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The Sanitary Aspects of a Cream Separator Cleaned and Sterilized by Centrifugal Force
T H E SANITARY A S P E C T S OF A CREAM S E P A R A T O R C L E A N E D AND S T E R I L I Z E D BY C E N T R I F U G A L FORCE E. O. H E R R E I D
Department of Food Technology, University of Illinois,
Urbana
The results of a number of trials indicate that a centrifugal separator can be cleaned and sterilized efficiently by centrifugal action without disassembling the machine. Similar results can be expected from a factory separator. Editor.
Cleaning equipment and pipe lines without disassembling them is an accepted practice in the dairy industry in this country. The commercial clarifier and cream :separator are disassembled for cleaning. A study was made of the sanitary efficiency of cleaning and sterilizing a .cream separator by centrifugal force. The pressure in the revolving bowl was found to be 109 p.s.i.g, at the instant the solutions were released into the cream and skimmilk assembly. The description of the separator and its mechanics has been reported (2). A similar study was made of another separator (3). PROCEDURES
Cleaning. The detergents were placed in the supply tank with 21/2 gal. of water at 120-125 ° F. When the bowl attained its maximum velocity, about one-third of the solution passed through the cream and skimmilk assembly. Then the power was shut off, and the bowl velocity decreased until the flexible rubber covered steel ring relaxed. At this instant another third of the solution passed ~hrongh and the power was turned on. The bowl velocity increased, but not enough for the flexible ring to close the openings in the bowl shell, and the last ~hird of the solution passed through the machine followed by a quart of water. Finally, the cream and skimmilk assembly was dried by allowing the bowl to attain maximum velocity and then shutting off the power. The detergents used are designated by number, and their composition follows : 1. NaeCO.~, 47.6% ; Na3POd, 9.5% ; Na2SiQ, 7.6% ; NadP207, 25.7% ; wetting agent, 4.8% ; and It20, 4.8%. 2. Na3P04, 10 H_~O. 3. Sodium salt of a sulfated monoglyceride, 43%; NadP207, 9%; Na2S04, 45% ; HeO, 3%. Figures are averages of a range given by manufacturer. 4. p H neutral, sodium alkyl benzene sulfonate, NaeS04, NaC1, H20, minor ingredients. 5. Organic a c i d - composition not available. Received for publication April 16, 1956. 1629
1630
~. o. HERREID
The detergents were used in concentrations of 0.1 to 0.2%, as recommended by the manufaeturer.
Sterilization. This means practical sterilization and not tile complete destruction of all microorganisms. Two procedures were used to sterilize the separator before it was used. I n method 1, 2 ~ gal. of water was passed through the machine and discharged at 170 ° to 175 ° F., and in method 2, the same volume of water at 115 ° to 120 ° F. containing 200 p.p.m, of chlorine was used. The solutions were passed through the machine in the same way as the detergent solutions. I n some experiments the machine was sterilized only at the beginning of the experimental trial and not before every separation. The containers for the cream and skimmilk were allowed to remain in eontaet with water at 170 ° to 175 ° F. for about 5 minutes. The hardness of the water varied frmn lSa to 196 p.p.m. Bacteriological a~mlysis. The samples for bacteriological analysis were taken with sterile pipettes and placed in sterile tubes in an ieed bath and were plated in duplicate within 2 hours according to standard methods (1). The samples for plating were warmeA to 95 ° to 100 ° Ie. in a to 5 minutes, dilutions were made immediately, and the medhun was poured into the plates within 10 to 15 minutes after tile first warming.
Separation, s. Twenty galhms of milk containing 3.8 to 4.27,, fat, obtained from the Uniw, vsity l~'arln, w a s used. The milk was well mixed, and half of it was separated in the morning and the other half was held at 40 ° to 45 ° F. and separated in the late afternoon. The separations were made at 95 ° to 100 ° F. on eonseeutive days for either 31/z, 7, or 14 days, and one trial lasted for 30 days. One quart of skimmilk from eaeh separation was used to remove sediment in the bowl after the flexible ring had relaxed and was eolleeted in the cream and i~ the sldmmilk.
Determinb~g the sa~dtary efficic,Jwg of the separator. The sanitary efficiency of the separator was determined by multil)lying the plate counts by the entire volume of cream and of skimmilk, and eomparino' them with the total plate count for all of the milk. F o r example, the plate eolmt of the milk was 5,600 per ee. (Trial 1, Table 1 ), then for all of the milk it was 37,836 × 5,600 = 211.8 millions ; for the skimmilk, 33,078 × 6,800 = 244.9 millions, and for the cream 4,740 × 3,200 = 15.1 millions (Trial 1, Table 2). The volmnes used for these caleulatim,s were obtained by separating seven lots of milk of 85.5-+-0.1 lb. each, obtaining an average of 10.46 ± 0.03 lb. of ereanl and 75.04 lb. of skimmilk ealeulated by difference. The avoirdupois weights of the three products were converted to metric weights and then to metric volumes, by using specific gravities of 1.025, 1.001, and 1.028 for milk, cream, and sldmmilk, respectively. The Westphal balanee was used to estinmte specific gravity at -97"5- ~ 0.2 1~. 77.0 Determining surface areas. A centrifugal cream separator has nmeh internal surface which eontaets milk, cream, or skimmilk. The surfaces of all parts were measured and calculated to obtain the total surface. The skimming discs, the
CLEANING AND STERILIZING A SEP.~kRATOR BY CENTRIFUGAL FORCE
1631
TABLE 1 Mea~ p l a t e counts o f milk, a n d of cream and s k i m m i l k obtained f r o m it, w h e n the separator was cleaned and sterilized
Trial No.
