STUDIES ON THE REFRACTIVE INDEX OF MILK BY K. S. RANGAPPA

Transcription

1 STUDES ON THE REFRACTVE NDEX OF MLK Part. Observations on Genuine Samples BY K. S. RANGAPPA (Department of Biochemistry, ndian nstitute of Science, Bangalore) Received November 8, 146 (Communicated by Mr. M. Sreenivasaya, B.A., F...S(~, F.A.SC.) VAROUS official tests, physical and chemical, have been devised for the determination of added water in milk. The principal ones are the well-known presumptive standards for fat and solids-not-fat of milk, the cryoscopic test (Beckmann, 184) and the refractive index of the sour (Leach and Lythgoe, 103), acetic (Leach and Lythgoe, 104) and copper sulphate serum (110) of milk, prepared under standard conditions. The preparation of the milk-serum has been resorted to owing to the opacity of milk when viewed through the immersion refractometer. The time-consuming chemical procedure of this method has brought about the popularity of the cryoscopic test, although the latter needs a considerable amount of skilled technique in its measurement. n this paper standardisation of a simple and quick method of determining the refractive index of milk, in contradistinction to that of milkserum, with the Abb6 Refractometer has been attempted. The cow and buffalo being equally common milch animals in ndia, the range of variation of R.. for each type of milk has been studied. Further, the relationship between the density and the R.. (i.e., Refractive constant, K) have also been calculated for a large number of samples. The data from all these determinations have been statistically analysed. EXPERMENTAL The R.. determinations with the Abb6 Refractometer were made, for the first few samples of milk, on whole milk. But it was found that the presence of fat in milk necessitated a very quick adjustment of the total line of reflection in the refractometer, as otherwise the line tended to blur and flow with delay. Defatted milk, therefore, gave a sharper and more permanent line of demarcation without affecting the measure of the R.., thus permitting a greater degree of accuracy and freedom in the determination. Table illustrates this fact, 86

2 Studies on the Refractive ndex o~ Milk-- 87 TABL~ R.L (40 ~ C.) of whole and skimmed milk C ow Buffalo Whole Skimmed Whole Skimmed After a few trials the following method was finally adopted. 10 c.c. of sample is pipetted into a Gerber butyrometer and centrifuged for 5 minutes when almost all the fat forms a plug on top leaving skimmed milk at bottom. A few c.c. of the latter is carefully collected in a test-tube without disturbing the disposition of the two layers, and the R.. determined on the skim milk. The readings were taken when the temperature of the instrument was steady at 40 ~ C., and repeated with fresh drops until the difference between consecutive readings did not exceed Samples of milk for examination were mostly obtained from the Military Dairy Farm, Hebbal, about 3 miles from the nstitute. About 1 to 2" 5 hours lapsed between milking and the analysis of the samples, the time lapse causing no detectable difference in the R.. The farm has about 400 milking animals, the cows belonging to Scindhi, Tharparkar, Ongole, Cross (Ayrshire ndian) and C.P. breeds, and the buffaloes to Delhi, Nagpur and Neeli breeds. Samples were collected both from individual animals as well as pooled milk, both chosen, as far as possible, at random from the herd. The bulk samples were collected from cans containing the yield of animals. About half the number of cows were suckled by calves before milking, but the buffaloes were all milked without this practice. The animals were in all stages of lactation from 15 days to about 8 months after parturition. Samples were collected in the morning ( to 11 in summer and 7 to 8 in the rainy season) by the laboratory attendant, in whose presence the animals were milked, and brought to the laboratory in sealed cans. The acidity of the samples lay between 0.0 and 0"11 per cent. Thus the data analysed in this paper cover a period of about 8 months, from March to June (dry summer) and July to end of September (rainy season) when plenty of green pasture is available.

