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The quality of skim-milk powder produced from raw milk stored at 2°C
Food Microbiology, 1988, 5, 89-96
The quality of skim-milk powder produced from raw milk stored at 2°C M. W. Griffiths*, J. D. Phillips, I. G. West, A. W. M. Sweetsur and D. D. Muir Hannah Research Institute, A yr KA6 5HL, Scotland Received 3 February 1988 Low-heat skimmed milk powder was manufactured from raw farm bulk tank and creamery silo milk which had been stored at 2°C for 24 and 72 h. During the storage period the psychrotroph count increased by about I log cycle after 24 h and 2 log cycles after 72 h. There was no increase in thermoduric or spore count of the milk under these storage conditions. The powder manufactured from these milks was of good bacteriological quality and conformed to ADMI recommendations regarding moisture content, titratable acidity and solubility. Storage of raw milk at 2°C had no detrimental effect on the heat stability of the powder manufactured from it when reconstituted to both 9 and 22% total solids concentrations.
Introduction Many factors, including alternate-day collection, the introduction of a five day working week and adoption of quotas by the EEC, have led to raw milk being stored for extended periods before processing. There is considerable interest, therefore, in ways of retarding the growth of psychrotrophic bacteria in raw milk to gain a significant increase in the storage life. Bacterial growth in milk can be inhibited by reducing the storage temperature (Swarthing 1967, Phillips and Griffiths 1987, Griffiths et al. 1987). A significant extension of the storage life (about 75%) was obtained by decreasing the temperature from 6 to 2°C (Griffiths et al. 1987). Such t r e a t m e n t had no practical effect on the quality of Cheddar cheese (Banks et al. 1988) or pasteurized and UHT milks produced (Griffiths et al. 1988). However, a relation was found *To whom correspondence should addressed. 0740-0020/88/020089 + 08 $02.00/0
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between the bacterial load in raw milk and the observed decline in protein stability of reconstituted milk powders on heating (Muir et al. 1986). Low-heat powders seem particularly susceptible to changes in the bacteriological quality of the raw milks from which they are manufactured (Lovell 1981, Muir et al. 1986). This paper examines the potential of storing raw milk at 2°C to extend the storage life of raw milk for the manufacture of milk powder in contrast to the more usual temperatures of 5 to 6°C encountered for raw milk storage in the UK at present.
Materials and Methods Milk samples and storage Raw milks from the Hannah Research Institute farm bulk tank (5 samples) and from the silos of a local creamery (4 samples) were transported to the Institute in churns. The temperature on receipt was noted. The raw milk was stored in churns (45 litres) at 2°C and the temperature monitored throughout its storage period using a Squirrel SQ8-4UAD temperature logger (Grant Instruments). Using this procedure the milk temperature (~) 1988 AcademicPress Limited
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could be maintained at 2° + 0.5°C. There was no agitation or mixing of the milk during storage except during sampling and processing when the milk was manually mixed using a plunger.
Powder production Raw milk which had previously been stored at 2°C for 1 or 3 days, was warmed to 45°C in a steam jacketed tank and separated using a small, open cream separator. The skimmed milk was pasteurized at 74°C for 16 s (lowheat treatment) and concentrated to 45% total solids (TS) in a falling film evaporator (Anhydro L ~ , Copenhagen, Denmark) with an evaporative capacity of 125 litres of water h-1 and a calandria temperature of less than 55°C. After concentration, the milk was spray dried in an experimental, tall-form spray drier fitted with nozzle atomization and heated by direct-gas firing (evaporative capacity 35 kg h-l; inlet air temperature 190°C; outlet air temperature 95°C). The dried skimmed milk was stored in airtight, sterile containers at 30°C for two weeks prior to analysis. This facilitated sample handling.
Analysis of some physical and chemical parameters of powders The moisture content of powders was estimated by drying to constant weight in a fan-assisted oven at 102°C. Powder solubility was estimated by an insolubility index method similar to that of the American Dry Milk Institute (ADMI) (1971). This method relies on determining the volume of sediment formed after centrifuging reconstituted milk powder (10% w/v). Titratable acidity was measured as described by Case et al. (1985). Non-fat dry milk (10 g) was mixed carefully with water (100 ml) to avoid foaming. After standing for 1 h at ambient temperature, an aliquot (18 g) was withdrawn after mixing and titrated against sodium hydroxide (0.1N) using phenophthalein (0-5 ml of 1% w/v solution in 95% ethanol) as an indicator. Acidity is expressed as % lactic acid. The heat stability of powders, reconstituted to 9% and 22% solids with distilled water, was determined by the time taken for visible clots to appear when the milks were heated at 140°C and 120°C, respectively. The pH of the milk was adjusted by the addition of HC1 or NaOH and the coagulation time (CT)-pH profiles determined by the method of Sweetsur and White (1974). Heat stability was
reported as the CT before pH adjustment (natural CT) and the maximum CT attained after pH adjustment.
Bacteriological analysis Raw milk. Samples were plated onto milk agar (Oxoid) plates using a Spiral Plate Maker (Don Whitley Scientific Ltd). When required, serial dilutions were made into sterile Maximum Recovery Diluent (Oxoid Ltd). Psychrotroph counts were determined after incubation of plates at 21°C for 25 h (Oliveria and Parmelee 1976). Mesophile and thermophile counts were obtained after incubation of inoculated plates at 30°C for 3 days and 55°C for 1 day, respectively. Spore and thermoduric counts were carried out as detailed in British Standard 4285 : 1968.
Powder. Milk powder (10 g) was reconstituted in 90 ml of sterile 2% (m/v) sodium citrate solution previously warmed to 50 + I°C in a water bath as directed in Supplement No. 1 to British Standard 4285:1968 (1970). Microbiological analyses and incubation conditions for plates were as detailed for raw milk samples.