No. of Detergent separations used
1 2 3 4 5 6 7 8 9 10
7 7 7 7 7 7 7 7 7 60
1 1 2 2 3 3 4 4 3 3
Milk
Sterilizer used c1 H~O C1 H~O C1 H.oO C1 It~O H~O/" C1
Cream
Skimmilk
Plate counts/cc. 5,600 6,700 5,200 5,000 5,100 6,000 1],900 ] 0,300 7,000 7,600
3,200 3,900 3,500 2,800 2,900 3,500 5,900 6,300 3,100 3,800
6,800 8,700 7,300 5,000 6,200 6,200 12,000 13,600 7,300 7,500
" Distilled water was used for both the detergent and sterilizing solutions. top disc, and parts of the bowl shell, bowl base, and circular part of the cream, skimmilk, and cover assembly were assumed, for the most part, to have the shape of a frustum of a right circuIar cone with larger lower and smaller upper radii. T h e t o t a l s u r f a c e o f e a c h of t h e s e p a r t s w a s c a l c u l a t e d a c c o r d i n g t o t h e e q u a t i o n : Lateral area = ~S(r =,r(r
+ R)
+ R) \/(R-
r) 2 + h e
S= ~/(R-r) 2+h e in which S = slant heio'ht h = height R a n d r = r t l d i i of b o t t o m a n d t o p , r e s p e c t i v e l y . The areas of the three holes and of the three notches in the skimming discs were d e d u c t e d . T h e l a t e r a l a r e a s a t t h e t o p a n d t h e b o t t o m of t h e d i s c s w e r e m e a s u r e d and calculated. The top disc has three ridges located at 120 ° around the circumference and they were assumed to be isosceles trapezoids. T h e c i r c u l a r p a r t o f t h e c r e a m s p o u t is a f r u s t u m w i t h t h e t o p a n d b o t t o m surfaces having the same area. The horizontal ring at the bottom has an inner a n d a n o u t e r s u r f i m e , w h i c h h a v e t h e s a m e a r e a . T h e v e r t i c a l r i n g , w h i c h fits TABLE 2 M e a n distribl~tion o f ~t~e total p l a t e eou~ts o f all the ~ilt,~, and o f all the cream and skimmilk" obtained front it, w h e n the separator was cleaned and sterilized
Trial No.
No. of separations
1 2 3 4 5
7 7 7 7 7 7 7 7 7 60
7 8 9 10
Milk 37,836 ec.
Cream 4,740 cc.
Skimmilk 33,078 cc.
Total plate count i × 106 211.8 253.5 196.7 189.2 192.9 227.0 450.2 389.7 264.8 287.5
15.1 18.5 16.6 ]3.3 13.7 16.6 28.0 29.9 14.7 18.0
244.9 287.8 241.5 165.4 205.1 205.1 396.9 449.9 241.5 248.1
Cream
Skimmilk
% of plate count of milk 7.1 7.3 8.4 7.0 7.1 7.3 6.2 7.7 5.6 6.3
106.2 113.5 122.8 87.4 106.3 90.4 88.2 115.4 91.2 86.3
1632
~. o. tIERREID
tightly into the frame of the separator, has one exposed surface. The groove for the rubber gasket has a lateral and a vertical surface. The surfaces of the skimmilk spout were calculated in essentially the same way. The spout assembly cover is the shape of a f r u s t u m with an outside groove which holds the rubber gasket and a center groove and a rubber gasket which seals the cover against the supply tank. The bowl base has a top and a bottom frustum, an inside and an outside rim,, and three posts at 120 ° intervals in the circumference to hold the discs. The bowl shell also has the shape of a frustum with top and bottom surfaces with inner' and outer horizontal and vertical bands of metal. The areas of the horizontal discharge ports on the shoulder of the shell were calculated and deducted from the total area. The metering valve assembly in the s u p p l y tank has a float, including a ring,. a stem, a rubber roller metering device, a ground valve with a horizontal rod support, a side wing, and four prongs. I t was necessary to use a planimeter to, calculate the surfaces of some of these parts. Since the supply tank and its lower gasket surface were composed of essentially circular parts of definite heights, the surfaces were not difficult to measure and calculate. The calculations are summarized in Table 5. RESULTS
