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Dry Whole Milk. I. A New Physical Form
DRY W H O L E MILK. I. A NE'W P H Y S I C A L FORM ]=L I. SINNAMON, N. C. ACETO, R. K. E S K E W , AND E. F. SCHOPPET
Eastern Regional Research Laboratory, ~ Philadelphia, Pennsylvania
This paper reports the properties of a new dry whole milk, dried under high vacuum and low temperatures in the form of an expanded sponge-like structure. Milk is homogenized at 145 ° F. and 2,500 p.s.i, and pasteurized at 162 ° F. for 16 seconds. It is concentrated to 47 to 50% total solids at 85 ° to 100 ° F. in a high-vacuum falling film evaporator, heated to 135 ° F. and homogenized at 4,000 and 500 p.s.i. Before the concentrate is homogenized, nitrogen is entrained in it and it is then flowed over drying pans, cooled to 55 ° F., and vacuum-dried as a foam. Finally, the dried mass is crushed lightly. This product disperses easily in cold water and has a natural flavor when reconstituted in the fresh state. Editor.
A long-range research program was recently initiated at this Laboratory, with the ultimate goal of developing a dried whole milk of easy dispersibility, good flavor, and storage stability. As presently conceived, this program will be carried forward in three phases : The first phase will be concerned with the preparation by batch methods of a dried whole milk which is initially easy to disperse and of good flavor. The second phase will include a study of the storage stability of the material and factors which influence stability. The third phase will be a translation of the essential processing conditions to a commercially feasible operation. The purpose of this paper, which deals with the first phase of the program, is to report the properties of whole milk dried in a new physical form and possessing excellent flavor and easy dispersibility in ice water. The processing variables found to be essential in its preparation are discussed. CHOICE OF THE DRYING METHOD
Puff-drying methods were investigated, because the authors believe them to be superior to spray- or roller-drying. Puff-drying has been successfully applied to heat-sensitive citrus juices and other food products in both batch and continuous processes (2-5, 7). Therefore, a systematic engineering study was made of the factors responsible for producing a product that would be easy to disperse and have good flavor. In general, puff-drying may be defined as the formation of a highly expanded, sponge-like structure of dried material from a thin film of liquid concentrate, under conditions of high vacuum and low temperature. The product would be expected to disperse rapidly because of its large surface area per unit weight. and to possess natural flavor because of the high-vacuum, low-temperature drying conditions employed. As applied to whole milk, drying at low temperatures Received for publication March 26, 1957. 1 A laboratory of the Eastern Utilization Research and Development Division, Agricultural Research Service, United States Department of Agriculture. 1036
DRY WHOLE MILK. I.
1037
would also be expected to inhibit protein destabilization, and dehydrating at low oxygen concentrations at low temperature would preclude atmospheric oxidation. BASIC CONSIDER.kTIONSIN PUFF-DRYING WHOLE 5IILK Puffed structures can be developed by various devices and, for some materials, such as those containing large amounts of sugars, the type of structure may not be of prime importance for rapid dispersibility. With whole milk, however, the structure has been found to influence markedly the dispersibility rate of the dried product. Two possible structures are illustrated (Figure 1). The puffed form on the top was developed by the evolution of water-vapor from a deaerated concentrate. This is characterized by large, nonuniform bubbles and a preponderance of unpuffed, intercellular material. Product obtained from such a form disperses very poorly compared to that shown (bottom of Figure 1). The latter was formed by the expansion of entrained gas of low solubility in the concentrated milk, and is characterized by small, uniform bubbles and a minimum of unexpanded intercellular material. To differentiate, the latter is hereafter referred to as a foam.
--USDA
photo by M. (7. A_~tdsley
FIG. 1. Typical foam structures obtained by water-vapor (top) and entrained gas of low solubility (bottom).
1038
H.i. SINNAMO~nT AT.
The mere use of an entrained gas does not insure a f o r m that will disperse readily. I n order for entrained gases to be effective in f o r m i n g a fine-grained foam, the concentrate must be held at a low t e m p e r a t u r e during the initial d r y i n g stage. A t t e m p e r a t u r e s above 55 ° F., and u n d e r the vacuums necessary for foaming, the concentrate will boil before it has expanded sufficiently, and boiling at this point will remove the gas f r o m the u n e x p a n d e d material. The dried structure will then be similar to a puff f o r m e d by water-vapor f r o m a deaerated concentrate. Initial experiments using only dissolved C02 or N20 indicate that these gases, because of their high solubility, give structures similar to the waterv a p o r type. W h e n the absolute pressure within the drier reaches the v a p o r pressure of the concentrate, vigorous agitation of the expanded s t r u c t u r e results f r o m the flashing of water-vapor. I f the desirable f o r m is to survive, it must be rigid enough to withstand this action. H e r e again, low t e m p e r a t u r e s are desirable because of the added viscosity. H i g h solids content is another factor which increases the viscosity of the concentrate, and it has been found t h a t materials containing up to about 50% solids can be readily dried as a foam. Beyond this concentration a gel forms which becomes difficult to spread evenly for drying. The conditions essential f o r producing a material of good dispersibility have been determined by experiments in a pilot-plant v a c u u m shelf drier. E x p l o r a t o r y studies conducted cooperatively with the Chain-Belt C o m p a n y 2 of Milwaukee, Wisconsin, have shown, moreover, t h a t these essential conditions can most p r o b a b l y be translated to a continuous method and thus, t h a t f o a m - d r y i n g of whole milk m a y have a f u t u r e in commerce. A public service p a t e n t is pending covering these principles. P R E P A R A T I O N OF T H E DRIED W H O L E ~ I I L K
F r e s h pasteurized, homogenized milk was obtained f r o m a local d a i r y for these studies. I t had been homogenized at 145 ° F. and 2,500 p.s.i, and then pasteurized at 162 ° F. f o r 16 seconds. I t contained 3.6 to 3.7% f a t a n d about 8.7% solids-not-fat. The milk was first concentrated to f r o m 47 to 50% total solids at a batch t e m p e r a t u r e of 85 to 100 ° F. in a high vacuum, falling-film evaporator. I t was then heated to 135 ° F. and homogenized, first at 4,000 p.s.i. and then at 500 p.s.i., through a single-stage homogenizer of the pulsator type. I m m e d i a t e l y p r i o r to homogenization, nitrogen was bubbled through the concent r a t e f r o m a fritted-glass sparger dispersing entrained gas through the material. The concentrated milk was then flowed over stainless steel d r y i n g p a n s to an average d e p t h of 1/16-inch, chilled to 55 ° F. or below, and dried as a f o a m in a v a c u u m shelf drier. The resulting dried mass was crushed lightly t h r o u g h stainless steel screens. A typical d r y i n g cycle is illustrated ( F i g u r e 2). 2Mention of companies and commercial products anywhere in the paper does not imply that they are endorsed or recommended by the Department of Agriculture over others of a similar nature not mentioned.
