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A g e - T h i c k e n i n g a n d G e l a t io n o f S t e ri li ze d E v a p o r a t e d M i l k 1V . R . H A R W A L K A R , D . C . B E C K E T T , R . C . M c K E L L A R , D . B . E M M O N SFood Re s e a rc h Ins t i tu te

Re s e arc h Bra nc h , Ag r ic u l ture C a na daO t t a w a , O n t a r i o , C an ad a K 1 A 0 C 6G . E . D O Y L E

P e r f e c t i o n F o o d s L t d .C h a r l o t t e t o w n , P r in c e E d w a r d I s l a n d , C a n ad a C 1 A 7 M 8

A B S T R A C TE v a p o r a t e d m i l k c o n t a i n i n g 2 6 % t o t a l

s o l id s ( 1 5 b a t c h e s ) a n d e v a p o r a t e d s k i mm i l k c o n t a i n i n g 1 8 % t o t a l s o l i d s ( 1 b a t c h )w e r e h e l d a t 4 C b e f o r e c a n n i n g a n ds t e r i l i za t i on . S t e r i l i zed samp les w eres t o r e d a t 2 8 C a n d e x a m i n e d p e r i o d i c a l l yf o r 1 y r f o r c h a n g e s i n r e l a t i v e v i s c o s i t y ,g e l a t i o n , g r a n u l a t i o n , a n d f a t s e p a r a t i o n .

S a m p l e s s t e r i l i z e d w i t h o u t c o l d s t o r a g er e m a i n e d l iq u i d f o r 1 2 to o . A s ti m e o fc o l d s t o r a g e o f c o n c e n t r a t e s b e f o r e s te r ii i-z a t i o n i n c r e a s e d , t h i s t e n d e n c y t o t h i c k e no r g e l d u r i n g s t o r a g e i n c r e a s e d . T e n o f 1 5b a t c h e s o f e v a p o r a t e d m i l k c o l d s t o r e df o r 3 d a y s b e f o r e s t e r i l iz a t i o n h a d g e l le da f t e r 1 2 m o a l o n g w i t h 6 t h a t h a d b e e nh e l d f o r 2 d a y s . S a m p l e s s t o r e d c o l d f o r3 d a y s t h a t d i d n o t g e l a f t e r 1 2 m o h a dt h i c k e n e d m o r e t h a n c o n t r o l s .

S a m p l e s t h a t g e l l e d c o n t a i n e d n om i c r o o r g a n i s m s c a p a b le o f g r o w t h a t3 0 C a n d s h o w e d n o a p p re c i a b le c h a n g ei n p r o t e o l y s i s o r p H .

H e a t i n g s a m p l e s a t 5 5 t o 6 0 C f o r 3 0r a in a f t e r 3 d a ~ cs c o l d s t o r a g e , a n d t h e nc o o l i n g t o 2 0 - C (s o - c a ll e d a n t i - c o o l -a g in g t r e a t m e n t " ) b e f o r e c a n n i n g a n ds t e r il i z a ti o n d i d n o t p r e v e n t g e l a ti o n o rinc rease i n v i scos i t y du r ing s t o rage .

O x i d i z i n g c o n d i t i o n s s u c h a s a e r a t i o na n d p e r o x i d e t r e a t m e n t s a c c e l e r a t e dw h e r e a s r e d u c i n g c o n d i t i o n s s u c h a s a n t i-o x i d a n t t r e a t m e n t s t e n d e d t o de l a y b u tn o t p r e v e n t a g e - t h i c k e n i n g a n d g e l a t i o n .

Received March 4, 1982.:Cont r ibut ion No. 494 from the Food ResearchInstitute.

I N T R O D U C T I O NE v a p o r a t e d m i lk p r e p a r e d b y c o n v e n t i o n a l

s t e ri li z in g m e t h o d s u s u a l l y r e m a i n s f r e e f r o ma g e - t h i c k e n i n g o r g e l a t i o n a f t e r l o n g s t o r a g e a ta m b i e n t t e m p e r a t u r e s ( o v e r 1 2 t o o ). H o w e v e r ,s p o r a d i c a p p e a r a n c e o f g e l a t io n i n s o m e e va -p o r a t e d m i l k s a m p l e s h a s c a u s e d s e r i o u s p r o b -l e m s f o r so m e m a n u f a c t u r e r s .