The results of 10 trials and a total of 123 separations are summarized in Table ]. I n all trials the separator was sterilized immediately before it was used. Detergent 1 and chlorine sterilization were used ill Trial 1. A f t e r seven separations the u p p e r part of the skimmilk assembly had a fihu of solid material on it. The inside surface of the bowl shell and its base had an adsorbed layer of material. All discs had thin fihns of white material on their underneath surfaces. The acid detergent removed most of this material. I n Trial 2, detergent 1 and water sterilization were used. All separating parts were clean except that the underneath surfaces of the discs had very slight films, which were renloved with the acid detergent. I n Trials 3 and 4, detergent 2 and chlorine and water sterilization were used. All parts were free of solids except that the underneath surfaces of the discs had thin loosely held films, which were easily rubbed off and were removed by flushing the acid detergent t h r o u g h the separating parts. I n Trial 5 detergent 3 and chlorine sterilization, and in Trial 6 the same detergent and water sterilization were used. Again, all parts were free of milk solids except that the underneath surfaces of the discs had slight films, which were with the acid detergent. I n Trials 3 and 4, detergent 2 and chlorine and water sterilization and detergent 4 were used. The results were the same as for trials 6 and 7. I n Trial 9 distilled water was used for both the detergent and sterilizing solutions. The discs and all surfaces of the bowl were clean and free of films of solids. Trial 10 represents 60 separations for 30 consecutive days, in which detergent 2 and chlorine sterilization were used. Examination of the separator after 7 days
C L E A N I N G AND S T E R I L I Z I N G
A SEPARATOR
BY C E N T R I F U G A L
1633
FORCE
s h o w e d t h a t all p a r t s were c l e a n e x c e p t t h e u n d e r n e a t h s u r f a c e s of t h e discs, w h i c h h a d v e r y s l i g h t films. A n e x a m i n a t i o n of all p a r t s a f t e r o p e r a t i n g t h e m a c h i n e f o r 14 d a y s s h o w e d a b o u t the s a m e c o n d i t i o n s as w e r e o b s e r v e d a f t e r 7 d a y s , e x c e p t f o r some l i g h t l y a d s o r b e d solids a r o u n d t h e l o w e r edge of t h e b o w l shell a n d i n t h e u p p e r p a r t o f t h e s k i m m i l k . A l l p a r t s w e r e e x a m i n e d a g a i n a f t e r 21 d a y s , a n d a s m a l l s p o t of solid m a t e r i a l was f o u n d on t h e c e n t e r of t h e c r e a m assembly. T h e discs h a d s l i g h t films of a d s o r b e d m a t e r i a l s on t h e i r u n d e r n e a t h s u r f a c e s . A f t e r 30 d a y s t h i s t r i a l was c o n c l u d e d a n d all p a r t s w e r e e x a m i n e d . T h e r e was a s l i g h t l a y e r of l i g h t l y a d s o r b e d m a t e r i a l on t h e u n d e r n e a t h s u r f a c e s o f t h e discs. S m a l l specks of solids a d h e r e d to t h e c e n t e r r i n g of t h e c r e a m assembly. T h e m i l k a n d c r e a m s p o u t s h a d s m a l l a r e a s of l i g h t l y a d s o r b e d s o l i d s in t h e i r i n n e r s u r f a c e s . The b o w l shell was clean, b u t t h e b o w l b a s e h a d some solid m a t e r i a l w h e r e the steel r i n g s n a p s i n t o t h e g r o o v e ; h o w e v e r , t h i s is n o t in the p a t h of c r e a m or s k i m m i l k . The r u b b e r g a s k e t s w e r e c l e a n a n d odorless. F l u s h i n g 2~/2 gal. of a n a c i d d e t e r g e n t t h r o u g h t h e m a c h i n e r e m o v e d a d s o r b e d solids on a l l s u r f a c e s . I n s p i t e of t h e i n h e r e n t v a r i a b i l i t y of t h e p l a t e c o u n t m e t h o d , t h e r e s u l t s i n T a b l e 2 i n d i c a t e f a i r a g r e e m e n t in c o m p a r i n g t o t a l p l a t e c o u n t s in t h e c r e a m a n d s k i m m i l k w i t h those i n t h e m i l k . I t c a n be a s s u m e d t h a t c o n t a m i n a t i o n f r o m t h e m a c h i n e was n e g l i g i b l e . F u r t h e r m o r e , the p l a t e c o u n t s f o r a l l o f t h e c r e a m v a r i e d w i t h i n o n l y 5.6 to 8.4% of those f o r all of t h e mi]k. T h e p l a t e c o u n t s p e r cubic c e n t i m e t e r of t h e c r e a m w e r e s l i g h t l y m o r e t h a n one h a l f those f o r t h e m i l k ( T a b l e 1). T h i s c r i t e r i o n f o r e v a l u a t i n g t h e c l e a n i n g efficiency of t h e s e p a r a t o r was c o m p a r e d in t r i a l s w h e r e t h e m a c h i n e was n o t s t e r i l i z e d b e f o r e each s e p a r a TABLE 3 M e a n p l a t e counts o / the ~dlk, and of cream and s~immilTc obtained f r o m it, w h e n the separator was cleaned but n o t s~er~lizcd
Trial No.
