acid recoveries and also overcame the potential interference from lactic acid. There are no published data on how well this method correlates with sensory results. A research group of the Royal Dutch Dairy Association modified the BDI method (1) by adjusting the pit of the reagent and changing the solvent system. These modifications improved reproducibility of the method, but did not increase speed significantly. The third modified method was developed by Koops and Klomp (4) at the Netherlands Institute for Dairy Resaerch. In this method the free fatty acids are converted to copper soaps, extracted, and the copper reacted with a color reagent. Whereas the other two methods involved titration of the free fatty acids, the copper color complex was measured colorimetrically. Because the sensitivity of colorimetric measurements is greater than titrimetric measurements and is more compatible with automation, it seemed to have the greatest potential for a sensitive rapid test. Therefore, investigation of this method was undertaken J Dairy Sci 63:

2 194 SHIPE ET AL. EXPERIMENTAL PROCEDURE Copper Soap Method (CSM) Reagents were certified A.C.S. Grade, Fisher Scientific Co., Pittsburgh, PA unless specified otherwise. The copper reagent was a mixture of 5 ml of triethanolamine and 10 ml 1 M aqueous Cu (NO3)2. 3H20, diluted to 100 ml with a saturated NaC1 solution. The ph was adjusted to 8.3 with 1N NaOH, and the mixture was stored in the dark at room temperature. (It should be stable for at least 5 too.). Color reagent was a.5% sodium diethyl dithiocarbamate solution in n-butanol. Solvent was chloroform-heptane-methanol (CHM) (49:49:2 vol: vol:vol). The solubilizing reagent was Ethylene diamuse tetra acetate (EDTA), disodium salt, 8% wt/vol in distilled water. The procedure was as follows: Initially the Koops and Klomp method (4) was used. This involved adding.5 ml milk to 16 x 125 mm test tubes containing 5 ml CHM,.1 ml.7 N HC1, and.1 ml EDTA solution. Samples were shaken vigorously for 30 rain and 2 ml copper reagent added, followed by shaking an additional 10 min. They were centrifuged for 10 min at 1200 x g, and 3 ml of clear supernatant were mixed with.5 ml color reagent in test tubes rendered copper free by acid washing. Absorbance was measured within 1 h at 440 nm in a 1-cm cell. After preliminary studies the following modified procedure (designated Variation li) was adopted. A.1-ml aliquot of.7 N HC1 was added to.5 ml milk in a 16 x 125 mm test tube. The mixture was shaken on a Vortex test tube mixer, and 2 ml of the copper reagent and 6 ml of solvent were added. The sample was shaken for 30 min in a rotary Eberbach shaker at 240 rpm and centrifuged for 10 min at 1940 x g in an International Model HN centrifuge. Then 3.5 ml of the solvent layer was transferred to an acid washed tube containing.1 ml of the color reagent. Color was measured after mixing at 440 nm within 1 h. Any spectrophotometer could be used to measure the color, but to speed the process, a Bausch and Lomb (Rochester, NY) Spectronic system was used. This system consists of an automatic sampler, a data processor, and spectrophotometer with a flow-thru cuvette (i.e., either a Spectronic 100 or 710). Racks containing up to 80 samples were placed in the automatic sampler which transfered aliquots to the flow-thru cuvette. Samples were analyzed at the rate of 3 to 4 per rain. Results in terms of absorbance or concentration of free fatty acids were printed on tape. BDI Method The procedure of Thomas et al. (6) was used except that the fat was weighed rather than measured volumetrically and a syringe microburet (Model SB82 Micro Metric Instruments Co., Cleveland, OH) was used for titrations. Flavor Evaluation Procedure A 6-member taste panel that had been trained in identifying lipolyzed flavor was used. The intensity of the lipolyzed flavor was scored on a 5-point scale with 0 indicating none, 1 = slight, 2 = moderate, 3 = strong, and 4 = very strong. A 24-member taste panel that had 6 wk instructions in sensory testing compared the intensity of lipolyzed flavor in nonhomogenized with homogenized milk. Four pairs of samples containing different amounts of FFA were prepared from a single lot of milk which was divided into two portions. One portion was pasteurized and homogenized in the University dairy plant. The two samples (one homogenized, one nonhomogenized) in each pair contained equal amounts of FFA as determined by the BDI method. The FFA were developed in the nonhomogenized samples by agitation in a Waring Blender for 0 to 4 min, followed by storage at 5 C for 48 h. The FFA were developed in the homogenized samples by adding from 0 to 2.5% raw milk to commercially pasteurized and homogenized samples. Samples were stored at 5 C for 24 h. After storage, the lipase in both sets of samples was inactivated by laboratory pasteurization. The four quantities of FFA were prepared by appropriate blending of samples. The panelists were asked to indicate which samples in each pair had the most intense lipolyzed flavor. They were provided with a labeled control and lipolyzed sample for reference. Milk Samples Milk samples for comparison of methods were from the Cornell University dairy plant. To produce a range of FFA concentrations, aliquots of these samples were treated in the Journal of Dairy Science Vol, 63, No, 2, 1980

