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Inhibition of the Oxidized Flavor of Milk with Chelating Compounds1
I N H I B I T I O N OF T H E O X I D I Z E D F L A V O R OF M I L K WITI~ CHELATING COMPOUNDS 1 L. R. ARRINGTON Department of A~dmal H~lsbandry and A~'utrition
AND W. A. K R I E N K E
Department of Dairy Science Florida Agrie~dt.nral Experiment Station, Gainesville
The relationship of the oxidized flavor of milk to the presence of copper is well established, and considerable data are available t h a t demonstrate the role of copper in its development. The experimental work relating to oxidized flavor was reviewed in 1948 b y Greenbank (6). References cited in this review r e p o r t the use of certain materials, which, when added to milk, act to r e t a r d or inhibit development of the oxidized flavor. Among the antioxidants used were: a dried culture of bacteria, a pancreatic enzyme, concentrated or dried milk, hydroquinone, ascorbic acid, and oat flour. I n 1944 L n n d b e r g (7) first r e p o r t e d the antioxidant p r o p e r t y of nordihydroguaiaretic acid ( N D G A ) in fats, and in 1948 Stull et al. (8) observed that this material retarded the development of oxidized flavor in milk and other d a i r y products. Additional data on the use of certain hydrolytic enzymes have been reported recently b y F o r s t e r et al. (3, 4), and an inhibitory effect of manganese was reported b y G a r r e t t (5). The research reported in this p a p e r deals with the inhibition of the oxidized flavor in fresh milk by certain chelating compounds which have the p r o p e r t y of complexing or t y i n g up metal ions when in solution. EXPERIMENTAL
Pasteurized homogenized milk processed in commercial p l a n t equipment was used for the experimental samples. No effort was made to isolate milk f r o m individual cows nor f r o m different sources which might be highly susceptible to development of oxidized flavor. The flavor defect was induced b y additions of copper in the f o r m of C u S Q • 5H~O in aqueous solution. The chelating agents used were three of the salts of ethylene diamine t e t r a acetic acid ( E D T A ) . These were: the disodium salt, the disodium calcium salt and the tetrasodium salt. Sodium diethyl dithiocarbamate was used in a few trials. Although not generally considered to be a chelating agent, this compound was included because of its p r o p e r t y of precipitating copper f r o m solutions. These materials were dissolved in Pyrex-distilled water a n d p r e p a r e d in concentrations such t h a t the additions did not appreciably dilute t h e milk. I n the p r e p a r a t i o n of some of the samples, copper was added first, followed by addition of the chelating agent, and to others it was added after the chelate Received for publication January 11, 1954. Florida Agricultural Experiment Station Journal Series, :No. 249. 819
TAI~LE 1
Effect of varying levels of chelating agents upon the copper induced oxidized flavor of mille Oxidized flavor a f t e r storage f o r 48-96 h o u r s "
M/1 of
Added Chelating a g e n t
copper
Nolle
1.5 × 10 -n
chelating a g e n t
6.5 × 10 -'~
1.25 × 10 -4
2.5 × 10 -~
5.0 X 10 -~
(3) + + (3) + + +
(5) 0 (4) + +
(2) 0 (6) 0
10 -'~
1.5 X 10 -s
(5) 0
(1) 0
(M/l) 5.0 x 10 -~ 8.0 × 10 ~ 10 -~
+++ ++-4-+ ++++
(3) + +
Disodium calcium salt of E D T A
5.0 X 10 -'~ 8.0 x 10 ~ 10 -~
+++ ++++ ++++
(1) + +
T e t r a s o d i u m salt of E D T A
5.0 x 1o -o 8.0 X 10 -~ 10 -~
+++ ++++ ++++
5.0 × 10 -5 8.0 X 10 -~
+++ ++++
1 0 -4
q-q-@@
Disodium salt of E D T A
Sodium diethyl dithioearbamate
(1) + (1) + + +
(!0) 0 (1) + (1) 0
(7) 0
(1) 0
(8) 0
(7) 0
(4) 0
(5)
(1) o
(4) 0
(1) 0
o
(5) 0 (1)b0 (2) 0
(1) 0 (i) 0
(2) 0
(I)
0
+ + + + badly oxidized, + + + oxidized, + + slightly oxidized, + questionable to slightly oxidized, 0 not oxidized b F i g u r e s in parentheses r e p r e s e n t n u m b e r s of samples.