No. of separatio;~s
11 12 13 14 15 16
14 14 14 14 14 14
Milk
Cream
Skimmilk
Plate count/cc. 13,500 8,500 24,300 59,000 78,000 113,000
] 5,900 10,100 31,800 63,000 91,600 205,000
18,900 9,700 30,200 55,000
105,000 180,000
TABLE 4 M e a n d i s t r i b u t i o n of the total p l a t e counts of all the milk, and of all the cream and s k i m m i l k obtained f r o m it, w h e n the separator was cleaned b~tt not sterilized
Trial No.
No. of separations
11 12 13 14 15 16
14 14 14 14 14 14
Milk 37,836 cc.
Cream 4,740 ec.
Skimmilk 33,078 ec.
Total plate counts I × 106 510.8 321.6 919.4 2,232.2 2,051.2 4,275.5
753.7 478.7 1,507.3 2,986.2 4.34i.8 9,717.0
625.2 320.8 998.9 1,852.4 3,473.2 5,954.0
Cream
Skimmilk
% of plate count of milk 147.5 148.8 163.9 133.8 147.1 227.3
122.4 99.8 108.6 83.0 117.7 139.3
E. O.
1634
I-IEI~REID
TABLE 5 The area of all parts of the centrifugal separator that contact mill6 cream, or s~immil~
Surface area
Name of part
(cm. 2)
Bowl base Bow1 shell Skimming discs, 22 Upper disc Cream collecting assembly Skim collecting assembly Cover, skim assembly ]Ketering assembly :Milk t'ud¢
401.9 557.0 3,201.0 359.5 1,353.8 1,200.4 636.3 380.5 1,915.7 10,006.1 era.-"= 1,550.9 in. 2 --10.8 ft?
tion. I n these t r i a l s ( T a b l e 3) t h e p l a t e c o u n t s of the c r e a m a n d s k i m m i l k exceeded those of the m i l k in p r a c t i c a l l y all cases a n d t h e c a l c u l a t e d d i s t r i b u t i o n of the t o t a l p l a t e counts in b o t h p r o d u c t s ( T a b l e 4) i n d i c a t e d c o n s i d e r a b l e c o n t a m i n a t i o n f r o m the m a c h i n e . DISCUSSION The r e s u l t s of this s t u d y i n d i c a t e t h a t t h e s e p a r a t o r can be c l e a n e d a n d s t e r i l i z e d efficiently b y c e n t r i f u g a l f o r c e a n d t h a t s i m i l a r r e s u l t s c a n be exp e c t e d f r o m a f a c t o r y s e p a r a t o r , p r o v i d e d such e q u i p m e n t becomes a v a i l a b l e . The a d s o r p t i o n of t h i n films, p r e s u m a b l y r e s i d u e s f r o m d e t e r g e n t s a n d f r o m salts in t h e w a t e r , d i d not affect t h e b a c t e r i o l o g i c a l q u a l i t y of t h e c r e a m a n d s k i m m i l k . A f t e r r i n s i n g a n d w a s h i n g , e x c e p t in T r i a l 9 ( T a b l e 1), t h e r e was a l w a y s a s l i g h t r e s i d u e w h i c h h a d to be r e m o v e d w i t h t h e a c i d d e t e r g e n t . T h e r e w a s no r e s i d u e a f t e r c o m p l e t i n g T r i a l 9, s u g g e s t i n g t h a t s a l t s in the w a t e r m a y h a v e been p a r t l y r e s p o n s i b l e f o r this c o n d i t i o n . I t is s u g g e s t e d t h a t a n a c i d d e t e r g e n t be u s e d daily after washing with the alkaline detergent. T h e s t e p w i s e p r o c e d u r e f o r c l e a n i n g a n d s t e r i l i z i n g the s e p a r a t o r s h o u l d be f o l l o w e d in d e t a i l a n d be done i m m e d i a t e l y a f t e r c o m p l e t i n g e v e r y s e p a r a t i o n because t h e r e v o l v i n g bowl t e n d s to r e m o v e m o i s t u r e a n d t h e r e b y leave, on t h e m e t a l s u r f a c e s , m i l k solids w h i c h m a k e c l e a n i n g m o r e difficult if d e l a y e d . SUMMARY AND CONCLUSIONS The c r e a m s e p a r a t o r can be c l e a n e d a n d s t e r i l i z e d b y t h e c e n t r i f u g a l force of a bowl r e v o l v i n g at a v e l o c i t y of 5,024 r.p.m., r e l e a s i n g s o l u t i o n s i n t o t h e c r e a m a n d s k i m m i l k a s s e l n b l y a t a c a l c u l a t e d p r e s s u r e of 109 p.s.i.g. REFERENCES
(1) AMEtClCANPUBLIC I~b;ALTIIASSOCIATIOn. Sta~tdard Methods for the Exa~nination of Dairy Products. 9th ed. Pub. Health Assoc. 1948. (2) lt~Rm-D, E. O., AND SV~SIKA, E. Tile Mechanics of a Cream Separator Cleaned and Sterilized by Centrifugal Force. J. Dairy S~i., 39: 1623. 1956. (3) HERBEID, E. O., TRACY, P. H., HVSSONG, R. V., AND TUCKEY, S. L. A Study of a Cream Separator Which Is Cleaned by Centrifugal Flushing. J. Dairy Sci., 30: 351. 1947.