T h e c a u s e o f t h i s d e f e c t i s n o t a p p a r e n t a n dh a s n o t b e e n r e l a t ed t o a n y o f t h e c u s t o m a r yp r o c e s s i n g v a r ia b l e s s u c h a s t i m e a n d t e m p e r a -t u r e o f f o r e w a r m i n g , h o m o g e n i z i n g p r e ss u r e,a d d e d o r t h o p h o s p h a t e s , e t c . H o w e v e r , m o s tb a t c h e s t h a t s h o w e d a g e - t h ic k e n i n g o r g e l a t i o ni n a p a r t i c u l a r d a i r y p l a n t i n C a n a d a w e r e p r e -p a r e d f r o m c o n c e n t r a t e s h e l d i n c o l d s t o r a g eb e f o r e c a n n i n g a n d s t e r il i z a ti o n .

S i m i l a r o b s e r v a t i o n s w e r e m a d e b y D u t c hw o r k e r s ( 3 ), w h o o b s e r v e d g e l a t i o n in s a m p l e so f e v a p o r a te d m i l k a n d d e m o n s t r a t e d a r el a-t i o n s h i p b e t w e e n g e l a t i o n a n d c o l d s t o r a g e o fc o n c e n t r a t e s b e f o r e s t er il iz a ti o n . T h e y d e m o n -s t r a t e d t h i s r e l a t i o n s h i p o n l y i n e v a p o r a t e dm i l k s c o n f o r m i n g t o E u r o p e a n s t a n d a r d s ( f a t /S N F ( s o l i d s -n o t - f a t ) o f 9 / 2 2 o r 1 0 / 2 3 ) . I n t h e i re x p e r i e n c e , e v a p o r a t e d m i l k m a d e t o N o r t hA m e r i c a n s t a n d a r d s ( f a t / S N F o f 8 / 1 8 ) d i d n o ts h o w t h i s d e f e c t .

I n vi e w o f t h e s i m i l a r it y o f t h e p r o b l e m d e -s c r ib e d b y t h e D u t c h w o r k e r s a n d t h e a g e-t h i c k e n i n g d e s c r i b e d , o u r f i r s t o b j e c t iv e w a s t os ee if t h e p h e n o m e n o n d e s c r i b e d b y t h e D u t c hw o r k e r s c o u l d b e o b s e r ve d i n e v a p o r a t e d m i l km a d e t o N o r t h A m e r i c a n s t a n d a r d s . T h e s e c o n do b j e c t i v e w a s t o f i n d s u i t a b l e e x p l a n a t i o n f o rt h e a g e - t h i c k e n i n g o r g e l a t i o n i n e v a p o r a t e dmi lk . M A T E R I A L S A N D M E T H O D S

E v a p o r a t e d m i l k w a s p r e p a r e d i n a n e v a -p o r a t e d m i l k p l a n t b y a c o n v e n t i o n a l m e t h o d .

1983 J Dai ry Sci. 66 :73 5- 74 2 735

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7 3 6 H A R W A L K A R E T A L .

B r i e fl y , r a w m i l k w a s f o r e w a r m e d t o 8 8 C a n dh e l d i n a " h o t w e l l " f o r 2 5 m i n . A h o t a q u e o u ss l u r ry o f s ta b i l i z in g s al t ( d i s o d i u m p h o s p h a t e )a n d c a rr a g e e n a n w e r e p u m p e d i n to t h e h o tw e l l. T h e a m o u n t o f s t a b i l i z in g sa l t a n d c a r ra -g e e n a n a d d e d w e r e u s u a l l y 1 1 .3 a n d . 6 8 k g p e rb a t c h o f 3 .1 7 x 1 0 4 k g r a w m i l k . T h e m i l k t h e nw a s c o n c e n t r a t e d i n a d o u b l e - e f f e c t e v a p o r a t o r ,f i rs t e f f e c t at 6 8 . 3 C w i t h 5 7 . 6 k P a o f v a c u u ma n d i n t h e s e c o n d e f f e c t a t 4 3 t o 4 5 C w i t h 9 5k P a o f v a c u u m . T h e c o n c e n t r a t e t h e n w a sh o m o g e n i z e d a t 2 2 .7 5 a n d 4 .8 M P a a t 4 3 C ,c o o l e d t o 6 . 7 C , a n d p u m p e d t o a h o l d i n g t a n kf o r s t a n d a r d i z a t i o n t o 8 % f a t a nd 2 6 % t o t a ls o l i ds . A s i m i l a r p r o c e d u r e w a s u s e d f o r p r e -p a r i n g e v a p o r a t e d s k i m m i l k .