INHIBITION
OF OXIDIZED
t~LAVOR WITH
CHELATING
COMPOUNDS
821
had been added. A f t e r storage at 36 to 40 ° F. for 48 to 96 hours, the samples were examined organolepticaUy by three experienced judges. No a t t e m p t was mgde to score the samples, but the judges reported their findings with r e g a r d to the presence or absence of the flavor defect and its relative intensity. An a r b i t r a r y rating as follows was established: + + + + badly oxidized, + + + oxidized, + + slightly oxidized, + questionable to v e r y slightly oxidized, 0 not oxidized. I t was anticipated that the milk would "~ary in the degree of susceptibility to induction of oxidized flavor; therefore, each series of samples was p r e p a r e d to include control samples to which copper only was added. The chelate-treated samples were then rated with reference to the controls in each trial. I n trials designed to observe the effect of pasteurization, both t h e copper and the chelates were added to samples of raw milk and the samples were l a b o r a t o r y pasteurized at 143 ° F. for 30 minutes. RESULTS
D a t a representing the inhibitory effect of the different chelates at the various concentrations are summarized in Table 1. The m a j o r portion of the s t u d y was conducted with levels of chelating agents at 2.5 × ] 0 -+ to 1.0 × 10 -3 M per liter, but lower concentrations were used in some trials in order to observe effects when the amount was insufficient for complete protection. The three salts of E D T A were a p p r o x i m a t e l y equally effective in their inhibitory action. The quantity required for complete protection was of the order of five tinies the amount of added copper on a molar basis. Samples to which 5.0 × 10- ~ M per liter of copper were added required 2.5 × 10:4 M per liter of the chelate. I n other samples, containing a similar concentration of copper, 1.25 × 10 -+ M per liter of the chelate appreciably reduced the susceptibility but did not give complete protection against development of the defect. The reduction in intensity was proportional to the amount of the chelate added. Although the results reported in Table ] represent examinations made at 48 to 96 hours, the m a j o r i t y of the samples did not change in flavor r a t i n g when storage was extended to 120 or 144 hours. I n a few of the samples which contained small quantities of the chelates, and in which the flavor defect developed slightly, there was a slight increase in intensity of the flavor after prolonged storage. Sodium diethyl dithiocarbamate a p p e a r e d to be more effective than the salts of E D T A since lower concentrations, on a molar basis, p r e v e n t e d development of the oxidized flavor. This compound may, however, i m p a r t a slight off-flavor to the milk. Pasteurization did not alter the effectiveness of the chelates in preventing development of the oxidized flavor (Table 2). W h e n samples of r a w milk were treated with copper and with a chelating agent, and the milk subsequently pasteurized, no change in the relative inhibitory action was observed.
TABLE 2
The effect of pasteurization upon the action of chelates in preventing oxidixed flavor O xi di z e d flavor r a t i n g a
M/1 of c h e l a t i n g a g e n t Chelating agent
Cu a d d e d
D i s o d i u m s a l t of E D T A
None
6.5 X 10 -~
1.25 X 10 -4
2.5 X 10 -4
5 . 0 × 1 0 -5 8 . 0 × 1 0 -~ 10 -4
+++ ++++ ++++
++
++ +++
0 ++
D i s o d i u m calcium s a l t of E D T A
5 . 0 X l O -5 8 . 0 × 1 0 -~ 10 ~
+++ ++++ ++++
++
T e t r a s o d i u m s a l t of E D T A
5 . 0 X l O -s 10 -~
+++ ++++
5.0 × 10 .4
10 -s
0 0
0
(U/l)
+ ++
0 +
" + + + + b a d l y oxidized, + + + oxidized, + + s l i g h t l y oxidized, + q u e s t i o n a b l e to s l i g h t l y oxidized, 0 n o t o x i d i z e d
INHIBITION OF OXIDIZED FLAVOR ~ r I T n CHELATING COMPOUNDS
823
TABLE 3
The effect of adding chelates prior to the addition of copper upon the action of the chelates in preventing oxidized flavor O x i d i z e d flavor r a t i n g *
M/1 of c h e l a t i n g a g e n t Chelating agent
None
2.5 × 10 -*
5.0 X 10 .4
10 -a
10 -~
++++
0
0
0
5.0 × 10 -'~ 10 -4
+++ ++++
0 0
0
0
5.0 × 10 -°"
+++
