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Feasibility of Ultrafiltration for Standardizing Protein in Milk
Feasibility of Ultrafiltration for Standardizing Protein in Milk P. R ~ N K I L D E POULSEN
The Danish Government Research Institute for Dairy Industry DK--3400 Hillerod, Denmark ABSTRACT
In the future the worldwide dairy industry will need more sophisticated standardization of market milk, protein standardization, along with the fat standardization which has been carried out in many countries for years. F o r many reasons, the standardization of protein content similar to that of fat content seems justified. Such action calls, however, for the establishment of standards of identity which could be a further elaboration of the universally accepted Code of Principles. Establishing such a standard is a major challenge which the dairy industry in all countries will have to meet, and the sooner negotiations are started the better. The naturally occurring variations in composition are known well; the possible modifications are beyond computation. Experiments show that ultrafiltration is a feasible method for standardizing protein content in milk. Protein adjustments by ultrafiltration are possible over a relatively broad range without detectable organoleptic consequences. We estimate that for whole milk (3.5% fat) the protein can vary from 1.5% to more than 6.5%; for half-skimmed milk (1.5 to 1.8% fat) from 1.75% to 6.5%, and for skim milk from 3.1% to 6.4%. This leaves a broad field for technological exploitation and regulatory decisions. The producers of ultrafiltration equipment believe that satisfactory industrial scale set ups are at hand, although some details need further improvement, especially automatic control systems. No technical obstacle seems to prevent the use of this technology. The major problem in standardizing the protein content of milk seems to be political.
INTRODUCTION
Milk is by nature complex. Because of its all-around content of nutritional constituents, it always has been considered a unique food for human beings. It is no wonder that milk as a food has been shown the greatest respect everywhere on earth, and that most people have a deep-rooted dislike of any adulteration of milk. The question is, what is to be considered an adulteration? Fat standardization now is common in most countries and people are getting used to it and hardly consider it an adulteration. One of the reasons for introducing fat standardization was that milk fat was the most valuable constituent of the milk at one time, and a reliable method was available for determining fat content. Since the introduction of fat standardization, the requirements for composition of foods and the demand and price of the individual food constituents have changed in favor of protein. Until now, however, it has not been possible to standardize the protein content of the milk on a commercial scale without affecting the other constituents o f the milk severely and thereby affecting the flavor. The development of the membrane filtration technique during the last few years now seems to make adjustment of milk protein possible. Because o f the rather large variation in the protein content of milk during the year (Table 1), it is uncertain how much longer it will be possible to define milk according to the Codex Alimentarius Commission of 1963 or in the E.E.C. I regulation 1411/71 for liquid milk. To provide for fair trade and to simplify marketing of liquid milk, I think a modern society should have a complete standard for liquid milk comprising definition, composition, and specification.
Actual Variations in the Content of Fat and Protein in Milk Received at Danish Dairies
According to statistics issued by The Danish Dairy Federation Feb., 1976, the total amount
Received July 27, 1977. 1E.E.C. = European Economic Cooperation. 1978 J Dairy Sci 61:807--814
807
808
POULSEN
TABLE 1. Fat and protein percentages in raw milk received for processing at fifty dairies (1974 to 1975).