A p o r t i o n o f th e c o n c e n t r a t e w a s h e ld a t4 C i n la r g e ( 1 0 0 l i t e r ) p l a s t i c c o n t a i n e r s f o rv a r y in g t i m e s . S o m e o f t hi s c o n c e n t r a t e w a st r e a t e d s p e c i a l ly a s d e s c r i b e d l a t er . F o l l o w i n gt h i s h o l d i n g p e r i o d a n d s p e c i a l t r e a t m e n t , t h ec o n c e n t r a t e w a s c a n n e d a n d s te r i li z e d a t 1 1 9 Cf o r 1 0 m i n i n a n F M C c o n t i n u o u s r o t a r ys t e ri li z e r. C a n s w e r e c o o l e d a n d s t o r e d u n -d i s t u rb e d a t ~ 2 8 C .

T h e s t o r e d s a m p l e s o f e v a p o r a t e d m i l k w e r ee x a m i n e d p e r i o d i c a l l y f o r c h a n g e s i n r e l a t iv ev i s c o s i t y , g r a n u l a t i o n , f a t s e p a r a t i o n , a n d g e l a -t i o n . V i s c o s i t y o f t h e s a m p l e s w a s m e a s u r e d a t2 1 C b y a B ro o k f ie l d v i s c o m e t e r m o u n t e d o ,a H e l i p a t h s t a n d w i t h t h e s p in d l e " A " a t 1 2r p m . R e a d i n g s w e r e t a k e n a f t e r e v e ry d o w n -

w a r d r o t a t i o n a n d a n a v e ra g e d e t e r m i n e d a f t e rt h e t w o t o p a n d b o t t o m r e a di n g s w e r e o m i t t e d .T h e a v e r a g e a r b i tr a r i ly w a s ta k e n a s r e l a t iv ev i s c o s i t y o f t h e s a m p l e ; a re l a t i v e v i sc o s i t y o f1 0 i s 1 6 6 c e n t i p o i s e .G r a n u l a t i o n w a s o b s e r v e d i n t h i n f i l m s o fs a m p l e d r a w n w i t h a w i r e l o o p ( 2 c m i. d. ). F a ts e p a r a t i o n a n d g e l a t i o n w e r e a s s e ss e d b y v is u a lo b s e r v a t i o n .

S t a n d a r d p l a t e c o u n t ( S P C ) w a s a s d e s c r i b e din Standard Metbods for tbe Examination ofDairy Products ( 6 ) w i t h t h e e x c e p t i o n t h a t d i lu -t i o n s w e r e w i t h .1 % p e p t o n e t o f a c i l i ta t e r e-c o v e r y o f o r g a n i s m s ( 7) . I n c u b a t i o n w a s f o r 4 8h a t 3 2 C .

D e g r e e o f p r o t e o l y s i s w a s m e a s u r e d a s t h ei n c re a s e i n t r i c h l o r o a c e t i c a c i d - s o l u b l e f r e ea m i n o g r o u p s ( 5 ) .

R E S U L T S A N D D I S C U S S I O NV i s c o s i ty a n d G e l a t i o n

D a t a i n T a b l e 1 s h o w t h e e f f e c t o f c o l d s t o r-a ge o f m i t k c o n c e n t r a t e b e f o r e s t e r i li z a t io n o nt h e r e l a ti v e v i s c o s i t y d u ri n g s t o r a g e o f s a m p l e so f c o n c e n t r a t e d m i l k f ro m 1 5 b a t c h e s . T h eu p p e r p o r t i o n o f T a b l e 1 s h o w s t h e m e a n r e l a-t i v e v i s c o s i t y o f u n g e l l e d s a m p l e s ; g e ll e d s a m -p l e s ( n u m b e r i n b r a c k e t s ) w e r e e x c l u d e d .B e c a u s e t h e h i g h v i s c o si t ie s o f g e l l e d s a m p l e sa re n o t i n c l u d e d, s o m e t r e a t m e n t s w i t h l o w

T A B L E 1 . E f f e c t o f c o l d s t o ra g e o f m i l k c o n c e n t r a t e s b e f o r e s t e r i l iz a t i o n u p o n c h a n g e s i n r e l at i v e v i s c o s it ydur ing s to rage .