Results and Discussion Bacteriological quality of raw milks stored at low temperatures and used for powder manufacture The effect of s t o r a g e on the a v e r a g e bacteriological q u a l i t y of r a w m i l k used in the m a n u f a c t u r e of low-heat, dried, s k i m m e d m i l k is s h o w n in Table 1. T h e r e was an a p p r o x i m a t e l y 100-fold increase in p s y c h r o t r o p h c o u n t after 3 d a y s ' storage at 2°C a n d this was c o n s i s t e n t w i t h previous findings (Griffiths et al. 1987). On only one occasion did the psychrot r o p h c o u n t exceed 1 × 107 cfu m1-1, the level at w h i c h levels of e x t r a c e l l u l a r e n z y m e s produced by p s y c h r o t r o p h i c b a c t e r i a are detectable (Law 1979, Cousin 1982). The initial p s y c h r o t r o p h counts r a n g e d from 2-6 × 102 to 4.0 × 105 cfu m1-1 a n d it was t h e s a m p l e h a v i n g the h i g h e s t initial c o u n t w h i c h h a d bacterial counts in excess of 1 × 107 cfu m1-1 after s t o r a g e at 2°C for 3 days. This was c o n s i s t e n t w i t h earlier findings
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T a b l e 1. Effect of s t o r a g e on m i l k q u a l i t y for l o w - h e a t p o w d e r p r o d u c t i o n . Mean a of count after storage time (h) at 2°C (cfu m l - 1)
Mesophile Psychrotroph Thermoduric Spore Thermophile
0
24
72
2.6 x 104 (2.0 x 103-6.0 x 105)5 6-5 x 103 (2-6 x 102-4.0 x 105) 5.2 x 102 (5.6 x 10t-5.1 x 103) 1.6 x 102 (2.8 x 101-1.6 x 103) 3-2 x 101 (<1.4 x 101-4-3 x 102)
5-4 x 104 (1.6 x 103-1.0 x 107) 3.7 x 104 (3.1 x 102-9.7 x 106) 4-6 x 102 (6-8.1 x 103) 2.3 x 102 (1.4 x 101-1-5 x 103) 4.0 x 101 (1-1.0 x 10 a)
4.5 x 105 (8.3 x 103-2.8 x l0 s) 4.3 x 105 (6.9 x 103-4.3 x l0 s) 4.3 x 102 (1.4 x 101-4-6 x 10 3) 7.7 x 101 (<1.4 x 101-1.3 x 103) 5.1 x 101 (<1-4 x 101-5-4 x 102)
a Geometric mean of 9 experiments. b Figures in parentheses are range of counts. which showed a strong relation between initial p s y c h r o t r o p h c o u n t a n d s t o r a g e life (Griffiths et al. 1987). T h e i n c r e a s e in m e s o p h i l e c o u n t w a s due to t h e g r o w t h of p s y c h r o t r o p h bacteria and these counts were strongly c o r r e l a t e d in r a w m i l k stored a t r e f r i g e r a t i o n t e m p e r a t u r e s ( K w e e et al. 1986b). T h e r e w a s no s u b s t a n t i a l c h a n g e of thermophile, thermoduric or spore counts of r a w m i l k s d u r i n g s t o r a g e a t 2°C a n d this w a s c o n s i s t e n t w i t h r e s u l t s found d u r i n g s t o r a g e of m i l k a t h i g h e r t e m p e r a t u r e s (5-6°C) (Muir et al. 1986, W e s t et al. 1986).
Effect of processing on bacterial load T h e effect of p r o c e s s i n g on b a c t e r i a l counts (Table 2) w a s s i m i l a r to t h a t o b s e r v e d p r e v i o u s l y d u r i n g p r o d u c t i o n of l o w - h e a t s k i m m e d m i l k p o w d e r (Muir et al. 1986, W e s t et al. 1986). T h e r e d u c t i o n in m e s o p h i l e c o u n t d u r i n g p r o c e s s i n g could be e x p l a i n e d b y t h e e l i m i n a t i o n of heat-sensitive psychrotrophic bacteria. T h e r e w a s little c h a n g e in t h e r m o d u r i c , spore or t h e r m o p h i l e c o u n t s w i t h e a c h p r o c e s s i n g step. H o w e v e r , s o m e w o r k e r s h a v e n o t e d a p p r e c i a b l e r e d u c t i o n s in t h e r m o d u r i c a n d spore counts of m i l k d u r i n g t h e m a n u f a c t u r e of l o w - h e a t
p o w d e r ( K w e e et al. 1986a). T h e s e w o r k ers also did not o b s e r v e a s i g n i f i c a n t c h a n g e in t h e r m o p h i l e c o u n t d u r i n g processing.
Bacteriological properties of powders 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 p o w d e r m a n u f a c t u r e d f r o m r a w m i l k s t o r e d for up to 3 d a y s a t 2°C is s h o w n in T a b l e 3. S t o r a g e of m i l k u n d e r t h e s e conditions did not a d v e r s e l y affect t h e b a c t e r i o l o g ical q u a l i t y of t h e final product. T h e r e w a s a s i g n i f i c a n t fall in spore c o u n t s in p o w d e r s p r o d u c e d f r o m r a w m i l k s stored a t 2°C for 3 d a y s b u t t h e r e a s o n for t h i s is unknown. A weak relation between thermoduric counts of t h e r a w m i l k a n d t h a t of t h e p o w d e r m a n u f a c t u r e d f r o m it h a s b e e n s h o w n to e x i s t for s o m e l o w - h e a t p o w d e r s (Muir et al. 1986). T h i s did n o t a p p e a r to be t h e case for t h e p r e s e n t series of e x p e r i m e n t s w h e r e t h e r e w a s no correlation b e t w e e n t h e s e two p a r a m e t e r s (r = 0-27; n = 18). T h e r e was, h o w e v e r , a w e a k c o r r e l a t i o n (r = 0-71; n = 18; P < 0 - 0 0 1 ) b e t w e e n spore c o u n t of r a w m i l k s a n d spore c o u n t of powder. I t h a s b e e n r e p o r t e d t h a t a c o r r e l a t i o n (r = 0.73) e x i s t e d b e t w e e n t h e t h e r m o d u r i c c o u n t of r a w m i l k a n d t h e t o t a l c o u n t of
1.6 × 105 (1-6 x 103-2-8 × 10s)c 1-3 x 105 (3.1 x 102-4.3 x l 0 s) 4.5 x 102 (6--8-1 X 103) 1-2 × 102 ( < 1 . 4 × 1 0 L 1 . 5 × 103 ) 1.9 × 101 ( 1 - 1 . 0 x 103)
Raw milk b 1.5 x 105 (2.1 x 103-2.8 x l 0 s) 1.1 x l 0 s (3.6 x 102-4.7 x I0 s) 3-5 × 102 (3-4"5 × 103} 9-6 x 101 ( 4 - 1 - 6 × 103) 2.1 × 101 (4--8.3 × 102 )
Skim milk b
Geometric mean of 18 samples (cfu ml ' or g) from 9 experiments. b Includes milk samples stored at 2°C for 24 and 72 h. c Figures in parentheses are range of counts.