DR~ W~OLE ~IIL~. ~. 22-0
1039 22
A SSOLU TE PRESSURE
200 PUFFING BEGINS
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, I00 TIME
r' 120 (MIN.}
, ...... 140 160
180
200
FIG. 2. Y a c u u m shelf d r y i n g cycle f o r whole milk.
I t has been found t h a t variations in foam structure were possible, depending on the r a t e of v a c u u m application. F o r instance, when the pressure inside the d r y i n g chamber was reduced f r o m 20 to 2 mm. in five minutes, the concentrate expanded r a p i d l y to about 24 times its original volume, and then became sufficiently rigid to prevent f u r t h e r expansion or collapse. The resulting structure was composed of t i n y bubbles a p p r o x i m a t e l y 1/]6-inch in diameter and had a relatively large proportion of u n e x p a n d e d intercellular material. W h e n the d r y i n g chamber pressure was reduced f r o m 20 to 2 ram. in about 20 minutes, on the other hand, the concentrate expanded slowly to a volumetric increase of f r o m 32- to 96-fold before it stabilized. Bubble sizes were larger in these structures, and the proportion of u n e x p a n d e d intercellular material was substantially reduced. Where slow-vacuunl application was used, variations in volume a p p e a r e d to be caused b y the viscosity of the concentrate. I n general, concentrates of higher viscosity contained more entrained gas and expanded to a g r e a t e r extent. Where v a c u u m was applied rapidly, on the other hand, concentrates of different viscosities all expanded to about the same degree, indicating that in those cases the r a p i d application of v a c u u m was the controlling variable. RATE-OF-DISPERSIBILITY TEST
Because the structure could be altered b y v a r y i n g the degree of expansion of the concentrate, a reliable method was required for determining the rates at which the several products would disperse. To fill this need, the ease-of-dispersion method of Stone et al. (6) was studied and modified. I n the original method, 15 gm. of material were added to 90 ml. of w a t e r at f r o m 70 to 75 ° F. in a 250-ml. beaker. The m i x t u r e was stirred by hand with a teaspoon for a design a t e d time-interval, dispersing as m u c h of the material as possible without
1040
H.I.
SINNAMON ET AL
splashing the dried milk or water out of the beaker. A f t e r stirring, the mixture was filtered through a coarse (40 to 60 microns) fritted-glass Buchner funnel. with the aid of about 16 inches of vacuum. The filtrate was then diluted to 200 ml. and grams of total solids were determined in a 20-ml. aliquot. When multiplied by ten, this weight represented the dispersion value for the corresponding stirring time. Several modifications in this test were made, in an effort to more nearly standardize the filtration step f r o m one material to another, to detect smaller differences among dried milks, and to improve the precision of the method. I n the original test it was found t h a t some materials bound the fritted-glass filter, whereas others did not. This condition, in turn, varied the holdup time in filtering f r o m a few seconds for readily dispersible materials to more t h a n two minutes for materials of poorer dispersibility. This wide variation is obviously undesirable. Two modifications in the procedure brought the filtration time for all materials to within about three to 15 seconds. The first was the use of scalper filters, consisting of cones of 100-mesh screen (140 microns) and 150-mesh screen (103 microns), just ahead of the v a c u u m filter to eliminate filter binding. The second provided that instead of collecting all the filtrate for analysis, only 35 ml. should be used. Collecting only the first 35 ml. of filtrate for analysis improved the precision of the method in two other ways. First, it was questionable that all of the filtrate could have been collected, and second, the initial fraction of the filtrate was p r o b a b l y more representative of the solution immediately a f t e r stirring than the total filtrate. Thus, b y removing only the m i n i m u m of filtrate necessary for analysis, the solution was separated f r o m the undispersed material more quickly, and any f u r t h e r dispersing of solids after stirring was minimized. Danger of splashing during stirring was reduced by substituting a 300-ml. tall-form beaker for the 250-ml. beaker used in the original test. I n order to detect smaller differences among dried milks, the filter screens and beaker were chilled in ice water p r i o r to use and the t e m p e r a t u r e of the stirring water was reduced to 38 ° F. The variation in dispersibility caused by changing the water t e m p e r a t u r e f r o m 75 to 38 ° F. is illustrated ( F i g u r e 3). F o r this experiment, materials f r o m the same respective batches were used. A comparison of the curves at the two t e m p e r a t u r e s for either material illustrates the effect of t e m p e r a t u r e on dispersibility rate, and a comparison of the curves for the two materials at 38 ° F. and then at 75 ° F. illustrates t h a t smaller differenees in dispersibility can be detected by using water at 38 ° F. I n summation, the rate-of-dispersibility test as used in this work was as follows: Fifteen g r a m s of powder at room t e m p e r a t u r e were added to 90 mi. of water at 38 ° F. in a 300-ml. tall-form beaker. The mixture was stirred with a teaspoon for a predetermined time-interval, dispersing as much of the powder as possible without splashing. The mixture was then poured through a 100-mesh cone contained in a glass funnel, dropped through a 150-mesh cone contained in another glass funnel, and finally dropped onto a coarse fritted-glass Buehner
DRY WHOLE M I L K .
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1041
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STIRRING TIME
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(SECS.)
Fz~. 3. D i s p e r s i b i l i t y r a t e s f o r d r i e d whole m i l k a nd effect of w a t e r t e m p e r a t u r e .
suction filter with about 16 inches of v a c u u m on the filtering flask. A f t e r 35 ml. of filtrate had been collected, the filtration was discontinued. An aliquot of 25 ml. was added to an a l u m i n u m p a n of known weight and concentrated to a plastic state in a B r a b e n d e r Moisture Tester,: using an air current at 275 ° F. The material was then brought to dryness overnight, in a vacuum oven operating at about 194 ° F., 0.4-inch absolute pressure, and with a bleed of d r y air. Percentage moisture in the filtrate was then determined, and dispersibility was calculated as the percentage of the total solids in the dried milk which had dispersed sufficiently in the given time to pass through the filters. This calculation was carried out as follows: Let x - % solids of the dried milk y = % moisture of the filtrate F - Grams of filterable solution (assuming the 35 ml. collected was representative)
1042
~. r. SINNA~ON ET .~L
G = Grams of solids contained in F T = Grams of solids m.f.b in the 15-gram sample S -=- % of the total solids in the dried milk that had dispersed Then, F --- (90/y) 100 G=F-90 15 × and T 100 -
-
Thus, S = - ~
× 100
Six stirring times were employed and each stirring time was run in triplicate. The filtering system is sketched (Figure 4).