T r e a t m e n t(days o fco ld s to rageSa m p l e a t 4 C) 0Re la t ive v iscos i tya a f te r s to rage ( too)(Mean of 15 ba tches)

3 6 9 12Al l 15 ba tch es 0 6 .5 8 .0 15 .5 21 .9 25 .9of evap ora ted mi lk 1 6 .2 9 .6 23 .0 22 .1 (2 ) 32 .0 (2 )2 7.4 12.9 33.0 (2) 48.0 (4) 37.0 (6)3 9 .7 21 .8 50 .0 6 ) 50 .0 (9 ) 49 .0 (10)Un gelled 0 8.0 7.4 10.9 11.8 12.4evap ora ted mi lkb 1 7 .9 9 .0 12 .0 17 .6 21 .02 9.3 10.8 23.0 38.0 33.4

3 9.8 12.2 34.6 48.1 48.6a Re l a t i v e v is c o s i ty d a t a a r e m e a n s o f t h e u n g e l l e d s a m p le s a n d t h e n u m b e r o f g e l le d s a m p l e s is i n d i c a te d i nbracke ts .bDa ta a re fo r the f ive ba tche s tha t d id no t ge l a t 12 mo in the 3 -day co ld- s to rage t r ea tm ent .

Jour na l o f Da i ry Sc ien ce Vol . 66 , No . 4 , :1 .983

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GELATION OF EVAPORATED MILK 737gelation (e.g., 2-day cold storage after 9 mo)show viscosities as high as another treatmentwith frequent gelation (e.g., 3-day cold storageafter 12 too). A better comparison is on thefive samples for which no gelation occurredduring 12 mo. These are in the bottom of Table1.

Viscosity of evaporated milk increasedduring storage. I n the 15 samples receiving nocold storage treatment, relative viscosity in-creased from 6.5 to 25.9. In the five samplesthat did not gel, relative viscosity of sampleswith zero cold storage increased from 8.0 to12.4 and that of samples with 3 days of coldstorage increased from 9.8 to 48.6.

It is normal for evaporated milk to show adecrease in viscosity after manufacture, afterwhich it increases. This was marginally apparentin Table 1 in the five samples that did not gel;relative viscosity at 3 mo was 7.4 compared to8.0 at 0 and 10.9 at 6 mo. Viscosity had de-creased and the n increased sufficien tly in the15 samples that the mean relative viscosity at3 mo was 8.0, compared with 6.5 at 0 time and15.5 at 6 mo.Cold storage of concentrate before steriliza-tion markedly influenced the relative viscosity,during storage. The 15 lots sterilized withoutcold storage remained liquid for over 12 mo ofstorage. Viscosity of samples stored cold beforesterilization showed greater increased in relativeviscosity and gelation during subsequent stor-age, the increase being propor tio nal to theperiod of cold storage. At 12 mo, the meanrelative viscosity with 0-clay cold storage treat-ment was 25.9. However, two of the 15 samplescold-stored for 1 day, 6 of the 15 samples cold-stored for 2 days, and 10 of the 15 samplescold stored for 3 days had gelled. At shorter

storage times, viscosity and frequency of gela-tion increased. Even after 3 mo storage therewas a regular increase in viscosity as cold stor-age treatment increased from 0 to 3 days.

In the five samples that remained ungelledafter 12 too, the mean relative viscosity had in-creased from 12.4 to 48.6 as the time of coldstorage before sterilization increased from 0 to3 days.

There were marked differences among lots inthe way in which cold storage period affectedviscosity and gelation during subsequent stor-age of the evaporated milk. It was typical thatviscosity increased with time of storage andwith time of cold storage before sterilization.Thus, susceptible lots gelled quickly with 3days of cold storage but less quickly with 1 daycold storage. Resistant samples did not gel butshowed typical increases of viscosity.