Department of Food Technology, University of Illinois,
Urbana
The results of a number of trials indicate that a centrifugal separator can be cleaned and sterilized efficiently by centrifugal action without disassembling the machine. Similar results can be expected from a factory separator. Editor.
Cleaning equipment and pipe lines without disassembling them is an accepted practice in the dairy industry in this country. The commercial clarifier and cream :separator are disassembled for cleaning. A study was made of the sanitary efficiency of cleaning and sterilizing a .cream separator by centrifugal force. The pressure in the revolving bowl was found to be 109 p.s.i.g, at the instant the solutions were released into the cream and skimmilk assembly. The description of the separator and its mechanics has been reported (2). A similar study was made of another separator (3). PROCEDURES
Cleaning. The detergents were placed in the supply tank with 21/2 gal. of water at 120-125 ° F. When the bowl attained its maximum velocity, about one-third of the solution passed through the cream and skimmilk assembly. Then the power was shut off, and the bowl velocity decreased until the flexible rubber covered steel ring relaxed. At this instant another third of the solution passed ~hrongh and the power was turned on. The bowl velocity increased, but not enough for the flexible ring to close the openings in the bowl shell, and the last ~hird of the solution passed through the machine followed by a quart of water. Finally, the cream and skimmilk assembly was dried by allowing the bowl to attain maximum velocity and then shutting off the power. The detergents used are designated by number, and their composition follows : 1. NaeCO.~, 47.6% ; Na3POd, 9.5% ; Na2SiQ, 7.6% ; NadP207, 25.7% ; wetting agent, 4.8% ; and It20, 4.8%. 2. Na3P04, 10 H_~O. 3. Sodium salt of a sulfated monoglyceride, 43%; NadP207, 9%; Na2S04, 45% ; HeO, 3%. Figures are averages of a range given by manufacturer. 4. p H neutral, sodium alkyl benzene sulfonate, NaeS04, NaC1, H20, minor ingredients. 5. Organic a c i d - composition not available. Received for publication April 16, 1956. 1629
1630
~. o. HERREID
The detergents were used in concentrations of 0.1 to 0.2%, as recommended by the manufaeturer.
Sterilization. This means practical sterilization and not tile complete destruction of all microorganisms. Two procedures were used to sterilize the separator before it was used. I n method 1, 2 ~ gal. of water was passed through the machine and discharged at 170 ° to 175 ° F., and in method 2, the same volume of water at 115 ° to 120 ° F. containing 200 p.p.m, of chlorine was used. The solutions were passed through the machine in the same way as the detergent solutions. I n some experiments the machine was sterilized only at the beginning of the experimental trial and not before every separation. The containers for the cream and skimmilk were allowed to remain in eontaet with water at 170 ° to 175 ° F. for about 5 minutes. The hardness of the water varied frmn lSa to 196 p.p.m. Bacteriological a~mlysis. The samples for bacteriological analysis were taken with sterile pipettes and placed in sterile tubes in an ieed bath and were plated in duplicate within 2 hours according to standard methods (1). The samples for plating were warmeA to 95 ° to 100 ° Ie. in a to 5 minutes, dilutions were made immediately, and the medhun was poured into the plates within 10 to 15 minutes after tile first warming.
Separation, s. Twenty galhms of milk containing 3.8 to 4.27,, fat, obtained from the Uniw, vsity l~'arln, w a s used. The milk was well mixed, and half of it was separated in the morning and the other half was held at 40 ° to 45 ° F. and separated in the late afternoon. The separations were made at 95 ° to 100 ° F. on eonseeutive days for either 31/z, 7, or 14 days, and one trial lasted for 30 days. One quart of skimmilk from eaeh separation was used to remove sediment in the bowl after the flexible ring had relaxed and was eolleeted in the cream and i~ the sldmmilk.
Determinb~g the sa~dtary efficic,Jwg of the separator. The sanitary efficiency of the separator was determined by multil)lying the plate counts by the entire volume of cream and of skimmilk, and eomparino' them with the total plate count for all of the milk. F o r example, the plate eolmt of the milk was 5,600 per ee. (Trial 1, Table 1 ), then for all of the milk it was 37,836 × 5,600 = 211.8 millions ; for the skimmilk, 33,078 × 6,800 = 244.9 millions, and for the cream 4,740 × 3,200 = 15.1 millions (Trial 1, Table 2). The volmnes used for these caleulatim,s were obtained by separating seven lots of milk of 85.5-+-0.1 lb. each, obtaining an average of 10.46 ± 0.03 lb. of ereanl and 75.04 lb. of skimmilk ealeulated by difference. The avoirdupois weights of the three products were converted to metric weights and then to metric volumes, by using specific gravities of 1.025, 1.001, and 1.028 for milk, cream, and sldmmilk, respectively. The Westphal balanee was used to estinmte specific gravity at -97"5- ~ 0.2 1~. 77.0 Determining surface areas. A centrifugal cream separator has nmeh internal surface which eontaets milk, cream, or skimmilk. The surfaces of all parts were measured and calculated to obtain the total surface. The skimming discs, the
CLEANING AND STERILIZING A SEP.~kRATOR BY CENTRIFUGAL FORCE
1631
TABLE 1 Mea~ p l a t e counts o f milk, a n d of cream and s k i m m i l k obtained f r o m it, w h e n the separator was cleaned and sterilized
Trial No.