0
10 -4
++++
0
0
Cu a d d e d
(Mfl) D i s o d i u m s a l t of E D T A D i s o d i u m c a l c i u m s a l t of E D T A T e t r a s o d i u m s a l t of E D T A
+ + + + b a d l y oxidized, + + + oxidized, + + s l i g h t l y oxidized, + q u e s t i o n a b l e t o s l i g h t l y oxidized, 0 n ot o xidized
The data ill Table 3 were obtained oil samples to which the chelating materials were added p r i o r to the additions of copper. The chelates were equally effective when added before or a f t e r the addition of copper. The results demonstrate that the order of addition is without effect. DISCUSSION
The results of this experiment demonstrate the inhibitory action of certain chelates against the copper-induced oxidized flavor of milk. The inhibition is a t t r i b u t e d to complexing or t y i n g up of the copper ions and p r o b a b l y other ions, rendering them ineffective as catalysts or as activators for certain enzyme systems. B y using the compounds in the m a n n e r described, it appears t h a t the antioxidant p r o p e r t y can be a t t r i b u t e d only to the mechanism of chelation of the copper. A n y consideration of the use of these compounds in m a r k e t milk or other d a i r y products must be based upon information to clearly establish their freedom from a n y h a r m f u l effects. Although no long-term feeding experiments with various species have been reported, there a p p e a r s to be no evidence f r o m other studies that E D T A is h a r m f u l to l a b o r a t o r y animals when consumed orally in quantities m a n y times greater t h a n would be present in milk if added to prevent the oxidized flavor. Child (2) fed rats up to 5% of the sodium salt of E D T A for 4 weeks, and the animals failed to develop abnormalities. Unpublished results (1) f r o m an experiment accompanying this s t u d y showed no detrimental effects when the disodium salt of E D T A was fed to rats. Weanling rats in this experiment were fed 300 and 900 p.p.m, of the salt in a mineralized milk sucrose diet. Over a 10-week period, no adverse effects on the r a t s were evident. SUMMARY
Samples of normal milk were treated with copper to induce oxidized flavor and at the same time were treated with certain chelating compounds. The chelates, which were the disodium salt, the disodium calcium salt, and the tetra-
824
L. R. ARRINGTON AND W. A. KRIENKE
sodium salt of ethylenediamine
tetraacetic
acid (EDTA),
were effective in pre-
venting development of the oxidized flavor. Quantities of the chelates which w e r e five t i m e s t h e a m o u n t o f a d d e d c o p p e r o n a m o l a r b a s i s p r o v i d e d c o m p l e t e protection, and smaller quantities of the chelates gave some inhibitory action. The chelates were equally effective when added either before or after pasteurization or when added prior to or after the addition of copper. ]~EFERENCES (1) AR~I~'GTO~, L. R., AND DAVIS, G. K. Unpublished data. Florida Agricultural Experiment: Station. 1953. (2) CHILD, G. P. The Inhibition of Hematopoietic Action of Cobalt by Ethylenedlamine Tetra Acetic Acid ( E D T A ) . Science, 114: 466. 1951. (3) FORSTER, T. L., -~D SO)~ER, H. H. Manganese, Trypsin, Milk Proteins and the Susceptibility of Milk to Oxidized Flavor Development. J. Dairy Sci., 34: 992. 1951. (4) FORSTER,T. L., JENSEN, C., AND PLATtt, EMILY. Influence of Protein Hydrolysis on t h e Susceptibility of Milk to Oxidized Flavor Development. J. Dairy Sci., 36: 98. 1953. (5) GARRETT, O. F. Some Factors Affecting the Stability of Certain Milk Properties. W. Interrelation of Certain Metals and Metallic Ions and the Development of Oxidized Flavor in Milk. J. Dairy Sci., 24: 103. 1941. (6) GREENBANK, G. ]:~. The Oxidized Flavor in Milk and Dairy Products: A Review. J. Dairy Sci., 31: 913. 1948. (7) LL~NDBEr~, W. O., ]=IALVO~SON, H. O., .~ND BI~RR, G. O. The Antioxidant Properties o f Nordihydroguaiaretic Acid. Oil and Soap, 21: 33. 1944. (8) STULL, J. W., HERREID, E. O., A~D TR.~C¥, P. H. A Study of the Use of the A n t i o x i d a n t Nordihydroguaiaretic Acid in Dairy Products. I. Its Antioxygenic Properties i n Milk. J. Dairy Sci., 31: 449. 1948.