Age-thickening also was accelerated whenevaporated skim milk was cold-stored beforesterilization (Table 2). At 6 too, the samplecold-stored for 3 days showed a substantial in-crease of viscosity and presumably would havegelled sooner than others. Evaporated skimmilks were more susceptible to gelation; theygelled between 6 and 9 mo of storage.Granu la t ion

Granulation appeared during storage as sam-ples thickened. These samples usually did notshow any granul ation in the freshly sterilizedsamples. The appearance of granulat ion usuallypreceded thickening or gelation during storage.The freq uency of appearance of granul ationduring storage increased with the time of coldstorage of the unsterili zed concent rate (Table3). There were some samples in which granula-

TABLE 2. Effect of cold storage of skim milk concentrate before sterilization upon changes in relative viscosityduring storage.Days ofcold storage Relative viscosity after storage (rno)at 4 C 0 3 6 9 120 2.5 4.0 3.9 Gelled Gelled1 2.4 3.7 4.0 Gelled Gelled2 3.0 3.9 7.8 Gelled Gelled3 2.9 3.8 31.2 Gelled Gelled

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738 HARWALKAR ET AL.

tion was observed in freshly sterilized samples.These samples showed increased tendency tothicken or gel during storage. However, lack ofsufficient number of such samples precludes aconclusive observation.Fat Separation

There was little or no visual fat separation inany of the untreated control or treated samples.Therefore, thickening or gelation of evaporatedmilk was n ot a result of any unusual fat separa-tion. However, samples of evaporated milk thatreceived cold storage treatment showed after6-mo storage more thickening at the top of thecan than at the bottom whereas gelation ofevaporated skim milk was accompanied by ex-cessive synerisis, and the gelled portion settledto the bottom of the can. Thus, greater thicken-ing at the top of the can is associated with fator fat protei n complexes.

We confirm the observation of Dutchworkers that gelation of evaporated milk iscaused by cold storage of concen trate beforesterilization. In contrast to their observations,we demons trate d this effect in evaporated milkmade to North American standards (lower totalsolids than Eu ropean evaporated milk).No suitable explanation is offered for thisphenomenon of age thickening or gelation ofevaporated milk. An explanation of the unusualgelation phenomenon was sought by examina-tion of the possible involv ement of 1) microbialquality of raw milk, 2) seasonal effects, 3)microbial growth and production and heat

stable proteases during cold storage, 4) cold-induced alteration of casein micelles, andoxidati on/redu ction reactions.

Qual i ty o f Raw M i lkThe microbial quality of the raw milk used

for preparing the evaporated milk did not seemrelated to age thickening or gelation. The rawmilk used for the 15 batches varied from 70 103 to 385 103 cfu/ml (Table 4). Regardlessof the initial count of the raw milk, evaporatedmilk thickened or gelled only when the un-sterilized concentrates were subjected to coldstorage. Corresponding batches of evaporatedmilk without cold storage remained liquid.Furthermore, some of the batches with higherinitial count of raw milk merely had thickenedwhereas others with lower initial counts gelledwithin 12 mo.Seasonal Effect

In commercial practice the problem of agethickening and gelation tends to be seasonal,more frequent in the early spring. This studytended to confirm this. Of the six batches pro-cessed between Fe bruary 22nd an d March 10th,1980, only one batch that was cold-stored for3 days had gelled by 12 mo. Of the ninebatches processed between March 14th andMay 12th, by 12 mo all batches with 3-day coldstorage and seven with 2-days cold storage hadgelled. By 6 m o five of the nine batches with 2-days cold storage had gelled. These nine batches

TABLE 3. Effect of cold storage by unsterilized concentrate upon granulation and gelation during storage ofsterilized evaporated milk.