No. of Detergent separations used
1 2 3 4 5 6 7 8 9 10
7 7 7 7 7 7 7 7 7 60
1 1 2 2 3 3 4 4 3 3
Milk
Sterilizer used c1 H~O C1 H~O C1 H.oO C1 It~O H~O/" C1
Cream
Skimmilk
Plate counts/cc. 5,600 6,700 5,200 5,000 5,100 6,000 1],900 ] 0,300 7,000 7,600
3,200 3,900 3,500 2,800 2,900 3,500 5,900 6,300 3,100 3,800
6,800 8,700 7,300 5,000 6,200 6,200 12,000 13,600 7,300 7,500
" Distilled water was used for both the detergent and sterilizing solutions. top disc, and parts of the bowl shell, bowl base, and circular part of the cream, skimmilk, and cover assembly were assumed, for the most part, to have the shape of a frustum of a right circuIar cone with larger lower and smaller upper radii. T h e t o t a l s u r f a c e o f e a c h of t h e s e p a r t s w a s c a l c u l a t e d a c c o r d i n g t o t h e e q u a t i o n : Lateral area = ~S(r =,r(r
+ R)
+ R) \/(R-
r) 2 + h e
S= ~/(R-r) 2+h e in which S = slant heio'ht h = height R a n d r = r t l d i i of b o t t o m a n d t o p , r e s p e c t i v e l y . The areas of the three holes and of the three notches in the skimming discs were d e d u c t e d . T h e l a t e r a l a r e a s a t t h e t o p a n d t h e b o t t o m of t h e d i s c s w e r e m e a s u r e d and calculated. The top disc has three ridges located at 120 ° around the circumference and they were assumed to be isosceles trapezoids. T h e c i r c u l a r p a r t o f t h e c r e a m s p o u t is a f r u s t u m w i t h t h e t o p a n d b o t t o m surfaces having the same area. The horizontal ring at the bottom has an inner a n d a n o u t e r s u r f i m e , w h i c h h a v e t h e s a m e a r e a . T h e v e r t i c a l r i n g , w h i c h fits TABLE 2 M e a n distribl~tion o f ~t~e total p l a t e eou~ts o f all the ~ilt,~, and o f all the cream and skimmilk" obtained front it, w h e n the separator was cleaned and sterilized
Trial No.
No. of separations
1 2 3 4 5
7 7 7 7 7 7 7 7 7 60
7 8 9 10
Milk 37,836 ec.
Cream 4,740 cc.
Skimmilk 33,078 cc.
Total plate count i × 106 211.8 253.5 196.7 189.2 192.9 227.0 450.2 389.7 264.8 287.5
15.1 18.5 16.6 ]3.3 13.7 16.6 28.0 29.9 14.7 18.0
244.9 287.8 241.5 165.4 205.1 205.1 396.9 449.9 241.5 248.1
Cream
Skimmilk
% of plate count of milk 7.1 7.3 8.4 7.0 7.1 7.3 6.2 7.7 5.6 6.3
106.2 113.5 122.8 87.4 106.3 90.4 88.2 115.4 91.2 86.3
1632
~. o. tIERREID
tightly into the frame of the separator, has one exposed surface. The groove for the rubber gasket has a lateral and a vertical surface. The surfaces of the skimmilk spout were calculated in essentially the same way. The spout assembly cover is the shape of a f r u s t u m with an outside groove which holds the rubber gasket and a center groove and a rubber gasket which seals the cover against the supply tank. The bowl base has a top and a bottom frustum, an inside and an outside rim,, and three posts at 120 ° intervals in the circumference to hold the discs. The bowl shell also has the shape of a frustum with top and bottom surfaces with inner' and outer horizontal and vertical bands of metal. The areas of the horizontal discharge ports on the shoulder of the shell were calculated and deducted from the total area. The metering valve assembly in the s u p p l y tank has a float, including a ring,. a stem, a rubber roller metering device, a ground valve with a horizontal rod support, a side wing, and four prongs. I t was necessary to use a planimeter to, calculate the surfaces of some of these parts. Since the supply tank and its lower gasket surface were composed of essentially circular parts of definite heights, the surfaces were not difficult to measure and calculate. The calculations are summarized in Table 5. RESULTS