AND W. A. K R I E N K E
Department of Dairy Science Florida Agrie~dt.nral Experiment Station, Gainesville
The relationship of the oxidized flavor of milk to the presence of copper is well established, and considerable data are available t h a t demonstrate the role of copper in its development. The experimental work relating to oxidized flavor was reviewed in 1948 b y Greenbank (6). References cited in this review r e p o r t the use of certain materials, which, when added to milk, act to r e t a r d or inhibit development of the oxidized flavor. Among the antioxidants used were: a dried culture of bacteria, a pancreatic enzyme, concentrated or dried milk, hydroquinone, ascorbic acid, and oat flour. I n 1944 L n n d b e r g (7) first r e p o r t e d the antioxidant p r o p e r t y of nordihydroguaiaretic acid ( N D G A ) in fats, and in 1948 Stull et al. (8) observed that this material retarded the development of oxidized flavor in milk and other d a i r y products. Additional data on the use of certain hydrolytic enzymes have been reported recently b y F o r s t e r et al. (3, 4), and an inhibitory effect of manganese was reported b y G a r r e t t (5). The research reported in this p a p e r deals with the inhibition of the oxidized flavor in fresh milk by certain chelating compounds which have the p r o p e r t y of complexing or t y i n g up metal ions when in solution. EXPERIMENTAL
Pasteurized homogenized milk processed in commercial p l a n t equipment was used for the experimental samples. No effort was made to isolate milk f r o m individual cows nor f r o m different sources which might be highly susceptible to development of oxidized flavor. The flavor defect was induced b y additions of copper in the f o r m of C u S Q • 5H~O in aqueous solution. The chelating agents used were three of the salts of ethylene diamine t e t r a acetic acid ( E D T A ) . These were: the disodium salt, the disodium calcium salt and the tetrasodium salt. Sodium diethyl dithiocarbamate was used in a few trials. Although not generally considered to be a chelating agent, this compound was included because of its p r o p e r t y of precipitating copper f r o m solutions. These materials were dissolved in Pyrex-distilled water a n d p r e p a r e d in concentrations such t h a t the additions did not appreciably dilute t h e milk. I n the p r e p a r a t i o n of some of the samples, copper was added first, followed by addition of the chelating agent, and to others it was added after the chelate Received for publication January 11, 1954. Florida Agricultural Experiment Station Journal Series, :No. 249. 819
TAI~LE 1
Effect of varying levels of chelating agents upon the copper induced oxidized flavor of mille Oxidized flavor a f t e r storage f o r 48-96 h o u r s "
M/1 of
Added Chelating a g e n t
copper
Nolle
1.5 × 10 -n
chelating a g e n t
6.5 × 10 -'~
1.25 × 10 -4
2.5 × 10 -~
5.0 X 10 -~
(3) + + (3) + + +
(5) 0 (4) + +
(2) 0 (6) 0
10 -'~
1.5 X 10 -s
(5) 0
(1) 0
(M/l) 5.0 x 10 -~ 8.0 × 10 ~ 10 -~
+++ ++-4-+ ++++
(3) + +
Disodium calcium salt of E D T A
5.0 X 10 -'~ 8.0 x 10 ~ 10 -~
+++ ++++ ++++
(1) + +
T e t r a s o d i u m salt of E D T A
5.0 x 1o -o 8.0 X 10 -~ 10 -~
+++ ++++ ++++
5.0 × 10 -5 8.0 X 10 -~
+++ ++++
1 0 -4
q-q-@@
Disodium salt of E D T A
Sodium diethyl dithioearbamate
(1) + (1) + + +
(!0) 0 (1) + (1) 0
(7) 0
(1) 0
(8) 0
(7) 0
(4) 0
(5)
(1) o
(4) 0
(1) 0
o
(5) 0 (1)b0 (2) 0
(1) 0 (i) 0
(2) 0
(I)
0
+ + + + badly oxidized, + + + oxidized, + + slightly oxidized, + questionable to slightly oxidized, 0 not oxidized b F i g u r e s in parentheses r e p r e s e n t n u m b e r s of samples.