ESH SCREEN
iSH SCREEN
COARSE ITTED GLASS
GUUM
Fro. 4. Filteri]]g system for dispersibility-rate test. [n an experiment designed to test the precision of the dispersibility-rate procedure used in this work, three different dried milks were r u n through the dispersibility-rate test in random order, using stirring times of 20, 40, 60, 80, 100, and 120 seconds. The data (see Table 1) for solids content of the filtrates were analyzed statistically, using the analysis of variance technique. The coefficient of variability for this experiment was 1.88%. Differenees between treatment means of 0.22% solids were significant at the 0.01 level of probability. Expressed as percentage of the total solids in the dried milk which have dispersed, a mean difference of 0.22% solids in the filtrate corresponds to plus or minus 1.8% total solids at the point of maximum dispersion. Thus, this is a precise method for measuring rates of dispersibility.
DRY
WIIOLE ~[ILK.
I.
1043
TABLE I Solids contents of dried whole mille filtrates, as meas~ere of dispersibility rate
Material ........................................ Stirring time (see.) 20 40 60 80 100 120 1Viean of three replicates.
A
B
C
Total solids ~ 10.31 12.27 13.11 13.45 13.51 13.87
10.03 12.03 12.46 12.67 13.13 12.97
10.26 12.47 13.11
13.50 13.67 13.65
DISPERSIBILITY RATES OF FOAM-DRIEDWHOLE .~[ILKS B a t c h e s of m a t e r i a l of the v a r i o u s t y p e s of foam s t r u c t u r e developed b y e n t r a i n e d gas were c r u s h e d l i g h t l y t h r o u g h 20-mesh (0.034-inch) a n d 40-mesh ( 0 . 0 ] 5 - i n c h ) screens, a n d d i s p e r s i b i l i t y rates were d e t e r m i n e d , u s i n g the modified test described above. D a t a f r o m these tests are s h o w n ( T a b l e 2). S t a t i s t i c a l c o m p a r i s o n s of the d a t a were made, u s i n g the a n a l y s i s of v a r i a n c e t e c h n i q u e . These showed t h a t 20-mesh s c r e e n i n g was s u p e r i o r to 40-mesh i n all cases, t h a t t h e r e was no real difference a p p a r e n t b e t w e e n 32- a n d 72-fold exp a n s i o n , t h a t 96-fold e x p a n s i o n was p o o r e r t h a n 32- a n d 72-folds, t h a t 24-fold e x p a n s i o n was poorer t h a n the other three, a n d t h a t differences caused b y degree of e x p a n s i o n , screen size, a n d s t i r r i n g time were all s i g n i f i c a n t b e y o n d the 0.01 p r o b a b i l i t y level. I n essence, these r e s u l t s show t h a t t h e o p t i m u m d i s p e r s i b i l i t y r a t e for f o a m - d r i e d m a t e r i a l is u n a f f e c t e d b y t h e degree of e x p a n s i o n of the conc e n t r a t e over the wide r a n g e f r o m 32-fold to a t least 72-fold; t h a t t h e r e are, however, u p p e r a n d lower limits, a n d t h a t 20-mesh s c r e e n i n g is more desirable t h a n 40-mesh. BULK DENSITIES OF FOAM-DRIED WHOLE MILKS B u l k densities of the m a t e r i a l s listed i n Table 2. Were d e t e r m i n e d , i n o r d e r to e x h i b i t the effects of v o l u m e t r i c e x p a n s i o n a n d screen size. These d a t a were f o u n d b y c a r e f u l l y p l a c i n g 100 ml. of m a t e r i a l i n a 250-ml. g r a d u a t e d c y l i n d e r , w e i g h i n g the m a t e r i a l , a n d c a l c u l a t i n g its b u l k d e n s i t y u n t a m p e d . T h e n the g r a d u a t e was t a p p e d b r i s k l y u p o n a t a b l e top, u n t i l the m a t e r i a l r e a c h e d a conTABLE
2
Mean percentage dispersibilities of dried whole milks
Volumetric expansion (fold) ................................... 24 Screen size (mesh) ........ 20
32 40
20
20.7 59.3 79,3 85.2 89.0 97.2
54.8 83.6 92.2 97.2 9P.2
Stirring time (see.) 10 25 40 55 70
120
72 40
20
96 40
20
40
24.6 69.8 83.5 90.0 91.0 98.0
-79.7 91.4 89.2 96.2 98.9
-70.2 80.0 85.0 92.8 99.5
Total solids 41.0 71.6 83.9 91.5 93.4 97.2
101.9
35.6 67.4 81.0 90.2 92.8 99.9
49.0 80.2 88.7 96.3 96.2 100.1
H. I. SINNAMON E T AL
1044
TABLE 3 B~dtc densities o f dried whole m.ilks, gin. per cc.
Volumetric expansion (fold) ........................ Screen size (mesh) .... 20
24
32 40
20
72 40
20
40
20
96 - 40
Commercial product ~
Density Untamped 0.17 0.19 0.18 0.30 0.15 Tamped 0.27 0.34 0.25 0.42 0.25 = A spray-dried product available commercially.