Days of Frequency of granulation and gelation acold storage during storage (mo)Sample at 4 C 0 3 6 9 12All 15 batches 0 0 0 0 0 3of evaporated milk 1 0 2 4 2 (2) 5 (2)2 0 4 7 (2) 8 (4) 6 (6)3 5 9 8 (6) 5 (9) 4 (10)Five batches of 0 0 0 0 0 1evaporated milk 1 0 0 0 1 2that did not 2 0 2 3 4 4gel at 12 mo 3 1 3 4 4 4

aThe additional number of batches that had gelled are shown in brackets.Journal of Dairy Science Vol. 66, No. 4, 1983

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GELATION OF EVAPORATED MILK 73 9with 0- or 1-day cold storage showed more agethickening than the corresponding samplesfrom the earlier six batches. Thus, seasonal fac-tors, although influencing the susceptibility ofthe evaporated milk to age thickening, alsoaccentuated the effect of cold storage upongelation.M i c r o b i a l G r o w t h

During cold storage of the concentrate,psychrotrophs may have grown and producedheat-stable proteases, which acted slowly duringstorage and caused gelation as in some ultra-high temp eratu re (UHT) sterilized milk pro-ducts (1, 4). This possibility seems unlikely forthe following reasons, a) The increase in stan-dard plate count (SPC) was usually slight after3 days of cold storage of the concentrates(

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740 HARWALKAR ET AL.

tion. Therefore, physicochemical changes of themicelles leading to storage gelation must be ofa different nature from changes affecting rennetcoagulation time.

O x i d a t i o n - R e d u c t i o nCold storage of milk concent rate may" in-

crease dissolved oxygen, which in turn mayinfluence gelation by oxidation reactions. Thispossibility was tested by treating milk concen-trates to accentuate or to reduce oxidizingconditions. The unsterilized concentrates weretreated as follows.

To accentuate oxidizing conditions: a) con-centrate, before or after cold storage, wasaerated by bubbling air through it for 5 rain.The air flow was controll ed to minimize exces-sive foaming, b) Hydrogen peroxide (H202)(.1%) was incorporated into the concentratebefore sterilization. The incorporation ofH202 in milk concentrates caused destabiliza-tion by normal sterilizing treat ment (11 9C for10 min). The sterilizing tempera ture, there-fore, was reduced to 115C. This temperaturein the presence of H202 was adequate tosterilize the sample. No such lowering ofsterilizing temperature was necessary for skimmilk concentr ates with added H20 , . These sam-ples were stable under normal sterilizing con-ditions. The reason for this difference of stabil-ity of milk and skim milk concentrates in thepresence of H202 is not known.

Reducing conditions were introduced bydeaeration and incorporation of antioxidants.Deaeration of concentrates was achieved by

first applying va cuum (38.3 kPa for 20 min)followed by bleeding in nitrogen and bubblingfor 20 rain. Incorp orat ion of antioxid ants wasachieved by stirring in .02% antioxidant in milkconcentrates at 55 to 60C. The two antioxi-dants were butylated hydroxytoluene (BHT)and propyl gallate.

The above samples treated to accentuate oxi-dation or reduction were transferred manuallyto individual cans, sealed, and sterilized asdescribed. Changes in viscosity and gelationduring storage of these samples are in Table 6.Samples 1 and 2 are controls with holding ofconcent rates for 0 and 3 days at 4 0 Samples 3and 4 show the effect of aeration. Aeration ofthe concen trate with out cold storage showedslight thickenin g (com pare samples 3 and 1).The aerated sample held 3 days showed nomore thickening than the unaerated control(comp are samples 4 and 2). Deaer atio n of thesamples held 3 days did not affect viscositychanges (compare samples 7 and 2). Treatmentwith hydrogen peroxide of samples held both0 and 3-day caused marked thickening or gela-tion compared to their controls (samples 5 and6 vs. 1 and 2). Holding for 3 days beforesterilizing markedly accentuated the effect oft h e H 2 0 2 .The presence of antioxida nts appeared todelay age-thickening (samples 8 to 11). In sam-pies 8 and 10 the antioxidants were added tothe 3-day-old concentrate. In samples 9 and 11the antioxidant was added to the concentrateon 0 day and canned. The canned concentrates,then were held for 3 days at 4C before sterili-zation. There was little difference between sam-

TABLE 5, Effect of anticool-aging treatment a of cold-stored milk concentrates before sterilization upon agethickening,Days ofcold storage Repli-at 4 C Treatment cates

Relative viscosity b after storage (mo)0 3 6 9 12

0 . . . 5 6.7 8.5 11.4 13.1 13.03 . , . 5 8.8 14.9 38.6 50.5 (3) 53.0 (3)3 Anticool aging 5 13.8 27.8 54.2 (3) Gelled (5) Gelled (5)aCold-stored samples were heated at 55 to 60C for 30 min and were cooled to -20C before canning andsterilization.bThe viscosity data are means of the ungelled samples, and the number of gelled samples is indicated inbrackets.