INHIBITION
OF OXIDIZED
t~LAVOR WITH
CHELATING
COMPOUNDS
821
had been added. A f t e r storage at 36 to 40 ° F. for 48 to 96 hours, the samples were examined organolepticaUy by three experienced judges. No a t t e m p t was mgde to score the samples, but the judges reported their findings with r e g a r d to the presence or absence of the flavor defect and its relative intensity. An a r b i t r a r y rating as follows was established: + + + + badly oxidized, + + + oxidized, + + slightly oxidized, + questionable to v e r y slightly oxidized, 0 not oxidized. I t was anticipated that the milk would "~ary in the degree of susceptibility to induction of oxidized flavor; therefore, each series of samples was p r e p a r e d to include control samples to which copper only was added. The chelate-treated samples were then rated with reference to the controls in each trial. I n trials designed to observe the effect of pasteurization, both t h e copper and the chelates were added to samples of raw milk and the samples were l a b o r a t o r y pasteurized at 143 ° F. for 30 minutes. RESULTS
D a t a representing the inhibitory effect of the different chelates at the various concentrations are summarized in Table 1. The m a j o r portion of the s t u d y was conducted with levels of chelating agents at 2.5 × ] 0 -+ to 1.0 × 10 -3 M per liter, but lower concentrations were used in some trials in order to observe effects when the amount was insufficient for complete protection. The three salts of E D T A were a p p r o x i m a t e l y equally effective in their inhibitory action. The quantity required for complete protection was of the order of five tinies the amount of added copper on a molar basis. Samples to which 5.0 × 10- ~ M per liter of copper were added required 2.5 × 10:4 M per liter of the chelate. I n other samples, containing a similar concentration of copper, 1.25 × 10 -+ M per liter of the chelate appreciably reduced the susceptibility but did not give complete protection against development of the defect. The reduction in intensity was proportional to the amount of the chelate added. Although the results reported in Table ] represent examinations made at 48 to 96 hours, the m a j o r i t y of the samples did not change in flavor r a t i n g when storage was extended to 120 or 144 hours. I n a few of the samples which contained small quantities of the chelates, and in which the flavor defect developed slightly, there was a slight increase in intensity of the flavor after prolonged storage. Sodium diethyl dithiocarbamate a p p e a r e d to be more effective than the salts of E D T A since lower concentrations, on a molar basis, p r e v e n t e d development of the oxidized flavor. This compound may, however, i m p a r t a slight off-flavor to the milk. Pasteurization did not alter the effectiveness of the chelates in preventing development of the oxidized flavor (Table 2). W h e n samples of r a w milk were treated with copper and with a chelating agent, and the milk subsequently pasteurized, no change in the relative inhibitory action was observed.
TABLE 2
The effect of pasteurization upon the action of chelates in preventing oxidixed flavor O xi di z e d flavor r a t i n g a
M/1 of c h e l a t i n g a g e n t Chelating agent
Cu a d d e d
D i s o d i u m s a l t of E D T A
None
6.5 X 10 -~
1.25 X 10 -4
2.5 X 10 -4
5 . 0 × 1 0 -5 8 . 0 × 1 0 -~ 10 -4
+++ ++++ ++++
++
++ +++
0 ++
D i s o d i u m calcium s a l t of E D T A
5 . 0 X l O -5 8 . 0 × 1 0 -~ 10 ~
+++ ++++ ++++
++
T e t r a s o d i u m s a l t of E D T A
5 . 0 X l O -s 10 -~
+++ ++++
5.0 × 10 .4
10 -s
0 0
0
(U/l)
+ ++
0 +
" + + + + b a d l y oxidized, + + + oxidized, + + s l i g h t l y oxidized, + q u e s t i o n a b l e to s l i g h t l y oxidized, 0 n o t o x i d i z e d
INHIBITION OF OXIDIZED FLAVOR ~ r I T n CHELATING COMPOUNDS
823
TABLE 3
The effect of adding chelates prior to the addition of copper upon the action of the chelates in preventing oxidized flavor O x i d i z e d flavor r a t i n g *
M/1 of c h e l a t i n g a g e n t Chelating agent
None
2.5 × 10 -*
5.0 X 10 .4
10 -a
10 -~
++++
0
0
0
5.0 × 10 -'~ 10 -4
+++ ++++
0 0
0
0
5.0 × 10 -°"