3 88 K.S. Rangappa The animals in the dairy farm being managed under standard conditions, it was thought advisable to test random samples from animals under widely differing conditions of management. The City of Bangalore is largely supplied by producers who own hardly a few animals each. Animals in the City are stall-fed, while those in nearby villages go out to pasture. While rich owners feed their cattle with concentrates like cottonseed, groundnut cake, etc., poorer ones supply mostly hay and grass, and perhaps a little rice bran. Thus, about 30 samples each of cow and buffalo milk were collected at random from all classes of owners for examination. The composition of a large number of samples eollected were also estimated. The density (Celsius lactometer, 20 ~ C.) and fat content (Gerber process) were used for computing the total solids (which closely agreed with the values of actual estimation) with the following formula for ndian milk: T.S.= 0.25 (D-1000) F The Refractive constant K, has been calculated according to the Lorenz and Lorentz formula: n ~- 1 nt~x 1/d----K, where n = R.. (40 ~ C.), d = density (20 ~ C.) of milk. More than 200 samples each of cow and buffalo milk have thus been analysed. The frequency distribution of R.. and K are represented in Figures 1 and 2. The relationship between S.N.F. and R.. and between S.N.F. and K are illustrated in Figures 3 and 4. Effect of De.fatting Milk on R.L-- DSCUSSON OF DATA The figures in Table show that removal of fat by centrifuging causes no difference in R.. of milk. This is to be expected as the fat is only a suspension in milk and forms no part of the solutes which contribute to the R.. Limits of R.L and K. of Milk.- While it is realised that a much larger number of analyses are to be completed before studying statistically the nature of the frequency curve, it is felt that the data collected so far are enough to warrant the fixing, at

4 S t u d i e s on the-refractive n d e x o f M i l k - - ~ least tentatively, of the extreme limits of variation of R.. and K for genuine cow and buffalo milk. 2O /.3440 n +o- 6o Jo 1.3~80 ReProo~,e l,,d.x(+o'c) 25 2O g,,o,d7 o.aoso 65 ~o 7s o.aoeo RePrao~;ue oons~an~ ~...o aul~ s=rnpl~a only, : b ~Satk'+lndtvJdual~a~ples F/S" f~equaney D/a6rLiu~/on o~ Rel't.di~/e n d e x ~ n d R"e#%acd/ve Co~,a~oa~ op Cow/wLk. Of all the samples examined about per cent. were made up of individual samples and the rest of bulk samples. Among these about half were analysed in the dry months, March to June, and the rest in the months July to October when lush vegetation was available for cattle. t is seen from Figs. 12 that the limits of R.. of individual samples of cow milk normally extend from 1 "344 to , buffalo milk from to The limits are, however, considerably narrowed down with bulk samples due to the ironing out of extremes of individuality by pooling milk. For cow milk these are 1" 34 to , and for buffalo milk 1" 3462

5 0 K.S. Rangappa to 1" t may here be repeated that these figures are inclusive of variations due to season, individuality and type of management (farm or villagebred cattle) and composition (cow milk, fat , total solids , S.N.F and ash "75 per cent.; buffalo milk, fat #.5 ~o L t.5~55 6"o /o ~o "o s45oo t]ept, ae~.ioe. ndex 8~ RO 0: ReP...ao~.;ve Cons~nn~ ~ ,/k Sotnp/es,, B-lk + ndlvldual $~mples ~',',e~uenw,q D/.~t,,',/~u[;ion op Rewawaed~ue ndex ~nd ~e/',,ac~/ve Cons/a.~ o~ 8..fPalo M/llc , total solids , S.N.F and ash per cent.). Within these limits the most frequently distributed value (the mode) is for cow and 1" 3480 for buffalo milk. But it will be noticed that there are secondary maxima in each of the frequency diagrams. Analysis of the data (which are not given in detail due to shortage of space) in the light of seasonal variations indicate that the R.. in the dry months has

6 Studies on the Re[motive ndex o1 c Milk-- 1 an average and a mode distinctly lower than in the months when a plentiful supply of green herbage is available for consumption. Thus the mode is 1" 34 for cow and 1" 3470 for buffalo milk in summer. Refractive Constant.--The frequency diagram of this constant brings out the fact that the range of variation of the constant is not only considerably narrower than that of R.. but is much less subject to changes due to external factors. For cow milk, K normally ranges from to , and for buffalo milk from to 0"2088. The modes, 0"2070 for cow and 0" 2080 for buffalo milk are also quite distinct from each other. Further, the range of K, unlike that of R.., is practically the same for both individual and bulk samples, which is an added advantage. Relationship between R.L and K in cow and buffalo milk.- t is noteworthy that although a certain degree of overlapping occurs in the ranges of R.. of the two types of milk, K is characteristically different for each milk in the overlapping range. Table brings out this fact clearly. TABLE Relationship between R.L and K in cow and buffalo milk Cow Buffalo Range of R.. (40" C.) K Range of R.. (40o C.) K O ~" r O With the help of K and the corresponding R.. it is thus possible to characterise the type of milk under examination with considerable certainty. Relationship between S.N.F. and R.L and K of Milk.- Fig. 3 illustrates the approximate relationship between R.. and S.N.F. of milk. Gross differences in S.N.F. are reflected, more or less, in corresponding changes of R.. Elsdon and Stubbs (12) observed a similar uz SO