3 FREE FATTY ACIDS 195 TABLE 1. Comparison of Bureau Dairy Industry (BDI) and copper soap method (CSM) results as meq FFA/liter milk. a Sample Copper soap procedures a no. BDI Original b Variation I c Variation I1 d Average SD H H H H H Average SD aall samples were tested 10 times. The H-series are homogenized samples. bthe original Koops and Klomp method (9). Cvariation I, solvent and copper reagent were added together rather than separately. dvariation 11, same as Variation I except the EDTA was omitted. same manner as described above for the taste panels. RESULTS Variations of the Koops and Klomp (4) procedure were tried to determine if the method could be simplified. One simplifying variation involved adding the copper reagent and solvent together to the HC1-EDTA-milk mixture. A second variation was based on the observation that EDTA addition was not necessary. These two modified procedures, Variations I and II, gave results closer to the BDI than the original procedure for the nonhomogenized samples. Variation II (Table 1) gave the best results for nonhomogenized milk. In this trial Variation I gave slightly better 1.00" J.75" / : t ,t m.50" O Qa.25" /' I I I I I FREE FATTy ACIDS Figure 1. Standard curve for palmitic acid. (Absorbance measured at 440 nm, palmitic acid expressed in meq/liter of milk.) 8 '; 2 / +,f +** +. / I I I I I l BDI Figure 2. Relationship between BDI and CSM for 124 nonhomogenized milk samples. BDI in meq/100 g of fat, CSM in meq/liter of milk.

4 196 SHIPE ET AL. results for homogenized milk. However, a second trial involving 19 samples indicated that Variations I and 11 have comparable results. Therefore, since Variation II is simpler, it was used in all subsequent experiments. To convert absorbance to meq FFA/liter milk, a standard curve was prepared with palmitic acid as specified by Koops and Klomp (4). A weighed quantity of palmitic acid was dissolved in the CHM solvent and made up to 250 ml in a volumetric flask. Aliquots of this stock (1.0 to 4.0 ml) were made up to 100 ml with additional CHM solvent to provide four concentrations of palmitic acid. The CHM solvent without added palmitic and the four palmitic acid solutions were used to prepare the standard curve. For the standard curves,.5 ml skim milk was used in place of whole milk, but the remainder of the procedure was the same as that specified for the regular milk samples. Data for the standard curves were based on three trials with quadruplicate determinations. The standard curve for Variation II is in Figure 1. The regression equation for this curve is y =.849x where y = absorbance at 440 nm and x = meq FFA/liter milk. Standard curves also were constructed for the original method and Variation I. The respective regression equations were: y =.785x and y =.787x The relationship between the BDI and CSM for 124 nonhomogenized milk samples is illustrated in Figure 2. The correlation coefficient for these data was.90. The corresponding correlation coefficient for 109 homogenized samples was.88 (Figure 3). Although the correlation between concentrations for homogenized milk was about the same as for nonhomogenized milk, the actual CSM meq/liter for homogenized milk were much lower. The same difference between CSM for homogenized and nonhomogenized milk also is shown in Table 1. Apparently, part of the free fatty acids in homogenized milk are bound in some way so that they are not extracted readily by the solvent. It is conceivable that protein absorbed on the surface of the fat globules in homogenized milk interferes with extraction. This theory is supported by the observation that recovery of fatty acids from homogenized milk could be increased by more prolonged shaking of the aqueous-csm mixture. The FFA recovery could be increased about 20% by increasing the shaking time from 30 to 90 min. Even though shaking 90 rain increased recovery, it did not alter the correlation between CSM and FFA content as determined by the BDI method. Therefore, with the appropriate regression equation one can predict the FFA content as accurately from the 30 as from the 90-rain data. On the average, 30-min shaking gave CSM meq/liter for homogenized milk that were 50% of the corresponding concentrations for nonhomogenized. The relationship between CSM and BDI are given by the following regression equations: For nonhomogenized milk CSM =.398 (BDI) +.002; for homogenized milk CSM =.207(BDI) The relationship between flavor evaluation and chemical analysis of 35 samples each of nonhomogenized and homogenized milk are in Figures 4 and 5. Flavor intensities are averages of scores by six panelists. Both the CSM and BDI are in meq FFAJliter milk. The relationship between the two chemical methods and flavor was about the same for nonhomogenized milk, but the CSM were considerably less for homogenized milk. The differences between CSM for nonhomogenized and homogenized milk have not been noted previously. Presumably concentrations by CSM have not been determined for homogenized milk. The regression equations relating the flavor score (FS) and BDI and CSM for nonhomogenized milk are, resp ectively: FS =.97 (BDI)-.43 and FS = 2.19 (CSM) -.27 The corresponding regression equations for homogenized milk are." FS =.82 (BDI) -.30 and FS = 3.89 (CSM)-.37 By these equations one can predict flavor score from BDI or CSM or vice versa. For example, the predicted BDI and CSM for a sample of nonhomogenized milk with a flavor score of 1.0 would be 1.47 and.58. The corresponding BDI and CSM for homogenized milk would be 1.59 and.35. These estimates can serve as guidelines for predicting whether a milk sample falls in the consumer complaint category. These specific regression equations are based on the sensory evaluations of six panelists, and, therefore, the accuracy of the prediction depends on the