Thermophile
Spore
Thermoduric
Psychrotroph
Mesophile
Bacterial count Concentrate
Powder
4.0 × 102 3-7 × 103 2.2 × 103 ( I . i x 3.8 x 103) ( I . I x 102-1.1 x 1 0 5 ) (2.1 x 102-2.5 x 104) < 1 . 4 x 101 8.1 x 101 7-4 x 101 (<1.4 x 101-8.3 x I0 I) (<1.4 x 101-6.7 x 104) (6.9 x 101-6.6 x 103) 2-3 × 102 2-3 x 102 1.8 x 103 (<1--2-5 X 103) (1"7 X 101--4.4 × 103} (4"2 × 102--1.3 X 104) 1.3 × 102 4-5 × 102 7.4 x 102 ( 2 - 8 × 1 0 1 - 1 . 0 × 103) ( 1 . 9 x 1 0 2 - 5 . 8 × 103 ) ( < 1 - 4 × 1 0 2 - i - I x 104) 1-5 × 101 3.8 × 101 4.2 x 101 (1-1-0 x 103 ) ( 1 - 1 . 5 × 103 ) ( < 1 . 4 × 101-2.2 × 103 )
Pasteurised milk
P r o c e s s step~
Table 2. Changes in bacteriological quality during manufacture of dried low-heat skim milk.
t~
Quality of skim-milk powder 93 Table 3. Bacteriological quality of low-heat s k i m m e d milk powder manufactured from raw milk stored at 2°C for 24 and 72 h.
Storage time of raw milka (h) Bacterial count Mesophile Thermoduric Spore Thermophile
24
72
5.6 x 103 (2-1 x 103-2-5 x 104) 3-3 x 103 (8.3 x 102-1.3 x 104) 2.7 x 103 (9-7 x 102-1.1 x 1 0 4 ) 3.5 x 101 (<1-4 x 101-1-8 x 103)
1-2 x 103 (2.1 x 102-5.1 x 103) 1.0 x 103 (3-5 x 102--4.7 x 103) 2-0 x 102 (<1-4 x 102-2-7 x 103) 7.9 x 101 (<1.4 x 101-2.2 x 103)
a Geometric mean of 9 experiments (cfu g ~). h Figures in parentheses are range of counts. the powder m a n u f a c t u r e d from it (Kwee et al. 1986b). No such r e l a t i o n existed b e t w e e n m i l k s and powders in this s t u d y (r = 0-23; n = 18).
Some physical and chemical properties of powders The effects of r a w m i l k s t o r a g e at 2°C on the physical p r o p e r t i e s of low-heat powders are s h o w n in Table 4. S t o r a g e of r a w milk at 2°C for up to 3 days h a d little effect on m o i s t u r e c o n t e n t or t i t r a t a b l e acidity of powders m a d e s u b s e q u e n t l y . T h e r e was a s l i g h t increase in the insolubility index of powders m a n u f a c t u r e d from m i l k s stored for 3 d a y s at 2°C as opposed to 1 day, b u t all the powders a n a l y z e d were of a n acceptable s t a n d a r d and w i t h i n A D M I (1971) r e c o m m e n d a tions. The increase in the insolubility index was u n l i k e l y to be due to excessive
g r o w t h of p s y c h r o t r o p h s as levels of these o r g a n i s m s in the stored r a w m i l k s did not r e a c h a p o i n t w h e r e a p p r e c i a b l e a m o u n t s of d e g r a d a t i v e e n z y m e s could be produced. M o i s t u r e c o n t e n t was g e n e r a l l y lower and insolubility g r e a t e r t h a n p r e v i o u s l y observed for l o w - h e a t powders m a n u f a c t u r e d at the I n s t i t u t e (Muir et al. 1986, West et al. 1986). The r e a s o n s for the differences b e t w e e n the r e s u l t s in the p r e s e n t s t u d y a n d t h o s e o b t a i n e d previously is u n c l e a r a n d m a y reside w i t h compositional c h a n g e s in the m i l k s used.
Heat stability of powders manufactured from raw milks stored at low temperatures U n l i k e the effect of s t o r a g e at t e m p e r a t u r e s of 5-6°C a n d above (Muir et al. 1986), s t o r a g e of r a w m i l k at 2°C h a d no
T a b l e 4. P r o p e r t i e s o f d r i e d s k i m m e d m i l k m a n u f a c t u r e d f r o m r a w m i l k s t o r e d at 2°C for 24 a n d 72 h.
Storage time of raw milk a (h) Property Insolubility index (ml) Titratable acidity (%) Moisture (%) a Mean of 9 experiments. b Figures in parentheses are standard errors.
24
72
0.36 (0.13) b 0.15 (0.003) 2.61 (0.06)
0.47 (0.09) 0.14 (0-001) 2.74 (0-08)
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M.W. Griffiths et al.
Table 5. Heat stability of dried milk reconstituted to 9% and 22% total solids.
Heat stability
Solids on reconstitution (%)
Natural CT Maximum CT Natural CT Maximum CT
9b 9b 22 c 22c
a Average of 9 experiments. b Coagulation time m e a s u r e d at 140°C.
d e t r i m e n t a l effect on CT at e i t h e r 9% or 22% TS c o n c e n t r a t i o n (Table 5). Maxim u m CTs at both 9% and 22% solids were similar to those obtained previously (Muir et al. 1986, West et al. 1986). E a r l i e r work showed t h a t t h e r e was a decline in stability of powder reconstit u t e d to 9% solids w h e n the corresponding count in the m i l k exceeded 3 × 107 cfu m1-1 (Muir et al. 1986). This v a l u e is typical of t h r e s h o l d v a l u e s for the d e t e r i o r a t i o n of raw milk and cream (Muir et al. 1978, Griffiths et al. 1981). On only one occasion in the p r e s e n t s t u d y did the bacterial count rise above this level, on storage. The powder produced from this milk showed a decrease in the n a t u r a l CT (Fig. 1). Significant reductions in the stability of powders r e c o n s t i t u t e d to 22% solids were found w h e n r a w milk counts exceeded 2 x 10 6 cfu m1-1 (Muir et al. 1986). This was suggested to be consist e n t with the view t h a t protein stability is p a r t i c u l a r l y sensitive to changes in milk composition. After storage at 2°C for 3 days, four of the milks in the p r e s e n t s t u d y had counts above 1 x 106 cfu ml-1, b u t on only one occasion was the h e a t stability of the powder affected. This powder had been m a n u f a c t u r e d from raw milk with a p s y c h r o t r o p h count of 4.3 x l 0 s cfu m1-1 (Fig. 1).