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GELAT1ON OF EVAPORATED MILK 741TABLE 6. Eff ect of oxidizing and reducing treatments of cold-stored milk concentra tes before sterilization uponage thickening.

Days ofstorage Relative viscosityb after storage (mo)No. at 4C Treatment a 0 3 6 9 12

1 0 . . . 6.3 8.2 10.5 12.4 9.82 3 9.5 14.8 38.6 (1) 50.0 (2) 49. 0(2 )3 0 Aceta te 11.4 16.1 15.2 33.0 33.54 3 Acet ate 9.8 15.3 34.9 39.0 47.0 (2)5 0 H202 5.4 7.1 21.0 48.0 36.0 (1)6 3 H202 6.1 48.6 Gelled (4) Gelled (4) Gelled (4)7 3 Deaerate 11.4 16.1 46.3 49.5 56.5 (2)8 3 BHT c 7.8 14.0 37.5 (1) 39.0 (2) 45.0 (2)9 0/3 BHT d 9.3 13.1 26.8 (1) 34.0 (1) 38.0 (2)10 3 Propyl c gallate 8.8 14.6 38.5 (1) 29.0 (2) 43.0 (2)11 0/3 Propyl d gallate 9.9 14.7 31.5 (1) 18.0 (2) 33.0 (2)aAll treatmen ts had four replicates except 5, which had only three replicates.bThe relative viscosity data are means o f ungelled samples, and the number of gelled samples is indicated inbrackets.C .Antmx~dant was incorporated after the 3-day cold storage.dAnt ioxid ant was incorporated before the 3-day cold storage.

pies to which ant ioxidant was added ei therbefore or af ter cold s torage; the addi t ion ofant ioxidant before cold s torage seemed to beslightly more beneficial. Evaluation of the ef-fect of ant ioxidants is complicated by the factthat evaporated milks vary in their susceptibil-i ty to gelat ion. Even though the viscosi ty of theless susceptible batches showed less viscosityincreases unde r reducing condi t ions, the num-ber of suscept ible samples that had geI led wasthe same. Therefore, def ini t ive conclusionsregarding the posi t ive effect of reducing condi-

t ions upo n age-gelat ion are not possible .The effect of ant i -cool-aging, hydr ogenperoxide, and ant ioxidant t reatment on eva-

por ate d skim milk is in Table 7. The eff ect ofthese t r eatme nts was s imilar to the effect onevaporated milk. However , skim milk sampleswere more suscept ible to gelat ion. Apparent ly,fat had a protect ive effect against age thicken-ing or gelat ion, presumably by interact i on withcasein micelles.

The abi li ty of hydro gen perox ide to aug-ment the gelation process is a definite indica-

TABLE 7. Effect of anticool aging, hydrogen peroxide, and antioxidant treatments of skim milk concentratescold-stored before sterilization upon age thickening.Days ofcold storageat 4 C

Relative viscosity after storage (too)Treatment 0 3 6 9 12

0 . . . 2.5 4.0 3.9 Gelled Gelled3 . . _ 2.9 3.8 31.2 Gelled Gelled0 H202 2.8 4.5 Gelled Gelled Gelled3 H202 3.1 Gelled Gelled Gelled Gelled3 Propyl gallate 2.8 4.0 27.6 Gelled Gelled3 Anti cool aging 3.0 3.8 25.5 Gelled Gelled

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7 4 2 H A R W A L K A R E T A L .

t i o n t h a t o x i d a t i o n r e a c t i o n s a r e im p o r t a n t i ng e l a ti o n . In c o r p o r a t i o n o f h y d r o g e n p e r o x i d ea ls o h a s t e n e d g e l a t i o n o f U H T - s t e r i l i z e d e v a -p o r a t e d s k i m m i l k ( 2) .