Period
Milk kg × 10 #
Average fat %
Average protein %
1974 9/27-10/10 10/11-10/24 10/25-11/7 11/8--11/21 11/22--12/5 12/6--12/19
57.98 57.02 56.07 55.38 56.13 57.00
4.46 4.48 4.47 4.45 4.43 4.40
3.66 3.66 3.66 3.63 3.57 3.53
58.54 60.14
4.36 4.32 4.28 4.26 4.27 4.27 4.28 4.29 4.26 4.24 4.21 4.17 4.15 4.12 4.11 4.14 4.17 4.15 4.24 4.33
3.50 3.46 3.40 3.37 3.37 3.35 3.34 3.29 3.29 3.28 3.33 3.43 3.44 3.39 3.35 3.34 3.36 3.33 3.43 3.53
4.27
3.42
1975 12/20--2/1 1/3--1/16 1/17--1/30 1/31--2/13 2/14--2/27 2/28--3/13 3/14--3/27
3/28--4/10 4/11--4/24 4/25--5/8 5/9--5/22 5/23--6/5 6/6--6/19 6/20--7/3 7/4--7/17 7/18--7/31 8tl--8/14 8/15--8/28 8/29-9/11 9/12--9/25
61.95
64.57 68.69 73.38 77.88 80.57 82.52 85.62 91.23 71.24 87.11 82.48 77.69 74.62 69.99 66.53 64.51 63.88
Average 9/27 (1974) 9/25 (1975)
of milk received at the Danish Dairies Oct. 1974 to Oct. 1975 was 4,628 million kg. The Applicability of Membrane Filtration Technique for Standardizing the Protein Content in Liquid Milk
The simplest way to alter protein c o n t e n t in liquid milk w o u l d be either to add or r e m o v e water. The c o n t e n t o f all the o t h e r milk constituents w o u l d be altered proportionally. The possibilities for specific a d j u s t m e n t o f the protein c o n t e n t in milk, however, have been limited until the m e m b r a n e t e c h n i q u e became technologically feasible near the beginning of this decade. Much i n f o r m a t i o n has been published in recent years concerning f u n d a m e n t a l principles of m e m b r a n e filtering technique, and m e m brane filtration already has reached c o m m e r c i a l Journal of Dairy Science Vol. 61, No. 6, 1978
scale, and m a n y different m e m b r a n e filtering units are in o p e r a t i o n for several purposes in different countries. By using one of the t w o m e m b r a n e filtering techniques, either the hyperfiltration (reverse osmosis, RO) or ultrafiltration, it is possible to adjust either the total solids-not-fat (SNF) or to carry o u t a specific a d j u s t m e n t of t h e protein. M e m b r a n e filtration represents a relatively simple physical separation which can be carried o u t at a sufficiently l o w t e m p e r a t u r e to avoid alteration in proteins and o t h e r heat sensitive constituents of the milk. A l t h o u g h the m e m brane filtering t e c h n i q u e , in itself, is a rather simple process, the e q u i p m e n t c a n n o t be claimed to be simple. The m e m b r a n e , which f o r m s the f u n d a m e n t a l part o f the e q u i p m e n t is a novel d e v e l o p m e n t which departs f r o m conventional w i s d o m regarding materials to be used
SYMPOSIUM: STANDARDIZING MILK FOR PROTEIN CONTENT for product contact surfaces. The manufacturers of membranes have succeeded in finding materials which can meet public health demands and which are able to resist detergents and cleaning temperatures ordinarily used. Experience has shown that satisfactory cleaning is feasible.
RETENTATE
Influence of SNF or Protein Adjustments on the Sensory Properties of Fluid Milk
Sensory properties are of the utmost importance in any adjustment of the constituents of fluid milk. To investigate to what extent adjustment of the content of SNF and/or protein can be accomplished without perceptible alteration of the character of the product, the following research has been carried out at the Danish Government Research Institute for Dairy Industry. The research comprised the three normal types of fluid milk on the Danish market: whole milk (minimum 3.5% fat), half-skimmed milk (minimum 1.5%, maximum 1.8% fat), and skim milk. These milks were standardized in the range of 1.5 to 6.5% protein at intervals of 1%. In three series, this was accomplished by hyperfiltration (RO) so that the SNF also was adjusted proportionally. In three other series the protein was adjusted by ultrafiltration so that the other SNF milk constituents were practically unchanged. All samples were prepared f r o m mixed herd milk, and all adjustments were in the skim milk fraction. All samples were finally high temperature-short time (HTST) heat treated (77 C, 15 s), and the samples of whole milk and half-skimmed milk were homogenized as well. After heat treatment, the milk samples were cooled to 8 C and stored at that temperature until evaluated. MATERIALS AND METHODS
The hyperfiltration (RO) was accomplished by a pilot module, manufactured by D.D.S. (The Danish Sugar Mills). It had a membrane area of .36 m 2 type D.D.S. 990 (Fig. 1). The typical operating conditions are in Table 2. For ultrafiltration we used a continuously operating plant unit made by The Danish Dairies Machine Works, Kolding, and D.D.S. (Table 3). The two modules are connected in series and have a membrane area of 4 m 2 each (D.D.S. type 600, celluloseacetate). Typical
809
) PERMEATE
FIG. 1. Principle of batch plant for hyperfiltration.