0.20 0.35
0.13 0.22
0.28 0.50
0.5q~ 0.72
s t a n t level i n t h e g r a d u a t e , a n d its n e w v o l u m e was d e t e r m i n e d as a basis f o r c a l c u l a t i n g i t s b u l k d e n s i t y t a m p e d . These d a t a a r e shown ( T a b l e 3). T h e r e is no c o n s i s t e n t t r e n d i n these d a t a w h i c h w o u l d i n d i c a t e a v a r i a t i o n of b u l k d e n s i t y w i t h d e g r e e of e x p a n s i o n of t h e c o n c e n t r a t e , e i t h e r a t 20- or 40-mesh s c r e e n i n g . A s w o u l d be e x p e c t e d , however, t h e p o w d e r s r u n t h r o u g h t h e 20-mesh s c r e e n a r e m o r e b u l k y t h a n those r u n t h r o u g h t h e 40-mesh screen. A s s u m i n g a n a v e r a g e d t a m p e d b u l k d e n s i t y of 0.25 g m / c c f o r 20-mesh, a n d 0.43 g m / c c f o r 40-mesh, a u n i t v o l u m e of m a t e r i a l w o u l d y i e l d 2 a n d 31/2 volumes, r e s p e c t i v e l y , of r e c o n s t i t u t e d fluid milk. O T H E R C H A R A C T E R I S T I C S OF F O A M - D R I E D W H O L E
5IILKS
F o a m - d r i e d whole m i l k s c o n t a i n e d o n l y t r a c e a m o u n t s of u n d i s p e r s e d mat e r i a l w h e n r u n t h r o u g h t h e s o l u b i l i t y i n d e x test ( 1 ) . C o m p a r i s o n s of t h e w h e y p r o t e i n s of f r e s h whole m i l k a n d f r e s h l y p r e p a r e d
25 SECONDS
:56 SECONDS
87
SECONDS --USDA
240
SECONDS
phota by M. G. Audsley
FIO. 5. Comparison of self-dispersing characteristics of foam-dried and spray-dried whole milks in cold water.
DRY WHOLE I~iILK. I.
1045
f o a m - d r i e d m a t e r i a l were m a d e . I n these studies, the d r i e d w i l k was r e c o n s t i t u t e d to t h r e e t i m e s t h e c o n c e n t r a t i o n of f l u i d milk, t h e f a t s e p a r a t e d b y cent r i f u g a t i o n , t h e casein p r e c i p i t a t e d b y acidification, t h e r e s u l t i n g w h e y s o l u t i o n d i a l y z e d a g a i n s t a p h o s p h a t e b u f f e r s o l u t i o n o f p H 7.5, a n d u l t r a c e n t r i f u g a l ins p e c t i o n c a r r i e d o u t a t 25 ° C. T h e f r e s h m i l k was t r e a t e d s i m i l a r l y , e x c e p t t h a t c o n c e n t r a t i o n of t h e m a t e r i a l was c a r r i e d o u t a f t e r t h e casein h a d b e e n p r e c i p i t a t e d . U l t r a c e n t r i f u g a l p a t t e r n s s h o w e d no difference b e t w e e n t h e w h e y p r o t e i n s of t h e f r e s h m i l k a n d t h e f o a m - d r i e d m a t e r i a l . A s t r i k i n g d e m o n s t r a t i o n of t h e ease of d i s p e r s i o n of a f o a m - d r i e d whole m i l k is g i v e n ( F i g u r e 5). F o r t h i s test, two 250-ml. clear, g r a d u a t e d c y l i n d e r s w e r e filled w i t h w a t e r of a b o u t 38 ° F . The w a t e r s u r f a c e of the r i g h t - h a n d g r a d u a t e was c a r e f u l l y c o v e r e d w i t h f o a m - d r i e d m a t e r i a l a n d t h e o t h e r w i t h a s p r a y - d r i e d c o m m e r c i a l p r o d u c t . P h o t o g r a p h s w e r e t a k e n a f t e r 25, 36, 87, a n d 240 seconds. T h e s u p e r i o r d i s p e r s i n g c h a r a c t e r i s t i c s of t h e f o a m - d r i e d whole m i l k a r e e v i d e n t . SU~MARY A d r i e d whole m i l k of e x c e l l e n t f r e s h flavor a n d r a p i d d i s p e r s i b i l i t y in cold water has been prepared in a vacuum shelf drier, using special foam-drying techniques. The i m p o r t a n t v a r i a b l e s s t u d i e d i n t h e b a t c h w i s e d r i e r c a n p r o b a b l y be t r a n s l a t e d to a c o n t i n u o u s m e t h o d . A m o d i f i e d t e s t f o r d e t e r m i n i n g t h e r a t e s a t w h i c h d r i e d whole m i l k s disp e r s e h a s been s h o w n to h a v e g o o d p r e c i s i o n . U s i n g t h i s test, f o a m - d r i e d whole m i l k s w i l l c o m p l e t e l y d i s p e r s e in w a t e r of 38 ° F . w i t h i n ] 0 0 seconds w i t h m a n u a l s t i r r i n g , a n d in w a t e r of 75 ° F . w i t h i n 50 seconds. ACKNOWLEDGMENTS The authors express their appreciation to C. L. Ogg of the Eastern Regional Research Laboratory for his chemical analyses, to R. E. Townend of the Eastern Regional Research Laboratory for his nltracentrifugal work, to M. D. Fiakner of the Biometrical Services, Agricultural Research Service, for his statistical analyses, and to G. F. Thompson and H. R. Freeman of the Eastern Regional Research Laboratory for their assistance with the dispersibility test. REFERENCES
(1) AMERIGAN DRY MILK IlVSTITUTt~,INC. The Grading of Dry Whole Milk and Sanitary (2) (3) (4~ (5) (6) (7)
and Quality Standards, Including Standard Methods of Analysis. Bulletin 913 (Revised). 1955. A~0N. Introduce Citrus Dehydrate in Crystalline Form. Food Eng., 27 (2) : 162. 1955. ANOX. Superior Dehydrated Juices from Continuous Vacuum Process. Food Eng., 27 (3) : 71. 1955. FOOD ENGINEFA~INOSTAFF. Speed-Dries Heat-Sensitive Liquids. Food Eng., 26 (1): 36. 1954. H~Y~AN, W. A. U. S. Patent No. 2,328,554 (Sept. 7, 1943). S~'oN~, W. K., CONLEY, T. F., AND MCINTm~, J. M. The Influence of Lipids on Self-Dispersion and on Ease of Dispersion of Milk Powder. Food Technol., 8, 367. 1954. STRASHUN, S. I., AND TALPURT, W. F. WRRL Develops Techniques for Making Puffed Powder from Juice. Food Eng., 25 (3) : 59. 1953.
Eastern Regional Research Laboratory, ~ Philadelphia, Pennsylvania
This paper reports the properties of a new dry whole milk, dried under high vacuum and low temperatures in the form of an expanded sponge-like structure. Milk is homogenized at 145 ° F. and 2,500 p.s.i, and pasteurized at 162 ° F. for 16 seconds. It is concentrated to 47 to 50% total solids at 85 ° to 100 ° F. in a high-vacuum falling film evaporator, heated to 135 ° F. and homogenized at 4,000 and 500 p.s.i. Before the concentrate is homogenized, nitrogen is entrained in it and it is then flowed over drying pans, cooled to 55 ° F., and vacuum-dried as a foam. Finally, the dried mass is crushed lightly. This product disperses easily in cold water and has a natural flavor when reconstituted in the fresh state. Editor.