T h e a n t i o x i d a n t s w e r e n o t e f f e c t i v e a g a in s ta g e t h i c k e n i n g o r g e l a t i o n ; o n l y s l i g h t p r o t e c -t i o n w a s o b s e r v e d . T h e a p p a r e n t i n a b i l i ty o f t h ea n t i o x i d a n t s t o p r e v e n t g e l a t i o n c o u l d h a v er e s u l t e d f r o m p o o r di s p e rs a l o f a n t i o x i d a n t s .A l t e r n a t i v e l y , c h a n g e s o t h e r th a n o x i d a t i o nm i g h t b e i n v o lv e d .

C h a n g e s d u r i n g c o l d s t o r a g e o f e v a p o r a t e dm i l k b e f o r e s t e r i l i z a t i o n s e e m t o p r e d i s p o s e t h em i c e l l a r s y s t e m o f t h e c o n c e n t r a t e s t o ag et h i c k e n i n g o r g e l a ti o n . G e l a t i o n w o u l d n o t b e as e r i o u s p r o b l e m i f c o l d s t o r a g e o f t h e c o n c e n -t r a t e i s a v o i d e d , b u t , w i t h i n c r e a s in g v o l u m e o fm i l k p r o c e s s e d c o u p l e d w i t h s h o r t e r w o r kw e e k s , s t o r a g e o f c o n c e n t r a t e s b e f o r e s te r i li z a -t i o n m a y b e u n a v o i d a b l e . I n t h i s s i t u a t i o n ab e t t e r d e f i n i t i o n a n d a c l e a r u n d e r s t a n d i n g o ft h e c h a n g e s i s e s s e n t i a l t o s o l u t i o n o f t h ep r o b l e m ,

A C K N O W L E D G M E N T

T h e a u t h o r s w o u l d l i k e t o th a n k B a r r y V e s s y

f o r h i s h e l p i n p r e p a r i n g t h e s a m p l e s a n d D .S i b b i t t f o r a n a l y si s o f t h e s a m p l e s .

R E F E R E N C E S1 Harwar lka r , V . R. 1982 . Age ge la t ion o f s te r i l izedmilks. In D e v e l o p m e n t s o f d a i r y c h e m i s t r y , Pa r t1: Prote ins . P. F. Fo x, ed. Appl. Sci. Publ. Co.,London , England ( in p re ss ) .2 Harwa lka r , V . R. , and H . J . Freem an . 1978 . Ef fe c to f added phosp ha te and s to rage on changes inu l t r a -h igh tempera tu re shor t - t ime s te r i l i zed concen-t r a t e d s k i m m i l k . N e t h e r l a n d s M i lk D a i r y J . 3 2 : 2 0 4 .3 He in tzbe rge r , H . , J. Koops , and D. We s te rbeek .1 9 7 2 . G e l a t i o n o f s te r i li z e d c a n n e d e v a p o r a t e dm i l k . N e t h e r l a n d s M i lk D a i r y J . 2 6 : 3 1 .4 Law , B. A. 1979. Revis ion on the p rogress o f da i rys c i e n c e : E n z y m e s o f p s y c h r o t r o p h i c b a c t e r i a a n dt h e i r e f f e c t s o n m i l k a n d m i l k p r o d u c t s . J . D a i r y

Res. 46:573.5 McKel la r , R. C. 1981 . De ve lo pm ent o f o f f - f lavor sin u l t r a -h igh tem pera tu re and pas teur ized mi lk as afunc t ion o f p ro teo lys i s . J . Da i ry Sc i . 64 :2138 .6 S t a n d a rd M e t h o d s f o r t h e E x a m i n a t i o n o f D a i r yProduc ts . 1978 . 14 th ed . E . M. Mar th , ed . Am .Publ. Hea l th Assoc . , Wa sh ing ton , DC 20036 . p .416 .7 TeW ha i t i, I. E ., and T . F . Frye r . 1977 . The enum -e ra t ion o f bac te r ia in r e f r ige ra ted mi lk . New Zea -land J . Da i ry Sc i . Teehnol . 12 :51 .8 Webb , B. H ., and A . H . John son . 1965 . Funda me n-ta l s o f da i ry chem is t ry . Avi Publ . Co . , W es tpor t ,CT.

J o u r n a l o f D a i r y Sc i e n c e V o l. 6 6 , N o . 4 , 1 9 8 3