operating conditions for the ultrafittration are in Table 3. The D.D.S. membrane modules used are tubular plate and frame modules with shortpass narrow channels that give essentially laminar flow and have low internal volume. Sensory Evaluation
Sensory evaluation of the milk samples w a s after 1, 3, and 8 days at 8 C. The evaluation was a triangle test. The judges were six departmental employees trained and experienced in milk judging. Their task was to identify the diverging sample out of three. RESULTS A N D DISCUSSION
The results of the sensory evaluations o f the milk samples from the two series of trials representing protein adjustments by hyperfihration and ultrafiltration are summarized in Fig. 3 and Fig. 4. F r o m Fig. 3 it appears that adjustment o f protein content without sensory discrimination by changing SNF is restricted to a limited range, approximately + .3% SNF from the normal. Below that limit, the samples achieved a flat, watery taste, and above, the taste became sahy-sweet. Contrary to expecta-
TABLE 2. Typical operating conditions for hyperfiltration of skim milk. Inlet pressure Inlet temperature
Batch size Recirculation rate Operation time
5.0 MPa 8C 130 liters 540 liters/h 18 h
Journal of Dairy Science Vol. 61, No. 6, 1978
810
POULSEN pErmEATE
FIG. 2. Principle of continuous ultrafiltration plant.
t i o n a n d t o r e p o r t s in t h e l i t e r a t u r e , increasing f a t c o n t e n t a c c e n t u a t e d t h e a b n o r m a l flavors. As seen f r o m t h e figure, c h a n g e s in S N F were m o r e easily d e t e c t e d b y t h e j u d g e s as t h e f a t c o n t e n t was increased. The standardization of protein content only b y u l t r a f i l t r a t i o n , was p r a c t i c a b l e w i t h i n a b r o a d interval w i t h o u t a f f e c t i n g t h e s e n s o r y
TABLE 3. Typical operating conditions for continuous ultrafiltration of skim milk. Skim milk Inlet temperature Inlet pressure Inlet flow
11 C .11 MPa 320 liters/h
Module 1 Inlet pressure Outlet pressure Inlet temperature Permeate flow
.46 MPa .08 MPa 11 C 75 liters/h
Module 2 Inlet pressure Outlet pressure Inlet temperature Permeate flow
.45 MPa .09 MPa 12 C 55 liters/h
properties of milk (Fig. 4). By e x t r a p o l a t i o n we estimate that th~ protein c o n t e n t could be varied within the following limits, before a significant response w~s reached: for skim milk 3.1 to 6.4%; for half-skimmed milk 1.75 to
1 % COR RECT RESPONSE 100 d
b % CORRECT AESPONSE
700.
9O
• C , ~ ~ - - - ~
•
SKIM MILK HALFSKIM MILK WHOLE MILK
80-
70.
70 6O.
60 50.
ONFiDENCE LIMIT FOR CORRECT RESPONSE 50-
. . . . . . . . . . . . . . . . . . . . . . . . .
40
95%=C=ONFI DENCE LIMIT FOR CORRECT RESPONSE
40.
30
+!,
4 35
123 45
•
•
SKIM M I L K
o
o
HALF SKiM MILK
O
O WHOLE MILK
16.8 5.5
2 .3 6.5
%$NF % PROTEIN
FIG. 3. Sensory detection of differences in milk standardized in S.N.F. by reverse osmosis (each point representing 36 evaluations). Journal of Dairy Science Voh 61, No. 6, 1978
30.
,'0
~!(
;+
°'++
+!+
01+
~'R.....
FIG. 4. Sensory detection of differences in milk standardized in protein by ultrafiltrafion (each point representing 48 evaluations).
SYMPOSIUM: STANDARDIZING MILK FOR PROTEIN CONTENT 6.5%, and for whole milk from 1.5% to more than 6.5%. Significant differences between paired samples were determined by Chisquare, and in Table 4 the Chi square values are given for the three types of liquid milk standardized for protein content. Table 4 shows that the panel was not able to differentiate with certainty protein in whole milk. In half-skimmed milk, only 1.5% could be detected with certainty, and in skim milk, 1.5%, 2.5%, and 6.5%. For the skim milk with low protein tactility, surface gloss, and differences in translucency played a more prominent part in differentiating it than taste or flavor. To estimate the significance of fat and protein in the sensory response and to evaluate any interaction between fat and protein, a statistical analysis of variance was performed. The results are in Table 5. Analysis shows that changes in both fat and protein could be detected by the panel, but no interaction between fat and protein seemed to occur. In evaluating the significance of the taste responses for milk with different protein con-
811
tents, it should be remembered that the protein itself is rather tasteless, and the observed response is undoubtedly due to constituents that accompany the proteins when it is concentrated by ultrafiltration. To get an idea of which nonprotein constituents may contribute to the sensory discrimination between milks standardized to different protein content, the standardized milk samples were analyzed for lactose, ash, sodium, potassium, calcium, and phosphorus. The results of these estimations are summarized in Fig. 5. The content of lactose is virtually constant within the protein intervals investigated. For sodium and potassium there is only a small variation amounting to about -+5% of the total. The content of calcium and phosphorus on the contrary increase in proportion to protein. This was expected because of protein-bound phosphorus and colloidal calcium and phosphorus. The variation in calcium and phosphorus accounts for about 60% of the variation in total ash. These data seem to indicate that the cause of the sensory differences was either a physical phenomenon such as tactility or some other ash constituents that were not determined.