A long-range research program was recently initiated at this Laboratory, with the ultimate goal of developing a dried whole milk of easy dispersibility, good flavor, and storage stability. As presently conceived, this program will be carried forward in three phases : The first phase will be concerned with the preparation by batch methods of a dried whole milk which is initially easy to disperse and of good flavor. The second phase will include a study of the storage stability of the material and factors which influence stability. The third phase will be a translation of the essential processing conditions to a commercially feasible operation. The purpose of this paper, which deals with the first phase of the program, is to report the properties of whole milk dried in a new physical form and possessing excellent flavor and easy dispersibility in ice water. The processing variables found to be essential in its preparation are discussed. CHOICE OF THE DRYING METHOD
Puff-drying methods were investigated, because the authors believe them to be superior to spray- or roller-drying. Puff-drying has been successfully applied to heat-sensitive citrus juices and other food products in both batch and continuous processes (2-5, 7). Therefore, a systematic engineering study was made of the factors responsible for producing a product that would be easy to disperse and have good flavor. In general, puff-drying may be defined as the formation of a highly expanded, sponge-like structure of dried material from a thin film of liquid concentrate, under conditions of high vacuum and low temperature. The product would be expected to disperse rapidly because of its large surface area per unit weight. and to possess natural flavor because of the high-vacuum, low-temperature drying conditions employed. As applied to whole milk, drying at low temperatures Received for publication March 26, 1957. 1 A laboratory of the Eastern Utilization Research and Development Division, Agricultural Research Service, United States Department of Agriculture. 1036
DRY WHOLE MILK. I.
1037
would also be expected to inhibit protein destabilization, and dehydrating at low oxygen concentrations at low temperature would preclude atmospheric oxidation. BASIC CONSIDER.kTIONSIN PUFF-DRYING WHOLE 5IILK Puffed structures can be developed by various devices and, for some materials, such as those containing large amounts of sugars, the type of structure may not be of prime importance for rapid dispersibility. With whole milk, however, the structure has been found to influence markedly the dispersibility rate of the dried product. Two possible structures are illustrated (Figure 1). The puffed form on the top was developed by the evolution of water-vapor from a deaerated concentrate. This is characterized by large, nonuniform bubbles and a preponderance of unpuffed, intercellular material. Product obtained from such a form disperses very poorly compared to that shown (bottom of Figure 1). The latter was formed by the expansion of entrained gas of low solubility in the concentrated milk, and is characterized by small, uniform bubbles and a minimum of unexpanded intercellular material. To differentiate, the latter is hereafter referred to as a foam.
--USDA
photo by M. (7. A_~tdsley
FIG. 1. Typical foam structures obtained by water-vapor (top) and entrained gas of low solubility (bottom).
1038
H.i. SINNAMO~nT AT.
The mere use of an entrained gas does not insure a f o r m that will disperse readily. I n order for entrained gases to be effective in f o r m i n g a fine-grained foam, the concentrate must be held at a low t e m p e r a t u r e during the initial d r y i n g stage. A t t e m p e r a t u r e s above 55 ° F., and u n d e r the vacuums necessary for foaming, the concentrate will boil before it has expanded sufficiently, and boiling at this point will remove the gas f r o m the u n e x p a n d e d material. The dried structure will then be similar to a puff f o r m e d by water-vapor f r o m a deaerated concentrate. Initial experiments using only dissolved C02 or N20 indicate that these gases, because of their high solubility, give structures similar to the waterv a p o r type. W h e n the absolute pressure within the drier reaches the v a p o r pressure of the concentrate, vigorous agitation of the expanded s t r u c t u r e results f r o m the flashing of water-vapor. I f the desirable f o r m is to survive, it must be rigid enough to withstand this action. H e r e again, low t e m p e r a t u r e s are desirable because of the added viscosity. H i g h solids content is another factor which increases the viscosity of the concentrate, and it has been found t h a t materials containing up to about 50% solids can be readily dried as a foam. Beyond this concentration a gel forms which becomes difficult to spread evenly for drying. The conditions essential f o r producing a material of good dispersibility have been determined by experiments in a pilot-plant v a c u u m shelf drier. E x p l o r a t o r y studies conducted cooperatively with the Chain-Belt C o m p a n y 2 of Milwaukee, Wisconsin, have shown, moreover, t h a t these essential conditions can most p r o b a b l y be translated to a continuous method and thus, t h a t f o a m - d r y i n g of whole milk m a y have a f u t u r e in commerce. A public service p a t e n t is pending covering these principles. P R E P A R A T I O N OF T H E DRIED W H O L E ~ I I L K
F r e s h pasteurized, homogenized milk was obtained f r o m a local d a i r y for these studies. I t had been homogenized at 145 ° F. and 2,500 p.s.i, and then pasteurized at 162 ° F. f o r 16 seconds. I t contained 3.6 to 3.7% f a t a n d about 8.7% solids-not-fat. The milk was first concentrated to f r o m 47 to 50% total solids at a batch t e m p e r a t u r e of 85 to 100 ° F. in a high vacuum, falling-film evaporator. I t was then heated to 135 ° F. and homogenized, first at 4,000 p.s.i. and then at 500 p.s.i., through a single-stage homogenizer of the pulsator type. I m m e d i a t e l y p r i o r to homogenization, nitrogen was bubbled through the concent r a t e f r o m a fritted-glass sparger dispersing entrained gas through the material. The concentrated milk was then flowed over stainless steel d r y i n g p a n s to an average d e p t h of 1/16-inch, chilled to 55 ° F. or below, and dried as a f o a m in a v a c u u m shelf drier. The resulting dried mass was crushed lightly t h r o u g h stainless steel screens. A typical d r y i n g cycle is illustrated ( F i g u r e 2). 2Mention of companies and commercial products anywhere in the paper does not imply that they are endorsed or recommended by the Department of Agriculture over others of a similar nature not mentioned.