+++
0
10 -4
++++
0
0
Cu a d d e d
(Mfl) D i s o d i u m s a l t of E D T A D i s o d i u m c a l c i u m s a l t of E D T A T e t r a s o d i u m s a l t of E D T A
+ + + + b a d l y oxidized, + + + oxidized, + + s l i g h t l y oxidized, + q u e s t i o n a b l e t o s l i g h t l y oxidized, 0 n ot o xidized
The data ill Table 3 were obtained oil samples to which the chelating materials were added p r i o r to the additions of copper. The chelates were equally effective when added before or a f t e r the addition of copper. The results demonstrate that the order of addition is without effect. DISCUSSION
The results of this experiment demonstrate the inhibitory action of certain chelates against the copper-induced oxidized flavor of milk. The inhibition is a t t r i b u t e d to complexing or t y i n g up of the copper ions and p r o b a b l y other ions, rendering them ineffective as catalysts or as activators for certain enzyme systems. B y using the compounds in the m a n n e r described, it appears t h a t the antioxidant p r o p e r t y can be a t t r i b u t e d only to the mechanism of chelation of the copper. A n y consideration of the use of these compounds in m a r k e t milk or other d a i r y products must be based upon information to clearly establish their freedom from a n y h a r m f u l effects. Although no long-term feeding experiments with various species have been reported, there a p p e a r s to be no evidence f r o m other studies that E D T A is h a r m f u l to l a b o r a t o r y animals when consumed orally in quantities m a n y times greater t h a n would be present in milk if added to prevent the oxidized flavor. Child (2) fed rats up to 5% of the sodium salt of E D T A for 4 weeks, and the animals failed to develop abnormalities. Unpublished results (1) f r o m an experiment accompanying this s t u d y showed no detrimental effects when the disodium salt of E D T A was fed to rats. Weanling rats in this experiment were fed 300 and 900 p.p.m, of the salt in a mineralized milk sucrose diet. Over a 10-week period, no adverse effects on the r a t s were evident. SUMMARY
Samples of normal milk were treated with copper to induce oxidized flavor and at the same time were treated with certain chelating compounds. The chelates, which were the disodium salt, the disodium calcium salt, and the tetra-
824
L. R. ARRINGTON AND W. A. KRIENKE
sodium salt of ethylenediamine
tetraacetic
acid (EDTA),
were effective in pre-
venting development of the oxidized flavor. Quantities of the chelates which w e r e five t i m e s t h e a m o u n t o f a d d e d c o p p e r o n a m o l a r b a s i s p r o v i d e d c o m p l e t e protection, and smaller quantities of the chelates gave some inhibitory action. The chelates were equally effective when added either before or after pasteurization or when added prior to or after the addition of copper. ]~EFERENCES (1) AR~I~'GTO~, L. R., AND DAVIS, G. K. Unpublished data. Florida Agricultural Experiment: Station. 1953. (2) CHILD, G. P. The Inhibition of Hematopoietic Action of Cobalt by Ethylenedlamine Tetra Acetic Acid ( E D T A ) . Science, 114: 466. 1951. (3) FORSTER, T. L., -~D SO)~ER, H. H. Manganese, Trypsin, Milk Proteins and the Susceptibility of Milk to Oxidized Flavor Development. J. Dairy Sci., 34: 992. 1951. (4) FORSTER,T. L., JENSEN, C., AND PLATtt, EMILY. Influence of Protein Hydrolysis on t h e Susceptibility of Milk to Oxidized Flavor Development. J. Dairy Sci., 36: 98. 1953. (5) GARRETT, O. F. Some Factors Affecting the Stability of Certain Milk Properties. W. Interrelation of Certain Metals and Metallic Ions and the Development of Oxidized Flavor in Milk. J. Dairy Sci., 24: 103. 1941. (6) GREENBANK, G. ]:~. The Oxidized Flavor in Milk and Dairy Products: A Review. J. Dairy Sci., 31: 913. 1948. (7) LL~NDBEr~, W. O., ]=IALVO~SON, H. O., .~ND BI~RR, G. O. The Antioxidant Properties o f Nordihydroguaiaretic Acid. Oil and Soap, 21: 33. 1944. (8) STULL, J. W., HERREID, E. O., A~D TR.~C¥, P. H. A Study of the Use of the A n t i o x i d a n t Nordihydroguaiaretic Acid in Dairy Products. I. Its Antioxygenic Properties i n Milk. J. Dairy Sci., 31: 449. 1948.