TABLE 4. Chi squares for differences between paired samples of three types of liquid milk standardized for protein.
Protein content %
Chi square corrected for continuity
Significance
Skim milk
1.5 2.5 3.5 4.5 5.5 6.5
38.25 17.85 .74 .59 .96 3.96
*** *** n.s. n.s. n.s. *
Half-skimmed milk
1.5 2.5 3.5 4.5 5.5 6.5
5.27 1.15 .00 .21 1.35 .06
* n.s. n.s. n.s. n.s. n.s.
Whole milk
1.5 2.5 3.5 4.5 5.5 6.5
2.84 2.30 .21 t .90 .30 .30
n.s. n.s. n.s. n.s. n.s. n.s.
Type of liquid milk
* *
*99.9% probability.
*95% probability.
812
POULSEN
TABLE 5. Analysis of variance of sensory response from fat, protein, and their interaction. Source of variation
Degrees of freedom
Mean square
Treatment Replication Protein (P) Fat (F) PXF Error
17 7 5 2 10 119
3.57"** 1.63 n.s. 6.05*** 7.38*** 1.46 n.s. .82
***99.9% probability.
Layout of Industrial Equipment for Protein-Standardization of Liquid Milk by Ultrafiltration
The problem of laying out of industrial equipment that could standardize protein continuously and under automatic control was turned over to The Danish Dairies Machine
o
% CONSTITUENT
SNF
11.0 IO.O
Works, Kolding, representing the D.D.S., manufacturers of the membrane equipment. They suggested the flow diagram, shown in Fig. 6. Figure 6 shows an arrangement for automatic standardization of both protein and fat in liquid milk. The milk is preheated in the regenerative section of the plate heat exchanger, separated into cream and skim milk and the skim milk is fed to the ultrafiltrationplant. The ultrafiltration-plant is arranged either to increase or decrease protein content by controlled removal of either permeate or retentate. By doing this, the retentate also would be protein-standardized, and this might be advantageous for other purposes. After the protein is standardized, cream is added with a Milko-tester and controller. Finally the standardized milk is homogenized, H.T.S.T.-pasteurized, and cooled. The following outline is given for a 25,000 liter/h unit (Table 6). When the protein content is decreased, skim milk and permeate are mixed in front of the final protein controller. The skim milk connection is shown as a stippled line in Fig. 6. In (2) the flow diagram was published (Fig. 7) for the manufacturing of milk with low fat and enriched protein. The authors were em-
9.O 8.O
5.u~^'
O
O
0
v
v
0 LACTOSE
04
4.
1.0 .9.8.7.
~
ASH
.6. .5. ,4
CALCIUM =o
o
3!6
415
--
1!5
21.5
~
POTASSIUM
o t 5.5
615
Ig PROTE~N %
FIG. 5. Variations in some of the milk constituents with protein as standardized by uhrafihration. Journal of Dairy Science Vol. 61, No. 6, 1978
FIG. 6. Plant for automatic standardization of protein and fat.