DR~ W~OLE ~IIL~. ~. 22-0
1039 22
A SSOLU TE PRESSURE
200 PUFFING BEGINS
180
~B E
16 E
160
f'~'\.~
~
AVERAGE PLATEN WATER TEMP,
141.U :b 12 U) (/)
140 w 12o n~ I00
~
80
i.i I1. 6(? I.I
I- 40 20
\ \
.;
LOWERCAKE TEMP.
|
,°..°°.o.°°°.o°°O~ o'oooeoooo'o'o°*
,,. I
:
\ i -" :.,.,.,,.i J .: ~..
~" . . . . . .'~"~ . . . . ......................... ./
"
,," M~Om.E CAKE TEMP. - - ~ " ~
°°'°~
~ •
,
1
/
'I
w 61-4o
~
2~ _J O3
2m ~ ' ~ - ~--'v..~. -"- ~ " ~ " " ' ~ 20 40 60 80 DRYING
, I00 TIME
r' 120 (MIN.}
, ...... 140 160
180
200
FIG. 2. Y a c u u m shelf d r y i n g cycle f o r whole milk.
I t has been found t h a t variations in foam structure were possible, depending on the r a t e of v a c u u m application. F o r instance, when the pressure inside the d r y i n g chamber was reduced f r o m 20 to 2 mm. in five minutes, the concentrate expanded r a p i d l y to about 24 times its original volume, and then became sufficiently rigid to prevent f u r t h e r expansion or collapse. The resulting structure was composed of t i n y bubbles a p p r o x i m a t e l y 1/]6-inch in diameter and had a relatively large proportion of u n e x p a n d e d intercellular material. W h e n the d r y i n g chamber pressure was reduced f r o m 20 to 2 ram. in about 20 minutes, on the other hand, the concentrate expanded slowly to a volumetric increase of f r o m 32- to 96-fold before it stabilized. Bubble sizes were larger in these structures, and the proportion of u n e x p a n d e d intercellular material was substantially reduced. Where slow-vacuunl application was used, variations in volume a p p e a r e d to be caused b y the viscosity of the concentrate. I n general, concentrates of higher viscosity contained more entrained gas and expanded to a g r e a t e r extent. Where v a c u u m was applied rapidly, on the other hand, concentrates of different viscosities all expanded to about the same degree, indicating that in those cases the r a p i d application of v a c u u m was the controlling variable. RATE-OF-DISPERSIBILITY TEST
Because the structure could be altered b y v a r y i n g the degree of expansion of the concentrate, a reliable method was required for determining the rates at which the several products would disperse. To fill this need, the ease-of-dispersion method of Stone et al. (6) was studied and modified. I n the original method, 15 gm. of material were added to 90 ml. of w a t e r at f r o m 70 to 75 ° F. in a 250-ml. beaker. The m i x t u r e was stirred by hand with a teaspoon for a design a t e d time-interval, dispersing as m u c h of the material as possible without
1040
H.I.
SINNAMON ET AL
splashing the dried milk or water out of the beaker. A f t e r stirring, the mixture was filtered through a coarse (40 to 60 microns) fritted-glass Buchner funnel. with the aid of about 16 inches of vacuum. The filtrate was then diluted to 200 ml. and grams of total solids were determined in a 20-ml. aliquot. When multiplied by ten, this weight represented the dispersion value for the corresponding stirring time. Several modifications in this test were made, in an effort to more nearly standardize the filtration step f r o m one material to another, to detect smaller differences among dried milks, and to improve the precision of the method. I n the original test it was found t h a t some materials bound the fritted-glass filter, whereas others did not. This condition, in turn, varied the holdup time in filtering f r o m a few seconds for readily dispersible materials to more t h a n two minutes for materials of poorer dispersibility. This wide variation is obviously undesirable. Two modifications in the procedure brought the filtration time for all materials to within about three to 15 seconds. The first was the use of scalper filters, consisting of cones of 100-mesh screen (140 microns) and 150-mesh screen (103 microns), just ahead of the v a c u u m filter to eliminate filter binding. The second provided that instead of collecting all the filtrate for analysis, only 35 ml. should be used. Collecting only the first 35 ml. of filtrate for analysis improved the precision of the method in two other ways. First, it was questionable that all of the filtrate could have been collected, and second, the initial fraction of the filtrate was p r o b a b l y more representative of the solution immediately a f t e r stirring than the total filtrate. Thus, b y removing only the m i n i m u m of filtrate necessary for analysis, the solution was separated f r o m the undispersed material more quickly, and any f u r t h e r dispersing of solids after stirring was minimized. Danger of splashing during stirring was reduced by substituting a 300-ml. tall-form beaker for the 250-ml. beaker used in the original test. I n order to detect smaller differences among dried milks, the filter screens and beaker were chilled in ice water p r i o r to use and the t e m p e r a t u r e of the stirring water was reduced to 38 ° F. The variation in dispersibility caused by changing the water t e m p e r a t u r e f r o m 75 to 38 ° F. is illustrated ( F i g u r e 3). F o r this experiment, materials f r o m the same respective batches were used. A comparison of the curves at the two t e m p e r a t u r e s for either material illustrates the effect of t e m p e r a t u r e on dispersibility rate, and a comparison of the curves for the two materials at 38 ° F. and then at 75 ° F. illustrates t h a t smaller differenees in dispersibility can be detected by using water at 38 ° F. I n summation, the rate-of-dispersibility test as used in this work was as follows: Fifteen g r a m s of powder at room t e m p e r a t u r e were added to 90 mi. of water at 38 ° F. in a 300-ml. tall-form beaker. The mixture was stirred with a teaspoon for a predetermined time-interval, dispersing as much of the powder as possible without splashing. The mixture was then poured through a 100-mesh cone contained in a glass funnel, dropped through a 150-mesh cone contained in another glass funnel, and finally dropped onto a coarse fritted-glass Buehner
DRY WHOLE M I L K .
too [ - 75: . . , . - -
rI // / 9o /
1041
I.
...._._
"
1lIt
C~ o
°/
//
_J
o
_J 8 0
I--
0
70
-
COMMERCIAL SPRAY-DRIED -----
602(
/
I
40
PUFF-DRIED
~
60
STIRRING TIME
l
80
I
IO0
I
120
(SECS.)