Conclusions The bacteriological q u a l i t y and physical properties, including h e a t stability, of
Milk storage time (h)a 24
72
13-6 (0.5) d 13.6 (0-5) 5-8 (0-3) 12.9 (0-4)
13.4 (0-6) 13.5 (0.5) 5-8 (0-5) 13.5 (0.4)
c Coagulation time m e a s u r e d at 120°C. d F i g u r e s in p a r e n t h e s e s are s t a n d a r d errors. I-3
>.
I-I
"5 .c
0'9
'6 0.7
0.5
~ 3
I 5
=
I 7
Log initial psychrotroph~c count
Fig. 1. Relation between raw milk psychrotroph count and heat stability of skimmed milk powder produced. Heat stability was determined as the coagulation time (CT) of powder manufactured from milks stored for 3 days relative to that manufactured from milks stored for 1 day at 2°C. The natural (0) and maximum (C)) CT for powder reconstituted to 9% total solids and natural (m) and maximum ([3) CT for powder reconstituted to 22% total solids are shown. low-heat powders was unaffected by storage of the raw m i l k from which it was m a d e at 2°C for a period up to 72 h. A l t h o u g h the storage s y s t e m used was not t r u l y r e p r e s e n t a t i v e of those found at processing sites, n e v e r t h e l e s s , substantial benefits were noted w h e n the results were c o m p a r e d w i t h those p r e v i o u s l y obtained with milks stored at 6°C (Muir
Quality of skim-milk powder 95 et al. 1986). T h e l a t t e r is t h e storage t e m p e r a t u r e c o m m o n l y associated w i t h commercial practices.
Acknowledgement This work was f u n d e d j o i n t l y by the Department of Agriculture and Fisheries for Scotland and the U n i t e d
K i n g d o m D a i r y I n d u s t r y R e s e a r c h Policy C o m m i t t e e . T h e a u t h o r s would like to t h a n k the Scottish Milk M a r k e t i n g Board for provision of c r e a m e r y silo milk. T h e skilled t e c h n i c a l assistance of Mrs S. H u t c h i s o n , Mrs E. Mitchell a n d Miss J. M c L e l l a n d is g r a t e f u l l y acknowledged.
References American Dry Milk Institute (1971) Standards for grades of dry milks. Bulletin 916. Chicago, ADMI. Banks, J. M., Griffiths, M. W., Phillips, J. D. and Muir, D. D. (1988) A comparison of the effect of storage of raw milk at 2°C and 6°C on the yield and quality of Cheddar cheese. Food Microbiol. 5, 9-16. British Standard 4285 (1968) Methods of microbiological examination for dairy purposes. London, British Standards Institution. Case, R. A., Bradley, R. L. J r and Williams, R. R. (1985) Chemical and physical methods. In Standard Methods for the Examination of Dairy Products, 15th edition (Ed. Richardson, G. H.) pp. 327-404. Washington D.C., American Public Health Association. Cousin, M. A. (1982) Presence and activity of psychrotrophic microorganisms in milk and dairy products. A review. J. Food Prot. 45, 172-207. Griffiths, M. W., Phillips, J. D. and Muir, D. D. (1981) Development of flavour defects in pasteurized double cream during storage at 6°C and 10°C. J. Soc. Dairy Technol. 34, 142-146. Griffiths, M. W., Phillips, J. D. and Muir, D. D. (1987) Effect of low temperature storage on the bacteriological quality of raw milk. Food Microbiol. 4, 285-291. Griffiths, M. W., Phillips, J. D., West, I. G. and Muir, D. D. (1988) The effect of extended low-temperature storage of raw milk on the quality of pasteurized and UHT milk. Food Microbiol. 5, 75-87. Kwee, W. S., Dommett, T. W., Giles, J. E., Roberts, R. and Smith, R. A. D. (1986a) Microbiological parameters during powdered milk manufacture. 1. Variation between processes and stages. Aust. J. Dairy Technol. 41, 3-6. Kwee, W. S., Dommett, T. W., Giles, J. E., Smith, R. A. D. and Roberts, R. (1986b) Microbiological parameters during powdered milk manufacture. 2. Relationships and predictability among counts. Aust. J. Dairy Technol. 41, 6-8. Law, B. A. (1979) Reviews of the progress of dairy science: Enzymes ofpsychrotrophic bacteria and their effects on milk and milk products. J. Dairy Res. 46, 573-588. Lovell, H. R. (1981) The microbiology of dried milk powders. In Dairy Microbiology, Volume 1, The Microbiology of Milk (Ed. Robinson, R. K.) pp. 209-231, London, Applied Science Publishers. Muir, D. D., Kelly, M. E. and Phillips, J. D. (1978) The effect of storage temperature on bacterial growth and lipolysis in raw milk. J. Soc. Dairy Technol. 31, 203-208. Muir, D. D., Griffiths, M. W., Phillips, J. D., Sweetsur, A. W. M. and West, I. G. (1986) Effect of the bacterial quality of raw milk on the bacterial quality and some other properties of low-heat and high-heat dried milk. J. Soc. Dairy Technol. 39, 115-118. Oliveria, J. S. and Parmelee, C. E. (1976) Rapid enumeration ofpsychrotrophic bacteria in raw and pasteurized milk. J. Milk Food Technol. 39, 269-272. Phillips, J. D. and Griffiths, M. W. (1987) The relation between temperature and growth of bacteria in dairy products. Food Microbiol. 4, 173-185. Supplement No. 1 to British Standard 4285:1968 (1970) Methods of microbiological examination for dairy purposes. Methods of microbiological examination of milk products. London British Standards Institution. Swarthing, P. (1967) The quality of milk with alternate day collection by tanker. Neth. Milk Dairy J. 21, 87-102.
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Sweetsur, A. W. M. and White, J. C. D. (1974) Studies on the heat stability of milk. 1. Interconversion of type A and type B milk heat-stability curves. J. Dairy Res. 41,349-358. West, I. G., Griffiths, M. W., Phillips, J. D., Sweetsur, A. W. M. and Muir, D. D. (1986) Production of dried skim milk from thermised milk. Dairy Ind. Int. 51 (6), 33-34.