SYMPOSIUM: STANDARDIZING MILK FOR PROTEIN CONTENT
813
TABLE 6. Examples of protein standardization in milk by use of ultrafiltration equipment. Increasing the protein content Actual protein % in raw milk Desired protein % in liquid milk Degree of concentration* (3.6-.2)/(3.4-.2) Amount of protein standardized liquid milk (25,000/1.0625) Amount of permeate (25.000--23,529) Membrane area Pt/P0 at 50C
3.4 3.6 1.0625 23,529 kg/h 1,471 kg/h 27 m 2 6.5/2.0 bar
Decreasing the protein content Actual protein % in raw milk Desired protein % in liquid milk Protein % in surplus retentate (adjusted to) Surplus retentate containing 8.0% protein I25,000 × (3.8-3.6)1/(8.0-3.6) Concentration degree* (8.0-.2)/(3.8-.2) Amount of protein standardized liquid milk (25,000-1,136) Amount of milk through the U.F. unit (1,136 × 2.1667) Membrane area Px/P0 at 50C
3.8 3.6 8.0 1,136 kg/h 2.1667 23,864 kg/h 2,461 kg/h 34 m 2 6.5/2.0 bar
*Corrected for nitrogen components in permeate (% N × 6.38 app. equal to .2).
ployed by the Swedish c o m p a n y Alfa-Laval, who kindly have m a d e it available for this presentation. Figure 7 shows t h e l a y o u t of a batch process designed for p r o d u c t i o n of 25,000 kg per day o f liquid milk with an i n c r e m e n t in protein
c o n t e n t o f a b o u t 1%. The u h r a f i l t r a t i o n unit is the D O R R - O L I V E R t y p e with a m e m b r a n e area of 20 m 2 described. Skim milk (4 C) is fed by the p u m p (P.U. 1) through the plate heat exchanger, where the t e m p e r a t u r e is raised to 30 C before entering the u h r a f i h r a t i o n unit.
e{,@ lll I! ,O
SKIM M I L K ,...r
I
i
[
!I
Rv~-~_UF 1-6HM
RETENTATE
I-(~
,
!
! 1
FIG. 7. Batch process layout for protein enrichment of liquid milk. Journal of Dairy Science Vol. 61, No. 6, 1978
814
POULSEN
The capacity is adjusted by the regulation valve (RV 1). In the ultrafiltration unit, the skim milk is circulated by the pump (P.U. 2) at a capacity of 100 mS/h. This gives a pressure difference across the membrane of approximately 1.8 bar. The permeate is sent to tank T 2. The surplus flow of skim milk returns through the plate cooler to the process tank. The pump P.U. 3 acts as a booster pump. When the desired concentration is reached, the filtering process is stopped and the filtration unit is emptied into the process tank. After fat standardizing, the product is pumped by P.U. 1 to homogenizing and HTST heat treatment. The permeate is removed continuously by pump P.U. 4. The whole process is claimed to be temperature, pressure, and flow controlled, and it has safety alarm for critical pressure, temperature, o r flow.
The entire installation is designed for C.I.P.
and experiments have shown that the protein content can be adjusted within a broad range without detectable organoleptic consequences. Units for ultrafihration have improved rapidly over recent years in construction, materials, and reliability and the units are coming closer to traditional and empirical sanitary requirements for dairy equipment in general. Further improvement, especially of the controlling methods and controlling instruments, is desirable. But there is no doubt that the manufacturers of dairy equipment in the near future will be able to present a fully reliable, easily operated, and automatically controlled commercial scale equipment for that purpose. Political considerations may create the greatest obstacles. It is a challenge to all dairy scientists and technologists to fill the technological gap so that an o p t i m u m utilization of the vast possibilities of the protein standardization will be possible when the legal aspects are clarified.
SUMMARY
Lack of uniformity in the milk supply has caused much trouble to the dairy industry. However, no dairymen would want alI milk to have the same composition. No composition would be o p t i m u m for all puposes. What the dairyman needs is suitable equipment and sufficient knowledge to adjust the composition of milk for various purposes and the permission to do so. The permission should be founded on approved standards of !dentity and controlled by a requirement for precise labeling. It would be highly advantageous if standards of identity for standardized liquid milk for consumption could be uniform world-wide. All the vital aspects involved need careful discussion in a gathering of experts who cover the whole range of dairy interests. It seems obvious that close cooperation between the I.D.F. and the A.D.S.A. would create the most ideal basis for preparing appropriate action. Ultrafiltration has proved feasible for standardizing the protein content in liquid milk,
Journal of Dairy Science Vol. 61, No. 6, 1978
REFERENCES
1 The EEC regulation for liquid milk and cream. Council Regulation (EEC), 1411/71 of June 29, 1971. Code of principles concerning milk and milk products and associated standards. FAO/WHO (5th ed.) Rome, 1966. 2 Gerhold, Hayer, and GUtter. 1975. Eiweissangereicherte Milchprodukte durch Ultrafiltration ernahrungsphysiologische hochwertige Nahrungsmittel. Deutsche Milchwirtschaft 26:265. 3 Pangbom, R. M. and W. L. Dunkley. 1964a. Sensory discrimination of fat and solids-not-fat in milk. J. Dairy Sci. 47:719. 4 Pangbom, R. M. and W. L. Dunkley. 1964b. Difference-preference evaluation of milk by trained judges. J. Dairy Sci. 47:1414. 5 Pangbom, R. M. and W. L. Dunkley. 1966. Sensory discrimination of milk salts, lactose, nondialyzable constituents, and algingum in milk. J. Dairy Sci. 49:1. 6 Wahid-UI-Hamid, S. S. and L. J. Manus. 1960. Effect of changing the fat and nonfat solids of milk. J. Dairy Sci. 43:1430. 7 Stull, J. W. and J. S. HiUman. 1960. Relation between composition and consumer acceptance of milk beverages. J. Dairy Sci. 43:945.