Fz~. 3. D i s p e r s i b i l i t y r a t e s f o r d r i e d whole m i l k a nd effect of w a t e r t e m p e r a t u r e .
suction filter with about 16 inches of v a c u u m on the filtering flask. A f t e r 35 ml. of filtrate had been collected, the filtration was discontinued. An aliquot of 25 ml. was added to an a l u m i n u m p a n of known weight and concentrated to a plastic state in a B r a b e n d e r Moisture Tester,: using an air current at 275 ° F. The material was then brought to dryness overnight, in a vacuum oven operating at about 194 ° F., 0.4-inch absolute pressure, and with a bleed of d r y air. Percentage moisture in the filtrate was then determined, and dispersibility was calculated as the percentage of the total solids in the dried milk which had dispersed sufficiently in the given time to pass through the filters. This calculation was carried out as follows: Let x - % solids of the dried milk y = % moisture of the filtrate F - Grams of filterable solution (assuming the 35 ml. collected was representative)
1042
~. r. SINNA~ON ET .~L
G = Grams of solids contained in F T = Grams of solids m.f.b in the 15-gram sample S -=- % of the total solids in the dried milk that had dispersed Then, F --- (90/y) 100 G=F-90 15 × and T 100 -
-
Thus, S = - ~
× 100
Six stirring times were employed and each stirring time was run in triplicate. The filtering system is sketched (Figure 4).
ESH SCREEN
iSH SCREEN
COARSE ITTED GLASS
GUUM
Fro. 4. Filteri]]g system for dispersibility-rate test. [n an experiment designed to test the precision of the dispersibility-rate procedure used in this work, three different dried milks were r u n through the dispersibility-rate test in random order, using stirring times of 20, 40, 60, 80, 100, and 120 seconds. The data (see Table 1) for solids content of the filtrates were analyzed statistically, using the analysis of variance technique. The coefficient of variability for this experiment was 1.88%. Differenees between treatment means of 0.22% solids were significant at the 0.01 level of probability. Expressed as percentage of the total solids in the dried milk which have dispersed, a mean difference of 0.22% solids in the filtrate corresponds to plus or minus 1.8% total solids at the point of maximum dispersion. Thus, this is a precise method for measuring rates of dispersibility.
DRY
WIIOLE ~[ILK.
I.
1043
TABLE I Solids contents of dried whole mille filtrates, as meas~ere of dispersibility rate
Material ........................................ Stirring time (see.) 20 40 60 80 100 120 1Viean of three replicates.
A
B
C
Total solids ~ 10.31 12.27 13.11 13.45 13.51 13.87
10.03 12.03 12.46 12.67 13.13 12.97
10.26 12.47 13.11
13.50 13.67 13.65
DISPERSIBILITY RATES OF FOAM-DRIEDWHOLE .~[ILKS B a t c h e s of m a t e r i a l of the v a r i o u s t y p e s of foam s t r u c t u r e developed b y e n t r a i n e d gas were c r u s h e d l i g h t l y t h r o u g h 20-mesh (0.034-inch) a n d 40-mesh ( 0 . 0 ] 5 - i n c h ) screens, a n d d i s p e r s i b i l i t y rates were d e t e r m i n e d , u s i n g the modified test described above. D a t a f r o m these tests are s h o w n ( T a b l e 2). S t a t i s t i c a l c o m p a r i s o n s of the d a t a were made, u s i n g the a n a l y s i s of v a r i a n c e t e c h n i q u e . These showed t h a t 20-mesh s c r e e n i n g was s u p e r i o r to 40-mesh i n all cases, t h a t t h e r e was no real difference a p p a r e n t b e t w e e n 32- a n d 72-fold exp a n s i o n , t h a t 96-fold e x p a n s i o n was p o o r e r t h a n 32- a n d 72-folds, t h a t 24-fold e x p a n s i o n was poorer t h a n the other three, a n d t h a t differences caused b y degree of e x p a n s i o n , screen size, a n d s t i r r i n g time were all s i g n i f i c a n t b e y o n d the 0.01 p r o b a b i l i t y level. I n essence, these r e s u l t s show t h a t t h e o p t i m u m d i s p e r s i b i l i t y r a t e for f o a m - d r i e d m a t e r i a l is u n a f f e c t e d b y t h e degree of e x p a n s i o n of the conc e n t r a t e over the wide r a n g e f r o m 32-fold to a t least 72-fold; t h a t t h e r e are, however, u p p e r a n d lower limits, a n d t h a t 20-mesh s c r e e n i n g is more desirable t h a n 40-mesh. BULK DENSITIES OF FOAM-DRIED WHOLE MILKS B u l k densities of the m a t e r i a l s listed i n Table 2. Were d e t e r m i n e d , i n o r d e r to e x h i b i t the effects of v o l u m e t r i c e x p a n s i o n a n d screen size. These d a t a were f o u n d b y c a r e f u l l y p l a c i n g 100 ml. of m a t e r i a l i n a 250-ml. g r a d u a t e d c y l i n d e r , w e i g h i n g the m a t e r i a l , a n d c a l c u l a t i n g its b u l k d e n s i t y u n t a m p e d . T h e n the g r a d u a t e was t a p p e d b r i s k l y u p o n a t a b l e top, u n t i l the m a t e r i a l r e a c h e d a conTABLE
2
Mean percentage dispersibilities of dried whole milks
Volumetric expansion (fold) ................................... 24 Screen size (mesh) ........ 20
32 40
20
20.7 59.3 79,3 85.2 89.0 97.2
54.8 83.6 92.2 97.2 9P.2
Stirring time (see.) 10 25 40 55 70
120
72 40
20
96 40
20
40
24.6 69.8 83.5 90.0 91.0 98.0
-79.7 91.4 89.2 96.2 98.9
-70.2 80.0 85.0 92.8 99.5
Total solids 41.0 71.6 83.9 91.5 93.4 97.2
101.9
35.6 67.4 81.0 90.2 92.8 99.9
49.0 80.2 88.7 96.3 96.2 100.1
H. I. SINNAMON E T AL
1044
TABLE 3 B~dtc densities o f dried whole m.ilks, gin. per cc.
Volumetric expansion (fold) ........................ Screen size (mesh) .... 20
24
32 40
20
72 40
20
40
20
96 - 40
Commercial product ~
Density Untamped 0.17 0.19 0.18 0.30 0.15 Tamped 0.27 0.34 0.25 0.42 0.25 = A spray-dried product available commercially.