The quality of skim-milk powder produced from raw milk stored at 2°C M. W. Griffiths*, J. D. Phillips, I. G. West, A. W. M. Sweetsur and D. D. Muir Hannah Research Institute, A yr KA6 5HL, Scotland Received 3 February 1988 Low-heat skimmed milk powder was manufactured from raw farm bulk tank and creamery silo milk which had been stored at 2°C for 24 and 72 h. During the storage period the psychrotroph count increased by about I log cycle after 24 h and 2 log cycles after 72 h. There was no increase in thermoduric or spore count of the milk under these storage conditions. The powder manufactured from these milks was of good bacteriological quality and conformed to ADMI recommendations regarding moisture content, titratable acidity and solubility. Storage of raw milk at 2°C had no detrimental effect on the heat stability of the powder manufactured from it when reconstituted to both 9 and 22% total solids concentrations.
Introduction Many factors, including alternate-day collection, the introduction of a five day working week and adoption of quotas by the EEC, have led to raw milk being stored for extended periods before processing. There is considerable interest, therefore, in ways of retarding the growth of psychrotrophic bacteria in raw milk to gain a significant increase in the storage life. Bacterial growth in milk can be inhibited by reducing the storage temperature (Swarthing 1967, Phillips and Griffiths 1987, Griffiths et al. 1987). A significant extension of the storage life (about 75%) was obtained by decreasing the temperature from 6 to 2°C (Griffiths et al. 1987). Such t r e a t m e n t had no practical effect on the quality of Cheddar cheese (Banks et al. 1988) or pasteurized and UHT milks produced (Griffiths et al. 1988). However, a relation was found *To whom correspondence should addressed. 0740-0020/88/020089 + 08 $02.00/0
be
between the bacterial load in raw milk and the observed decline in protein stability of reconstituted milk powders on heating (Muir et al. 1986). Low-heat powders seem particularly susceptible to changes in the bacteriological quality of the raw milks from which they are manufactured (Lovell 1981, Muir et al. 1986). This paper examines the potential of storing raw milk at 2°C to extend the storage life of raw milk for the manufacture of milk powder in contrast to the more usual temperatures of 5 to 6°C encountered for raw milk storage in the UK at present.
Materials and Methods Milk samples and storage Raw milks from the Hannah Research Institute farm bulk tank (5 samples) and from the silos of a local creamery (4 samples) were transported to the Institute in churns. The temperature on receipt was noted. The raw milk was stored in churns (45 litres) at 2°C and the temperature monitored throughout its storage period using a Squirrel SQ8-4UAD temperature logger (Grant Instruments). Using this procedure the milk temperature (~) 1988 AcademicPress Limited
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M.W. Griffiths et al.
could be maintained at 2° + 0.5°C. There was no agitation or mixing of the milk during storage except during sampling and processing when the milk was manually mixed using a plunger.
Powder production Raw milk which had previously been stored at 2°C for 1 or 3 days, was warmed to 45°C in a steam jacketed tank and separated using a small, open cream separator. The skimmed milk was pasteurized at 74°C for 16 s (lowheat treatment) and concentrated to 45% total solids (TS) in a falling film evaporator (Anhydro L ~ , Copenhagen, Denmark) with an evaporative capacity of 125 litres of water h-1 and a calandria temperature of less than 55°C. After concentration, the milk was spray dried in an experimental, tall-form spray drier fitted with nozzle atomization and heated by direct-gas firing (evaporative capacity 35 kg h-l; inlet air temperature 190°C; outlet air temperature 95°C). The dried skimmed milk was stored in airtight, sterile containers at 30°C for two weeks prior to analysis. This facilitated sample handling.
Analysis of some physical and chemical parameters of powders The moisture content of powders was estimated by drying to constant weight in a fan-assisted oven at 102°C. Powder solubility was estimated by an insolubility index method similar to that of the American Dry Milk Institute (ADMI) (1971). This method relies on determining the volume of sediment formed after centrifuging reconstituted milk powder (10% w/v). Titratable acidity was measured as described by Case et al. (1985). Non-fat dry milk (10 g) was mixed carefully with water (100 ml) to avoid foaming. After standing for 1 h at ambient temperature, an aliquot (18 g) was withdrawn after mixing and titrated against sodium hydroxide (0.1N) using phenophthalein (0-5 ml of 1% w/v solution in 95% ethanol) as an indicator. Acidity is expressed as % lactic acid. The heat stability of powders, reconstituted to 9% and 22% solids with distilled water, was determined by the time taken for visible clots to appear when the milks were heated at 140°C and 120°C, respectively. The pH of the milk was adjusted by the addition of HC1 or NaOH and the coagulation time (CT)-pH profiles determined by the method of Sweetsur and White (1974). Heat stability was
reported as the CT before pH adjustment (natural CT) and the maximum CT attained after pH adjustment.
Bacteriological analysis Raw milk. Samples were plated onto milk agar (Oxoid) plates using a Spiral Plate Maker (Don Whitley Scientific Ltd). When required, serial dilutions were made into sterile Maximum Recovery Diluent (Oxoid Ltd). Psychrotroph counts were determined after incubation of plates at 21°C for 25 h (Oliveria and Parmelee 1976). Mesophile and thermophile counts were obtained after incubation of inoculated plates at 30°C for 3 days and 55°C for 1 day, respectively. Spore and thermoduric counts were carried out as detailed in British Standard 4285 : 1968.
Powder. Milk powder (10 g) was reconstituted in 90 ml of sterile 2% (m/v) sodium citrate solution previously warmed to 50 + I°C in a water bath as directed in Supplement No. 1 to British Standard 4285:1968 (1970). Microbiological analyses and incubation conditions for plates were as detailed for raw milk samples.