5 FREE FATTY ACIDS 197 1, , >r I I I I I BDI Figure 3. Relationship between BDI and CSM for 109 homogenized milk samples. BDI in meq/100 g of fat, CSM in meq/liter of milk. similarity between panelists and consumers responses to this off-flavor. To determine if nonhomogenized and homogenized milk with the same concentrations by BDI would have the same sensory impact, four pairs of samples were tested. Each pair consisted of a nonhomogenized and a homogenized sample with equivalent BDI. One pair had subthreshold concentrations of FFA whereas the other three pairs had superthreshold amounts. The flavor intensity (Table 2) in the superthreshold samples was significantly higher for nonhomogenized samples. No difference was significant between samples in the pair with the subthreshold FFA. The nonhomogenized and homogenized samples themselves did not have significantly different flavors. Some of the FFA in homogenized milk may be bound or entrapped in a way that they do not contribute to lipolyzed flavor. The difference in sensory impact of FFA in nonhomogenized and homogenized was also in the previous experiment, i.e., a BDI of 1.47 gave a flavor score of 1.0 for nonhomogenized milk whereas a higher BDI (1.59) was required to elicit a flavor score of 1.0 for the homogenized milk. In other words, the flavor threshold for free fatty acids in homogenized milk was slightly higher than the threshold in nonhomogenized milk. CONCLUSION A modified copper soap method was developed that provides a sensitive rapid method for determining the FFA content of milks and estimating the intensity of lipolyzed flavor. The correlation between CSM meq/liter and flavor scores is slightly larger than the correlation between BDI meq/liter and flavor scores. Only a small sample (.5 ml) of milk is needed for this colorimetric test, so it would be feasible to take aliquots from a universal sample. The method can be made semi-automatic by an automatic sampler and spectrophotometer with a flow through cell to test up to 200 samples per hour. It would take approximately 2 days to run the equivalent number by the BDI method. 30' BOI O---- CSM 2,0. o ~ I J ,,s g > 1.0 o *o~'~f a a + m oj" is m + o ~," + o " o o~+o~o ", I I t I I FREE FATTY ACIDS Figure 4. Relationship between flavor scores and BDI and CSM in meqlliter of nonhomogenized milk. Flavor scores: 0 = no lipolyzed flavor to 4 = very strong lipolyzed flavor. Correlation coefficients: Flavor vs. BDI =.67. Flavor vs. CSM =.83. s ~ BDI // --o--- CSM / 2.5 o +it,, /s e t + z/i 2.0 a u / + + ~3 /" e Z o csmad _z 1.5 = Y-- +/ + 11 y+.5 ~ 1 +,-----~o I I I! , FREE FA'FFY ACIDS Figure 5. Relationship between flavor scores and BD1 and CSM in meq/liter of homogenized milk. Flavor scores: 0 = no lipolyzed flavor to 4 = very strong lipolyzed flavor. Correlation coefficients: Flavor vs. BDI =.78. Flavor vs CSM =.82.

6 198 SHIPE ET AL. TABLE 2. A c6mparison of the relative intensity of the lipolyzed flavor in nonhomogenized and homogenized milk. BDI Flavor paired comparisons a No. of Non- Nonpair homogenized Homogenized homogenized Homogenized (meq/liter) anumber of 24 panelists who thought the particular sample in each pair had the most intense lipolyzed flavor. Results for second and third pair were significant at 1% and results for fourth pair were significant at 5%. The availability of FFA is lower in homogenized than in nonhomogenized milk, since FFA is recovered by extraction with a chloroform-heptane-methanol solvent and the FFA in homogenized milk have a lower sensory impact. Presumably some of the FFA in homogenized milk is bound or entrapped. ACKNOWLEDGMENT The authors are grateful for the support of this research by Dairy Research, Inc. REFERENCES 1 Driessen, F. M., A. Jellema, F. J. P. van Luin, J. Stadhouders, and G. J. M. Walhers The estimation of the fat acidity in raw milk. An adaptation of the BDI method, suitable for routine assays. Netherlands Milk Dairy J. 31:40. 2 International Dairy Federation Symposium Proc. Lipolysis IDF Annu. Bull. Doc. No International Dairy Federation Symposium Proc. Lipolysis IDF Annu. Bull. Doc. No Koops, J., and H. Klomp Rapid colorimetric determination of free fatty acids (lipolysis) in milk by the copper soap method. Netherlands Milk Dairy J. 31:56. 5 Salih, A. M. A., M. Anderson, and B. Tuckley The determination of short- and long-chain free fatty acids in milk. J. Dairy Res. 44: Thomas, E. L., A. J. Nielson, and J. C. Olson, Jr tlydrolytic rancidity-a simplified method for estimating the extent of its development. Am. Milk Rev. 77(1):50.