7 2 K, S. Rangappa.S St.O aappalo mil~ 10.4 "~ 3"4f : * e~ 8"~.i q~ 8~0 u ":,,t,l" ;,.i:?.. ~ 7.8 ot.~. a.~ o " 7.0/ /..~'440SO 60!!! 1.3'~60?0/.,Y4~ ~./. 0~o~ #~'q.j. R,d~ R.Z. (',fo'c) ~o~,o~, s,!/ds.,,o~.p=t ~ t?epe,ne~l"~e s op M/lh. A.PPa/o milk Q i %1 eoee ~0 N ~ SJ o ~L" t~ me8 B 7.O 8.0 e oee Qo ee Q~ ee oe ~Q 14 e~eee,0 $o l l d.s. no~ - Pa ~!.S ~ 0"2~ S!o $ollds F/8. 4t m e oe~ eeee O.tOSS "~ Rel~al~/ons,~ 1~e~oee~ 8.N.F'. a n d 'to-o ' - n o d - Pa& o~o K /n Coco a n d B - P~=lo m Hh, relationship between S.N.F. of milk and R.. of milk-serum. But it is of advantage that no such relationship (Fig. 4) exists between S.N.F. and K. The figure shows that in spite of abnormal changes in the S.N.F. content of milk, K remains within normal limits.

8 S/udies on lhe Refractive index o f Milk--~ 3 TABLE ll Relationship between certain constants of milk low in S.N.F. Density (20oC.) S.N.F. % R.. (40 ~ C.) K Cow Milk Buffalo Milk ~ t can be seen from the above figures that samples of milk low in S.N.F. (with less than 8.5~o) are usually associated with low R.. and values of K considerably above the minimum for normal milk. t is perhaps possible that such a correspondence of the two constants is, as a rule, characteristic of genuine samples with low S.N.F. content. n the light of the above data it is concluded that samples of cow milk with R.. less than and K less than and buffalo milk with R.. less than and K less than 0"2076 can be considered to be adulterated. SUMMARY A simple method of determining the refractive index of milk with the Abb~ refractometer has been devised. About 10 c.c. of sample in a Gerber butyrometer is centrifuged for 5 minutes in an ordinary milk centrifuge. A few c.c. of the defatted milk is carefully collected without disturbing the fat layer and tested for R.. t takes less than 30 minutes to test the R.. of a dozen samples in this fashion; and these values represent, unlike those of milk-sera, the true refractive index of milk. From the density and R.. the refractive constant, K, has been calculated.

9 4 K.S. Rangappa The R.. and K of more than 200 samples each of cow and buffalo milk have been tested over a period of 8 months. The limits of R.. for cow milk lie between 1" 344 and , the mode being , and for buffalo milk between 1 "3461 and 1.30, the mode being K is distinct and lies within much narrower limits for each type of milk-cow milk , and buffalo milk These limits, unlike those of R.., are independent of the solids-not-fat content of milk. From the available data it is concluded that samples of cow milk with R..,~ and K <: , and buffalo milk with R.. < and K < can be considered to be adulterated. ACKNOWLEDGMENT am indebted to Mrs. P. Rangappa, B.Sc., for the statistical analysis of the data. My thanks are due to Mr. B. N. Banerjee and Prof. V. Subrahmanyan for their kind interest. REFERENCES Bcckmann.. Milch. Zig., 184, 23, 702 (Original not seen). Elsdon and Stubbs.. Analyst, 12, 5, 14. Leach and Lythgor.. Ztsch. f Offentliche Chem.,, 173 (Original not seen)... J. Am. Chem. Soc., 26, U.S. Dept. Agr. Bur. Chem. Bull., 1t0, pl'luted at The ~n~ajowe ~'eess. ~4y.~o~.s Road K~-~,~lo,-~ Cuoy ay (3. ~nlv~sm R~o. ~,J;)emnt~ndea~, sn~ P~]~hs~*~l oy T'n, [~4,aa AQLA~,ny e! S0 enqos. Bs.age.loro.