The Danish Government Research Institute for Dairy Industry DK--3400 Hillerod, Denmark ABSTRACT
In the future the worldwide dairy industry will need more sophisticated standardization of market milk, protein standardization, along with the fat standardization which has been carried out in many countries for years. F o r many reasons, the standardization of protein content similar to that of fat content seems justified. Such action calls, however, for the establishment of standards of identity which could be a further elaboration of the universally accepted Code of Principles. Establishing such a standard is a major challenge which the dairy industry in all countries will have to meet, and the sooner negotiations are started the better. The naturally occurring variations in composition are known well; the possible modifications are beyond computation. Experiments show that ultrafiltration is a feasible method for standardizing protein content in milk. Protein adjustments by ultrafiltration are possible over a relatively broad range without detectable organoleptic consequences. We estimate that for whole milk (3.5% fat) the protein can vary from 1.5% to more than 6.5%; for half-skimmed milk (1.5 to 1.8% fat) from 1.75% to 6.5%, and for skim milk from 3.1% to 6.4%. This leaves a broad field for technological exploitation and regulatory decisions. The producers of ultrafiltration equipment believe that satisfactory industrial scale set ups are at hand, although some details need further improvement, especially automatic control systems. No technical obstacle seems to prevent the use of this technology. The major problem in standardizing the protein content of milk seems to be political.
INTRODUCTION
Milk is by nature complex. Because of its all-around content of nutritional constituents, it always has been considered a unique food for human beings. It is no wonder that milk as a food has been shown the greatest respect everywhere on earth, and that most people have a deep-rooted dislike of any adulteration of milk. The question is, what is to be considered an adulteration? Fat standardization now is common in most countries and people are getting used to it and hardly consider it an adulteration. One of the reasons for introducing fat standardization was that milk fat was the most valuable constituent of the milk at one time, and a reliable method was available for determining fat content. Since the introduction of fat standardization, the requirements for composition of foods and the demand and price of the individual food constituents have changed in favor of protein. Until now, however, it has not been possible to standardize the protein content of the milk on a commercial scale without affecting the other constituents o f the milk severely and thereby affecting the flavor. The development of the membrane filtration technique during the last few years now seems to make adjustment of milk protein possible. Because o f the rather large variation in the protein content of milk during the year (Table 1), it is uncertain how much longer it will be possible to define milk according to the Codex Alimentarius Commission of 1963 or in the E.E.C. I regulation 1411/71 for liquid milk. To provide for fair trade and to simplify marketing of liquid milk, I think a modern society should have a complete standard for liquid milk comprising definition, composition, and specification.
Actual Variations in the Content of Fat and Protein in Milk Received at Danish Dairies
According to statistics issued by The Danish Dairy Federation Feb., 1976, the total amount
Received July 27, 1977. 1E.E.C. = European Economic Cooperation. 1978 J Dairy Sci 61:807--814
807
808
POULSEN
TABLE 1. Fat and protein percentages in raw milk received for processing at fifty dairies (1974 to 1975).