Results and Discussion Bacteriological quality of raw milks stored at low temperatures and used for powder manufacture The effect of s t o r a g e on the a v e r a g e bacteriological q u a l i t y of r a w m i l k used in the m a n u f a c t u r e of low-heat, dried, s k i m m e d m i l k is s h o w n in Table 1. T h e r e was an a p p r o x i m a t e l y 100-fold increase in p s y c h r o t r o p h c o u n t after 3 d a y s ' storage at 2°C a n d this was c o n s i s t e n t w i t h previous findings (Griffiths et al. 1987). On only one occasion did the psychrot r o p h c o u n t exceed 1 × 107 cfu m1-1, the level at w h i c h levels of e x t r a c e l l u l a r e n z y m e s produced by p s y c h r o t r o p h i c b a c t e r i a are detectable (Law 1979, Cousin 1982). The initial p s y c h r o t r o p h counts r a n g e d from 2-6 × 102 to 4.0 × 105 cfu m1-1 a n d it was t h e s a m p l e h a v i n g the h i g h e s t initial c o u n t w h i c h h a d bacterial counts in excess of 1 × 107 cfu m1-1 after s t o r a g e at 2°C for 3 days. This was c o n s i s t e n t w i t h earlier findings
Quality of skim-milk powder
91
T a b l e 1. Effect of s t o r a g e on m i l k q u a l i t y for l o w - h e a t p o w d e r p r o d u c t i o n . Mean a of count after storage time (h) at 2°C (cfu m l - 1)
Mesophile Psychrotroph Thermoduric Spore Thermophile
0
24
72
2.6 x 104 (2.0 x 103-6.0 x 105)5 6-5 x 103 (2-6 x 102-4.0 x 105) 5.2 x 102 (5.6 x 10t-5.1 x 103) 1.6 x 102 (2.8 x 101-1.6 x 103) 3-2 x 101 (<1.4 x 101-4-3 x 102)
5-4 x 104 (1.6 x 103-1.0 x 107) 3.7 x 104 (3.1 x 102-9.7 x 106) 4-6 x 102 (6-8.1 x 103) 2.3 x 102 (1.4 x 101-1-5 x 103) 4.0 x 101 (1-1.0 x 10 a)
4.5 x 105 (8.3 x 103-2.8 x l0 s) 4.3 x 105 (6.9 x 103-4.3 x l0 s) 4.3 x 102 (1.4 x 101-4-6 x 10 3) 7.7 x 101 (<1.4 x 101-1.3 x 103) 5.1 x 101 (<1-4 x 101-5-4 x 102)
a Geometric mean of 9 experiments. b Figures in parentheses are range of counts. which showed a strong relation between initial p s y c h r o t r o p h c o u n t a n d s t o r a g e life (Griffiths et al. 1987). T h e i n c r e a s e in m e s o p h i l e c o u n t w a s due to t h e g r o w t h of p s y c h r o t r o p h bacteria and these counts were strongly c o r r e l a t e d in r a w m i l k stored a t r e f r i g e r a t i o n t e m p e r a t u r e s ( K w e e et al. 1986b). T h e r e w a s no s u b s t a n t i a l c h a n g e of thermophile, thermoduric or spore counts of r a w m i l k s d u r i n g s t o r a g e a t 2°C a n d this w a s c o n s i s t e n t w i t h r e s u l t s found d u r i n g s t o r a g e of m i l k a t h i g h e r t e m p e r a t u r e s (5-6°C) (Muir et al. 1986, W e s t et al. 1986).
Effect of processing on bacterial load T h e effect of p r o c e s s i n g on b a c t e r i a l counts (Table 2) w a s s i m i l a r to t h a t o b s e r v e d p r e v i o u s l y d u r i n g p r o d u c t i o n of l o w - h e a t s k i m m e d m i l k p o w d e r (Muir et al. 1986, W e s t et al. 1986). T h e r e d u c t i o n in m e s o p h i l e c o u n t d u r i n g p r o c e s s i n g could be e x p l a i n e d b y t h e e l i m i n a t i o n of heat-sensitive psychrotrophic bacteria. T h e r e w a s little c h a n g e in t h e r m o d u r i c , spore or t h e r m o p h i l e c o u n t s w i t h e a c h p r o c e s s i n g step. H o w e v e r , s o m e w o r k e r s h a v e n o t e d a p p r e c i a b l e r e d u c t i o n s in t h e r m o d u r i c a n d spore counts of m i l k d u r i n g t h e m a n u f a c t u r e of l o w - h e a t
p o w d e r ( K w e e et al. 1986a). T h e s e w o r k ers also did not o b s e r v e a s i g n i f i c a n t c h a n g e in t h e r m o p h i l e c o u n t d u r i n g processing.
Bacteriological properties of powders 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 p o w d e r m a n u f a c t u r e d f r o m r a w m i l k s t o r e d for up to 3 d a y s a t 2°C is s h o w n in T a b l e 3. S t o r a g e of m i l k u n d e r t h e s e conditions did not a d v e r s e l y affect t h e b a c t e r i o l o g ical q u a l i t y of t h e final product. T h e r e w a s a s i g n i f i c a n t fall in spore c o u n t s in p o w d e r s p r o d u c e d f r o m r a w m i l k s stored a t 2°C for 3 d a y s b u t t h e r e a s o n for t h i s is unknown. A weak relation between thermoduric counts of t h e r a w m i l k a n d t h a t of t h e p o w d e r m a n u f a c t u r e d f r o m it h a s b e e n s h o w n to e x i s t for s o m e l o w - h e a t p o w d e r s (Muir et al. 1986). T h i s did n o t a p p e a r to be t h e case for t h e p r e s e n t series of e x p e r i m e n t s w h e r e t h e r e w a s no correlation b e t w e e n t h e s e two p a r a m e t e r s (r = 0-27; n = 18). T h e r e was, h o w e v e r , a w e a k c o r r e l a t i o n (r = 0-71; n = 18; P < 0 - 0 0 1 ) b e t w e e n spore c o u n t of r a w m i l k s a n d spore c o u n t of powder. I t h a s b e e n r e p o r t e d t h a t a c o r r e l a t i o n (r = 0.73) e x i s t e d b e t w e e n t h e t h e r m o d u r i c c o u n t of r a w m i l k a n d t h e t o t a l c o u n t of
1.6 × 105 (1-6 x 103-2-8 × 10s)c 1-3 x 105 (3.1 x 102-4.3 x l 0 s) 4.5 x 102 (6--8-1 X 103) 1-2 × 102 ( < 1 . 4 × 1 0 L 1 . 5 × 103 ) 1.9 × 101 ( 1 - 1 . 0 x 103)
Raw milk b 1.5 x 105 (2.1 x 103-2.8 x l 0 s) 1.1 x l 0 s (3.6 x 102-4.7 x I0 s) 3-5 × 102 (3-4"5 × 103} 9-6 x 101 ( 4 - 1 - 6 × 103) 2.1 × 101 (4--8.3 × 102 )
Skim milk b
Geometric mean of 18 samples (cfu ml ' or g) from 9 experiments. b Includes milk samples stored at 2°C for 24 and 72 h. c Figures in parentheses are range of counts.