The results of 10 trials and a total of 123 separations are summarized in Table ]. I n all trials the separator was sterilized immediately before it was used. Detergent 1 and chlorine sterilization were used ill Trial 1. A f t e r seven separations the u p p e r part of the skimmilk assembly had a fihu of solid material on it. The inside surface of the bowl shell and its base had an adsorbed layer of material. All discs had thin fihns of white material on their underneath surfaces. The acid detergent removed most of this material. I n Trial 2, detergent 1 and water sterilization were used. All separating parts were clean except that the underneath surfaces of the discs had very slight films, which were renloved with the acid detergent. I n Trials 3 and 4, detergent 2 and chlorine and water sterilization were used. All parts were free of solids except that the underneath surfaces of the discs had thin loosely held films, which were easily rubbed off and were removed by flushing the acid detergent t h r o u g h the separating parts. I n Trial 5 detergent 3 and chlorine sterilization, and in Trial 6 the same detergent and water sterilization were used. Again, all parts were free of milk solids except that the underneath surfaces of the discs had slight films, which were with the acid detergent. I n Trials 3 and 4, detergent 2 and chlorine and water sterilization and detergent 4 were used. The results were the same as for trials 6 and 7. I n Trial 9 distilled water was used for both the detergent and sterilizing solutions. The discs and all surfaces of the bowl were clean and free of films of solids. Trial 10 represents 60 separations for 30 consecutive days, in which detergent 2 and chlorine sterilization were used. Examination of the separator after 7 days
C L E A N I N G AND S T E R I L I Z I N G
A SEPARATOR
BY C E N T R I F U G A L
1633
FORCE
s h o w e d t h a t all p a r t s were c l e a n e x c e p t t h e u n d e r n e a t h s u r f a c e s of t h e discs, w h i c h h a d v e r y s l i g h t films. A n e x a m i n a t i o n of all p a r t s a f t e r o p e r a t i n g t h e m a c h i n e f o r 14 d a y s s h o w e d a b o u t the s a m e c o n d i t i o n s as w e r e o b s e r v e d a f t e r 7 d a y s , e x c e p t f o r some l i g h t l y a d s o r b e d solids a r o u n d t h e l o w e r edge of t h e b o w l shell a n d i n t h e u p p e r p a r t o f t h e s k i m m i l k . A l l p a r t s w e r e e x a m i n e d a g a i n a f t e r 21 d a y s , a n d a s m a l l s p o t of solid m a t e r i a l was f o u n d on t h e c e n t e r of t h e c r e a m assembly. T h e discs h a d s l i g h t films of a d s o r b e d m a t e r i a l s on t h e i r u n d e r n e a t h s u r f a c e s . A f t e r 30 d a y s t h i s t r i a l was c o n c l u d e d a n d all p a r t s w e r e e x a m i n e d . T h e r e was a s l i g h t l a y e r of l i g h t l y a d s o r b e d m a t e r i a l on t h e u n d e r n e a t h s u r f a c e s o f t h e discs. S m a l l specks of solids a d h e r e d to t h e c e n t e r r i n g of t h e c r e a m assembly. T h e m i l k a n d c r e a m s p o u t s h a d s m a l l a r e a s of l i g h t l y a d s o r b e d s o l i d s in t h e i r i n n e r s u r f a c e s . The b o w l shell was clean, b u t t h e b o w l b a s e h a d some solid m a t e r i a l w h e r e the steel r i n g s n a p s i n t o t h e g r o o v e ; h o w e v e r , t h i s is n o t in the p a t h of c r e a m or s k i m m i l k . The r u b b e r g a s k e t s w e r e c l e a n a n d odorless. F l u s h i n g 2~/2 gal. of a n a c i d d e t e r g e n t t h r o u g h t h e m a c h i n e r e m o v e d a d s o r b e d solids on a l l s u r f a c e s . I n s p i t e of t h e i n h e r e n t v a r i a b i l i t y of t h e p l a t e c o u n t m e t h o d , t h e r e s u l t s i n T a b l e 2 i n d i c a t e f a i r a g r e e m e n t in c o m p a r i n g t o t a l p l a t e c o u n t s in t h e c r e a m a n d s k i m m i l k w i t h those i n t h e m i l k . I t c a n be a s s u m e d t h a t c o n t a m i n a t i o n f r o m t h e m a c h i n e was n e g l i g i b l e . F u r t h e r m o r e , the p l a t e c o u n t s f o r a l l o f t h e c r e a m v a r i e d w i t h i n o n l y 5.6 to 8.4% of those f o r all of t h e mi]k. T h e p l a t e c o u n t s p e r cubic c e n t i m e t e r of t h e c r e a m w e r e s l i g h t l y m o r e t h a n one h a l f those f o r t h e m i l k ( T a b l e 1). T h i s c r i t e r i o n f o r e v a l u a t i n g t h e c l e a n i n g efficiency of t h e s e p a r a t o r was c o m p a r e d in t r i a l s w h e r e t h e m a c h i n e was n o t s t e r i l i z e d b e f o r e each s e p a r a TABLE 3 M e a n p l a t e counts o / the ~dlk, and of cream and s~immilTc obtained f r o m it, w h e n the separator was cleaned but n o t s~er~lizcd
Trial No.