Period
Milk kg × 10 #
Average fat %
Average protein %
1974 9/27-10/10 10/11-10/24 10/25-11/7 11/8--11/21 11/22--12/5 12/6--12/19
57.98 57.02 56.07 55.38 56.13 57.00
4.46 4.48 4.47 4.45 4.43 4.40
3.66 3.66 3.66 3.63 3.57 3.53
58.54 60.14
4.36 4.32 4.28 4.26 4.27 4.27 4.28 4.29 4.26 4.24 4.21 4.17 4.15 4.12 4.11 4.14 4.17 4.15 4.24 4.33
3.50 3.46 3.40 3.37 3.37 3.35 3.34 3.29 3.29 3.28 3.33 3.43 3.44 3.39 3.35 3.34 3.36 3.33 3.43 3.53
4.27
3.42
1975 12/20--2/1 1/3--1/16 1/17--1/30 1/31--2/13 2/14--2/27 2/28--3/13 3/14--3/27
3/28--4/10 4/11--4/24 4/25--5/8 5/9--5/22 5/23--6/5 6/6--6/19 6/20--7/3 7/4--7/17 7/18--7/31 8tl--8/14 8/15--8/28 8/29-9/11 9/12--9/25
61.95
64.57 68.69 73.38 77.88 80.57 82.52 85.62 91.23 71.24 87.11 82.48 77.69 74.62 69.99 66.53 64.51 63.88
Average 9/27 (1974) 9/25 (1975)
of milk received at the Danish Dairies Oct. 1974 to Oct. 1975 was 4,628 million kg. The Applicability of Membrane Filtration Technique for Standardizing the Protein Content in Liquid Milk
The simplest way to alter protein c o n t e n t in liquid milk w o u l d be either to add or r e m o v e water. The c o n t e n t o f all the o t h e r milk constituents w o u l d be altered proportionally. The possibilities for specific a d j u s t m e n t o f the protein c o n t e n t in milk, however, have been limited until the m e m b r a n e t e c h n i q u e became technologically feasible near the beginning of this decade. Much i n f o r m a t i o n has been published in recent years concerning f u n d a m e n t a l principles of m e m b r a n e filtering technique, and m e m brane filtration already has reached c o m m e r c i a l Journal of Dairy Science Vol. 61, No. 6, 1978
scale, and m a n y different m e m b r a n e filtering units are in o p e r a t i o n for several purposes in different countries. By using one of the t w o m e m b r a n e filtering techniques, either the hyperfiltration (reverse osmosis, RO) or ultrafiltration, it is possible to adjust either the total solids-not-fat (SNF) or to carry o u t a specific a d j u s t m e n t of t h e protein. M e m b r a n e filtration represents a relatively simple physical separation which can be carried o u t at a sufficiently l o w t e m p e r a t u r e to avoid alteration in proteins and o t h e r heat sensitive constituents of the milk. A l t h o u g h the m e m brane filtering t e c h n i q u e , in itself, is a rather simple process, the e q u i p m e n t c a n n o t be claimed to be simple. The m e m b r a n e , which f o r m s the f u n d a m e n t a l part o f the e q u i p m e n t is a novel d e v e l o p m e n t which departs f r o m conventional w i s d o m regarding materials to be used
SYMPOSIUM: STANDARDIZING MILK FOR PROTEIN CONTENT for product contact surfaces. The manufacturers of membranes have succeeded in finding materials which can meet public health demands and which are able to resist detergents and cleaning temperatures ordinarily used. Experience has shown that satisfactory cleaning is feasible.
RETENTATE
Influence of SNF or Protein Adjustments on the Sensory Properties of Fluid Milk
Sensory properties are of the utmost importance in any adjustment of the constituents of fluid milk. To investigate to what extent adjustment of the content of SNF and/or protein can be accomplished without perceptible alteration of the character of the product, the following research has been carried out at the Danish Government Research Institute for Dairy Industry. The research comprised the three normal types of fluid milk on the Danish market: whole milk (minimum 3.5% fat), half-skimmed milk (minimum 1.5%, maximum 1.8% fat), and skim milk. These milks were standardized in the range of 1.5 to 6.5% protein at intervals of 1%. In three series, this was accomplished by hyperfiltration (RO) so that the SNF also was adjusted proportionally. In three other series the protein was adjusted by ultrafiltration so that the other SNF milk constituents were practically unchanged. All samples were prepared f r o m mixed herd milk, and all adjustments were in the skim milk fraction. All samples were finally high temperature-short time (HTST) heat treated (77 C, 15 s), and the samples of whole milk and half-skimmed milk were homogenized as well. After heat treatment, the milk samples were cooled to 8 C and stored at that temperature until evaluated. MATERIALS AND METHODS
The hyperfiltration (RO) was accomplished by a pilot module, manufactured by D.D.S. (The Danish Sugar Mills). It had a membrane area of .36 m 2 type D.D.S. 990 (Fig. 1). The typical operating conditions are in Table 2. For ultrafiltration we used a continuously operating plant unit made by The Danish Dairies Machine Works, Kolding, and D.D.S. (Table 3). The two modules are connected in series and have a membrane area of 4 m 2 each (D.D.S. type 600, celluloseacetate). Typical
809
) PERMEATE
FIG. 1. Principle of batch plant for hyperfiltration.