Thermophile
Spore
Thermoduric
Psychrotroph
Mesophile
Bacterial count Concentrate
Powder
4.0 × 102 3-7 × 103 2.2 × 103 ( I . i x 3.8 x 103) ( I . I x 102-1.1 x 1 0 5 ) (2.1 x 102-2.5 x 104) < 1 . 4 x 101 8.1 x 101 7-4 x 101 (<1.4 x 101-8.3 x I0 I) (<1.4 x 101-6.7 x 104) (6.9 x 101-6.6 x 103) 2-3 × 102 2-3 x 102 1.8 x 103 (<1--2-5 X 103) (1"7 X 101--4.4 × 103} (4"2 × 102--1.3 X 104) 1.3 × 102 4-5 × 102 7.4 x 102 ( 2 - 8 × 1 0 1 - 1 . 0 × 103) ( 1 . 9 x 1 0 2 - 5 . 8 × 103 ) ( < 1 - 4 × 1 0 2 - i - I x 104) 1-5 × 101 3.8 × 101 4.2 x 101 (1-1-0 x 103 ) ( 1 - 1 . 5 × 103 ) ( < 1 . 4 × 101-2.2 × 103 )
Pasteurised milk
P r o c e s s step~
Table 2. Changes in bacteriological quality during manufacture of dried low-heat skim milk.
t~
Quality of skim-milk powder 93 Table 3. Bacteriological quality of low-heat s k i m m e d milk powder manufactured from raw milk stored at 2°C for 24 and 72 h.
Storage time of raw milka (h) Bacterial count Mesophile Thermoduric Spore Thermophile
24
72
5.6 x 103 (2-1 x 103-2-5 x 104) 3-3 x 103 (8.3 x 102-1.3 x 104) 2.7 x 103 (9-7 x 102-1.1 x 1 0 4 ) 3.5 x 101 (<1-4 x 101-1-8 x 103)
1-2 x 103 (2.1 x 102-5.1 x 103) 1.0 x 103 (3-5 x 102--4.7 x 103) 2-0 x 102 (<1-4 x 102-2-7 x 103) 7.9 x 101 (<1.4 x 101-2.2 x 103)
a Geometric mean of 9 experiments (cfu g ~). h Figures in parentheses are range of counts. the powder m a n u f a c t u r e d from it (Kwee et al. 1986b). No such r e l a t i o n existed b e t w e e n m i l k s and powders in this s t u d y (r = 0-23; n = 18).
Some physical and chemical properties of powders The effects of r a w m i l k s t o r a g e at 2°C on the physical p r o p e r t i e s of low-heat powders are s h o w n in Table 4. S t o r a g e of r a w milk at 2°C for up to 3 days h a d little effect on m o i s t u r e c o n t e n t or t i t r a t a b l e acidity of powders m a d e s u b s e q u e n t l y . T h e r e was a s l i g h t increase in the insolubility index of powders m a n u f a c t u r e d from m i l k s stored for 3 d a y s at 2°C as opposed to 1 day, b u t all the powders a n a l y z e d were of a n acceptable s t a n d a r d and w i t h i n A D M I (1971) r e c o m m e n d a tions. The increase in the insolubility index was u n l i k e l y to be due to excessive
g r o w t h of p s y c h r o t r o p h s as levels of these o r g a n i s m s in the stored r a w m i l k s did not r e a c h a p o i n t w h e r e a p p r e c i a b l e a m o u n t s of d e g r a d a t i v e e n z y m e s could be produced. M o i s t u r e c o n t e n t was g e n e r a l l y lower and insolubility g r e a t e r t h a n p r e v i o u s l y observed for l o w - h e a t powders m a n u f a c t u r e d at the I n s t i t u t e (Muir et al. 1986, West et al. 1986). The r e a s o n s for the differences b e t w e e n the r e s u l t s in the p r e s e n t s t u d y a n d t h o s e o b t a i n e d previously is u n c l e a r a n d m a y reside w i t h compositional c h a n g e s in the m i l k s used.
Heat stability of powders manufactured from raw milks stored at low temperatures U n l i k e the effect of s t o r a g e at t e m p e r a t u r e s of 5-6°C a n d above (Muir et al. 1986), s t o r a g e of r a w m i l k at 2°C h a d no
T a b l e 4. P r o p e r t i e s o f d r i e d s k i m m e d m i l k m a n u f a c t u r e d f r o m r a w m i l k s t o r e d at 2°C for 24 a n d 72 h.
Storage time of raw milk a (h) Property Insolubility index (ml) Titratable acidity (%) Moisture (%) a Mean of 9 experiments. b Figures in parentheses are standard errors.
24
72
0.36 (0.13) b 0.15 (0.003) 2.61 (0.06)
0.47 (0.09) 0.14 (0-001) 2.74 (0-08)
94
M.W. Griffiths et al.
Table 5. Heat stability of dried milk reconstituted to 9% and 22% total solids.
Heat stability
Solids on reconstitution (%)
Natural CT Maximum CT Natural CT Maximum CT
9b 9b 22 c 22c
a Average of 9 experiments. b Coagulation time m e a s u r e d at 140°C.
d e t r i m e n t a l effect on CT at e i t h e r 9% or 22% TS c o n c e n t r a t i o n (Table 5). Maxim u m CTs at both 9% and 22% solids were similar to those obtained previously (Muir et al. 1986, West et al. 1986). E a r l i e r work showed t h a t t h e r e was a decline in stability of powder reconstit u t e d to 9% solids w h e n the corresponding count in the m i l k exceeded 3 × 107 cfu m1-1 (Muir et al. 1986). This v a l u e is typical of t h r e s h o l d v a l u e s for the d e t e r i o r a t i o n of raw milk and cream (Muir et al. 1978, Griffiths et al. 1981). On only one occasion in the p r e s e n t s t u d y did the bacterial count rise above this level, on storage. The powder produced from this milk showed a decrease in the n a t u r a l CT (Fig. 1). Significant reductions in the stability of powders r e c o n s t i t u t e d to 22% solids were found w h e n r a w milk counts exceeded 2 x 10 6 cfu m1-1 (Muir et al. 1986). This was suggested to be consist e n t with the view t h a t protein stability is p a r t i c u l a r l y sensitive to changes in milk composition. After storage at 2°C for 3 days, four of the milks in the p r e s e n t s t u d y had counts above 1 x 106 cfu ml-1, b u t on only one occasion was the h e a t stability of the powder affected. This powder had been m a n u f a c t u r e d from raw milk with a p s y c h r o t r o p h count of 4.3 x l 0 s cfu m1-1 (Fig. 1).