No. of separatio;~s
11 12 13 14 15 16
14 14 14 14 14 14
Milk
Cream
Skimmilk
Plate count/cc. 13,500 8,500 24,300 59,000 78,000 113,000
] 5,900 10,100 31,800 63,000 91,600 205,000
18,900 9,700 30,200 55,000
105,000 180,000
TABLE 4 M e a n d i s t r i b u t i o n of the total p l a t e counts of all the milk, and of all the cream and s k i m m i l k obtained f r o m it, w h e n the separator was cleaned b~tt not sterilized
Trial No.
No. of separations
11 12 13 14 15 16
14 14 14 14 14 14
Milk 37,836 cc.
Cream 4,740 ec.
Skimmilk 33,078 ec.
Total plate counts I × 106 510.8 321.6 919.4 2,232.2 2,051.2 4,275.5
753.7 478.7 1,507.3 2,986.2 4.34i.8 9,717.0
625.2 320.8 998.9 1,852.4 3,473.2 5,954.0
Cream
Skimmilk
% of plate count of milk 147.5 148.8 163.9 133.8 147.1 227.3
122.4 99.8 108.6 83.0 117.7 139.3
E. O.
1634
I-IEI~REID
TABLE 5 The area of all parts of the centrifugal separator that contact mill6 cream, or s~immil~
Surface area
Name of part
(cm. 2)
Bowl base Bow1 shell Skimming discs, 22 Upper disc Cream collecting assembly Skim collecting assembly Cover, skim assembly ]Ketering assembly :Milk t'ud¢
401.9 557.0 3,201.0 359.5 1,353.8 1,200.4 636.3 380.5 1,915.7 10,006.1 era.-"= 1,550.9 in. 2 --10.8 ft?
tion. I n these t r i a l s ( T a b l e 3) t h e p l a t e c o u n t s of the c r e a m a n d s k i m m i l k exceeded those of the m i l k in p r a c t i c a l l y all cases a n d t h e c a l c u l a t e d d i s t r i b u t i o n of the t o t a l p l a t e counts in b o t h p r o d u c t s ( T a b l e 4) i n d i c a t e d c o n s i d e r a b l e c o n t a m i n a t i o n f r o m the m a c h i n e . DISCUSSION The r e s u l t s of this s t u d y i n d i c a t e t h a t t h e s e p a r a t o r can be c l e a n e d a n d s t e r i l i z e d efficiently b y c e n t r i f u g a l f o r c e a n d t h a t s i m i l a r r e s u l t s c a n be exp e c t e d f r o m a f a c t o r y s e p a r a t o r , p r o v i d e d such e q u i p m e n t becomes a v a i l a b l e . The a d s o r p t i o n of t h i n films, p r e s u m a b l y r e s i d u e s f r o m d e t e r g e n t s a n d f r o m salts in t h e w a t e r , d i d not affect t h e b a c t e r i o l o g i c a l q u a l i t y of t h e c r e a m a n d s k i m m i l k . A f t e r r i n s i n g a n d w a s h i n g , e x c e p t in T r i a l 9 ( T a b l e 1), t h e r e was a l w a y s a s l i g h t r e s i d u e w h i c h h a d to be r e m o v e d w i t h t h e a c i d d e t e r g e n t . T h e r e w a s no r e s i d u e a f t e r c o m p l e t i n g T r i a l 9, s u g g e s t i n g t h a t s a l t s in the w a t e r m a y h a v e been p a r t l y r e s p o n s i b l e f o r this c o n d i t i o n . I t is s u g g e s t e d t h a t a n a c i d d e t e r g e n t be u s e d daily after washing with the alkaline detergent. T h e s t e p w i s e p r o c e d u r e f o r c l e a n i n g a n d s t e r i l i z i n g the s e p a r a t o r s h o u l d be f o l l o w e d in d e t a i l a n d be done i m m e d i a t e l y a f t e r c o m p l e t i n g e v e r y s e p a r a t i o n because t h e r e v o l v i n g bowl t e n d s to r e m o v e m o i s t u r e a n d t h e r e b y leave, on t h e m e t a l s u r f a c e s , m i l k solids w h i c h m a k e c l e a n i n g m o r e difficult if d e l a y e d . SUMMARY AND CONCLUSIONS The c r e a m s e p a r a t o r can be c l e a n e d a n d s t e r i l i z e d b y t h e c e n t r i f u g a l force of a bowl r e v o l v i n g at a v e l o c i t y of 5,024 r.p.m., r e l e a s i n g s o l u t i o n s i n t o t h e c r e a m a n d s k i m m i l k a s s e l n b l y a t a c a l c u l a t e d p r e s s u r e of 109 p.s.i.g. REFERENCES
(1) AMEtClCANPUBLIC I~b;ALTIIASSOCIATIOn. Sta~tdard Methods for the Exa~nination of Dairy Products. 9th ed. Pub. Health Assoc. 1948. (2) lt~Rm-D, E. O., AND SV~SIKA, E. Tile Mechanics of a Cream Separator Cleaned and Sterilized by Centrifugal Force. J. Dairy S~i., 39: 1623. 1956. (3) HERBEID, E. O., TRACY, P. H., HVSSONG, R. V., AND TUCKEY, S. L. A Study of a Cream Separator Which Is Cleaned by Centrifugal Flushing. J. Dairy Sci., 30: 351. 1947.