0.20 0.35
0.13 0.22
0.28 0.50
0.5q~ 0.72
s t a n t level i n t h e g r a d u a t e , a n d its n e w v o l u m e was d e t e r m i n e d as a basis f o r c a l c u l a t i n g i t s b u l k d e n s i t y t a m p e d . These d a t a a r e shown ( T a b l e 3). T h e r e is no c o n s i s t e n t t r e n d i n these d a t a w h i c h w o u l d i n d i c a t e a v a r i a t i o n of b u l k d e n s i t y w i t h d e g r e e of e x p a n s i o n of t h e c o n c e n t r a t e , e i t h e r a t 20- or 40-mesh s c r e e n i n g . A s w o u l d be e x p e c t e d , however, t h e p o w d e r s r u n t h r o u g h t h e 20-mesh s c r e e n a r e m o r e b u l k y t h a n those r u n t h r o u g h t h e 40-mesh screen. A s s u m i n g a n a v e r a g e d t a m p e d b u l k d e n s i t y of 0.25 g m / c c f o r 20-mesh, a n d 0.43 g m / c c f o r 40-mesh, a u n i t v o l u m e of m a t e r i a l w o u l d y i e l d 2 a n d 31/2 volumes, r e s p e c t i v e l y , of r e c o n s t i t u t e d fluid milk. O T H E R C H A R A C T E R I S T I C S OF F O A M - D R I E D W H O L E
5IILKS
F o a m - d r i e d whole m i l k s c o n t a i n e d o n l y t r a c e a m o u n t s of u n d i s p e r s e d mat e r i a l w h e n r u n t h r o u g h t h e s o l u b i l i t y i n d e x test ( 1 ) . C o m p a r i s o n s of t h e w h e y p r o t e i n s of f r e s h whole m i l k a n d f r e s h l y p r e p a r e d
25 SECONDS
:56 SECONDS
87
SECONDS --USDA
240
SECONDS
phota by M. G. Audsley
FIO. 5. Comparison of self-dispersing characteristics of foam-dried and spray-dried whole milks in cold water.
DRY WHOLE I~iILK. I.
1045
f o a m - d r i e d m a t e r i a l were m a d e . I n these studies, the d r i e d w i l k was r e c o n s t i t u t e d to t h r e e t i m e s t h e c o n c e n t r a t i o n of f l u i d milk, t h e f a t s e p a r a t e d b y cent r i f u g a t i o n , t h e casein p r e c i p i t a t e d b y acidification, t h e r e s u l t i n g w h e y s o l u t i o n d i a l y z e d a g a i n s t a p h o s p h a t e b u f f e r s o l u t i o n o f p H 7.5, a n d u l t r a c e n t r i f u g a l ins p e c t i o n c a r r i e d o u t a t 25 ° C. T h e f r e s h m i l k was t r e a t e d s i m i l a r l y , e x c e p t t h a t c o n c e n t r a t i o n of t h e m a t e r i a l was c a r r i e d o u t a f t e r t h e casein h a d b e e n p r e c i p i t a t e d . U l t r a c e n t r i f u g a l p a t t e r n s s h o w e d no difference b e t w e e n t h e w h e y p r o t e i n s of t h e f r e s h m i l k a n d t h e f o a m - d r i e d m a t e r i a l . A s t r i k i n g d e m o n s t r a t i o n of t h e ease of d i s p e r s i o n of a f o a m - d r i e d whole m i l k is g i v e n ( F i g u r e 5). F o r t h i s test, two 250-ml. clear, g r a d u a t e d c y l i n d e r s w e r e filled w i t h w a t e r of a b o u t 38 ° F . The w a t e r s u r f a c e of the r i g h t - h a n d g r a d u a t e was c a r e f u l l y c o v e r e d w i t h f o a m - d r i e d m a t e r i a l a n d t h e o t h e r w i t h a s p r a y - d r i e d c o m m e r c i a l p r o d u c t . P h o t o g r a p h s w e r e t a k e n a f t e r 25, 36, 87, a n d 240 seconds. T h e s u p e r i o r d i s p e r s i n g c h a r a c t e r i s t i c s of t h e f o a m - d r i e d whole m i l k a r e e v i d e n t . SU~MARY A d r i e d whole m i l k of e x c e l l e n t f r e s h flavor a n d r a p i d d i s p e r s i b i l i t y in cold water has been prepared in a vacuum shelf drier, using special foam-drying techniques. The i m p o r t a n t v a r i a b l e s s t u d i e d i n t h e b a t c h w i s e d r i e r c a n p r o b a b l y be t r a n s l a t e d to a c o n t i n u o u s m e t h o d . A m o d i f i e d t e s t f o r d e t e r m i n i n g t h e r a t e s a t w h i c h d r i e d whole m i l k s disp e r s e h a s been s h o w n to h a v e g o o d p r e c i s i o n . U s i n g t h i s test, f o a m - d r i e d whole m i l k s w i l l c o m p l e t e l y d i s p e r s e in w a t e r of 38 ° F . w i t h i n ] 0 0 seconds w i t h m a n u a l s t i r r i n g , a n d in w a t e r of 75 ° F . w i t h i n 50 seconds. ACKNOWLEDGMENTS The authors express their appreciation to C. L. Ogg of the Eastern Regional Research Laboratory for his chemical analyses, to R. E. Townend of the Eastern Regional Research Laboratory for his nltracentrifugal work, to M. D. Fiakner of the Biometrical Services, Agricultural Research Service, for his statistical analyses, and to G. F. Thompson and H. R. Freeman of the Eastern Regional Research Laboratory for their assistance with the dispersibility test. REFERENCES
(1) AMERIGAN DRY MILK IlVSTITUTt~,INC. The Grading of Dry Whole Milk and Sanitary (2) (3) (4~ (5) (6) (7)
and Quality Standards, Including Standard Methods of Analysis. Bulletin 913 (Revised). 1955. A~0N. Introduce Citrus Dehydrate in Crystalline Form. Food Eng., 27 (2) : 162. 1955. ANOX. Superior Dehydrated Juices from Continuous Vacuum Process. Food Eng., 27 (3) : 71. 1955. FOOD ENGINEFA~INOSTAFF. Speed-Dries Heat-Sensitive Liquids. Food Eng., 26 (1): 36. 1954. H~Y~AN, W. A. U. S. Patent No. 2,328,554 (Sept. 7, 1943). S~'oN~, W. K., CONLEY, T. F., AND MCINTm~, J. M. The Influence of Lipids on Self-Dispersion and on Ease of Dispersion of Milk Powder. Food Technol., 8, 367. 1954. STRASHUN, S. I., AND TALPURT, W. F. WRRL Develops Techniques for Making Puffed Powder from Juice. Food Eng., 25 (3) : 59. 1953.