Conclusions The bacteriological q u a l i t y and physical properties, including h e a t stability, of
Milk storage time (h)a 24
72
13-6 (0.5) d 13.6 (0-5) 5-8 (0-3) 12.9 (0-4)
13.4 (0-6) 13.5 (0.5) 5-8 (0-5) 13.5 (0.4)
c Coagulation time m e a s u r e d at 120°C. d F i g u r e s in p a r e n t h e s e s are s t a n d a r d errors. I-3
>.
I-I
"5 .c
0'9
'6 0.7
0.5
~ 3
I 5
=
I 7
Log initial psychrotroph~c count
Fig. 1. Relation between raw milk psychrotroph count and heat stability of skimmed milk powder produced. Heat stability was determined as the coagulation time (CT) of powder manufactured from milks stored for 3 days relative to that manufactured from milks stored for 1 day at 2°C. The natural (0) and maximum (C)) CT for powder reconstituted to 9% total solids and natural (m) and maximum ([3) CT for powder reconstituted to 22% total solids are shown. low-heat powders was unaffected by storage of the raw m i l k from which it was m a d e at 2°C for a period up to 72 h. A l t h o u g h the storage s y s t e m used was not t r u l y r e p r e s e n t a t i v e of those found at processing sites, n e v e r t h e l e s s , substantial benefits were noted w h e n the results were c o m p a r e d w i t h those p r e v i o u s l y obtained with milks stored at 6°C (Muir
Quality of skim-milk powder 95 et al. 1986). T h e l a t t e r is t h e storage t e m p e r a t u r e c o m m o n l y associated w i t h commercial practices.
Acknowledgement This work was f u n d e d j o i n t l y by the Department of Agriculture and Fisheries for Scotland and the U n i t e d
K i n g d o m D a i r y I n d u s t r y R e s e a r c h Policy C o m m i t t e e . T h e a u t h o r s would like to t h a n k the Scottish Milk M a r k e t i n g Board for provision of c r e a m e r y silo milk. T h e skilled t e c h n i c a l assistance of Mrs S. H u t c h i s o n , Mrs E. Mitchell a n d Miss J. M c L e l l a n d is g r a t e f u l l y acknowledged.
References American Dry Milk Institute (1971) Standards for grades of dry milks. Bulletin 916. Chicago, ADMI. Banks, J. M., Griffiths, M. W., Phillips, J. D. and Muir, D. D. (1988) A comparison of the effect of storage of raw milk at 2°C and 6°C on the yield and quality of Cheddar cheese. Food Microbiol. 5, 9-16. British Standard 4285 (1968) Methods of microbiological examination for dairy purposes. London, British Standards Institution. Case, R. A., Bradley, R. L. J r and Williams, R. R. (1985) Chemical and physical methods. In Standard Methods for the Examination of Dairy Products, 15th edition (Ed. Richardson, G. H.) pp. 327-404. Washington D.C., American Public Health Association. Cousin, M. A. (1982) Presence and activity of psychrotrophic microorganisms in milk and dairy products. A review. J. Food Prot. 45, 172-207. Griffiths, M. W., Phillips, J. D. and Muir, D. D. (1981) Development of flavour defects in pasteurized double cream during storage at 6°C and 10°C. J. Soc. Dairy Technol. 34, 142-146. Griffiths, M. W., Phillips, J. D. and Muir, D. D. (1987) Effect of low temperature storage on the bacteriological quality of raw milk. Food Microbiol. 4, 285-291. Griffiths, M. W., Phillips, J. D., West, I. G. and Muir, D. D. (1988) The effect of extended low-temperature storage of raw milk on the quality of pasteurized and UHT milk. Food Microbiol. 5, 75-87. Kwee, W. S., Dommett, T. W., Giles, J. E., Roberts, R. and Smith, R. A. D. (1986a) Microbiological parameters during powdered milk manufacture. 1. Variation between processes and stages. Aust. J. Dairy Technol. 41, 3-6. Kwee, W. S., Dommett, T. W., Giles, J. E., Smith, R. A. D. and Roberts, R. (1986b) Microbiological parameters during powdered milk manufacture. 2. Relationships and predictability among counts. Aust. J. Dairy Technol. 41, 6-8. Law, B. A. (1979) Reviews of the progress of dairy science: Enzymes ofpsychrotrophic bacteria and their effects on milk and milk products. J. Dairy Res. 46, 573-588. Lovell, H. R. (1981) The microbiology of dried milk powders. In Dairy Microbiology, Volume 1, The Microbiology of Milk (Ed. Robinson, R. K.) pp. 209-231, London, Applied Science Publishers. Muir, D. D., Kelly, M. E. and Phillips, J. D. (1978) The effect of storage temperature on bacterial growth and lipolysis in raw milk. J. Soc. Dairy Technol. 31, 203-208. Muir, D. D., Griffiths, M. W., Phillips, J. D., Sweetsur, A. W. M. and West, I. G. (1986) Effect of the bacterial quality of raw milk on the bacterial quality and some other properties of low-heat and high-heat dried milk. J. Soc. Dairy Technol. 39, 115-118. Oliveria, J. S. and Parmelee, C. E. (1976) Rapid enumeration ofpsychrotrophic bacteria in raw and pasteurized milk. J. Milk Food Technol. 39, 269-272. Phillips, J. D. and Griffiths, M. W. (1987) The relation between temperature and growth of bacteria in dairy products. Food Microbiol. 4, 173-185. Supplement No. 1 to British Standard 4285:1968 (1970) Methods of microbiological examination for dairy purposes. Methods of microbiological examination of milk products. London British Standards Institution. Swarthing, P. (1967) The quality of milk with alternate day collection by tanker. Neth. Milk Dairy J. 21, 87-102.
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Sweetsur, A. W. M. and White, J. C. D. (1974) Studies on the heat stability of milk. 1. Interconversion of type A and type B milk heat-stability curves. J. Dairy Res. 41,349-358. West, I. G., Griffiths, M. W., Phillips, J. D., Sweetsur, A. W. M. and Muir, D. D. (1986) Production of dried skim milk from thermised milk. Dairy Ind. Int. 51 (6), 33-34.