operating conditions for the ultrafittration are in Table 3. The D.D.S. membrane modules used are tubular plate and frame modules with shortpass narrow channels that give essentially laminar flow and have low internal volume. Sensory Evaluation
Sensory evaluation of the milk samples w a s after 1, 3, and 8 days at 8 C. The evaluation was a triangle test. The judges were six departmental employees trained and experienced in milk judging. Their task was to identify the diverging sample out of three. RESULTS A N D DISCUSSION
The results of the sensory evaluations o f the milk samples from the two series of trials representing protein adjustments by hyperfihration and ultrafiltration are summarized in Fig. 3 and Fig. 4. F r o m Fig. 3 it appears that adjustment o f protein content without sensory discrimination by changing SNF is restricted to a limited range, approximately + .3% SNF from the normal. Below that limit, the samples achieved a flat, watery taste, and above, the taste became sahy-sweet. Contrary to expecta-
TABLE 2. Typical operating conditions for hyperfiltration of skim milk. Inlet pressure Inlet temperature
Batch size Recirculation rate Operation time
5.0 MPa 8C 130 liters 540 liters/h 18 h
Journal of Dairy Science Vol. 61, No. 6, 1978
810
POULSEN pErmEATE
FIG. 2. Principle of continuous ultrafiltration plant.
t i o n a n d t o r e p o r t s in t h e l i t e r a t u r e , increasing f a t c o n t e n t a c c e n t u a t e d t h e a b n o r m a l flavors. As seen f r o m t h e figure, c h a n g e s in S N F were m o r e easily d e t e c t e d b y t h e j u d g e s as t h e f a t c o n t e n t was increased. The standardization of protein content only b y u l t r a f i l t r a t i o n , was p r a c t i c a b l e w i t h i n a b r o a d interval w i t h o u t a f f e c t i n g t h e s e n s o r y
TABLE 3. Typical operating conditions for continuous ultrafiltration of skim milk. Skim milk Inlet temperature Inlet pressure Inlet flow
11 C .11 MPa 320 liters/h
Module 1 Inlet pressure Outlet pressure Inlet temperature Permeate flow
.46 MPa .08 MPa 11 C 75 liters/h
Module 2 Inlet pressure Outlet pressure Inlet temperature Permeate flow
.45 MPa .09 MPa 12 C 55 liters/h
properties of milk (Fig. 4). By e x t r a p o l a t i o n we estimate that th~ protein c o n t e n t could be varied within the following limits, before a significant response w~s reached: for skim milk 3.1 to 6.4%; for half-skimmed milk 1.75 to
1 % COR RECT RESPONSE 100 d
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SKIM MILK HALFSKIM MILK WHOLE MILK
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ONFiDENCE LIMIT FOR CORRECT RESPONSE 50-
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16.8 5.5
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FIG. 3. Sensory detection of differences in milk standardized in S.N.F. by reverse osmosis (each point representing 36 evaluations). Journal of Dairy Science Voh 61, No. 6, 1978
30.
,'0
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FIG. 4. Sensory detection of differences in milk standardized in protein by ultrafiltrafion (each point representing 48 evaluations).
SYMPOSIUM: STANDARDIZING MILK FOR PROTEIN CONTENT 6.5%, and for whole milk from 1.5% to more than 6.5%. Significant differences between paired samples were determined by Chisquare, and in Table 4 the Chi square values are given for the three types of liquid milk standardized for protein content. Table 4 shows that the panel was not able to differentiate with certainty protein in whole milk. In half-skimmed milk, only 1.5% could be detected with certainty, and in skim milk, 1.5%, 2.5%, and 6.5%. For the skim milk with low protein tactility, surface gloss, and differences in translucency played a more prominent part in differentiating it than taste or flavor. To estimate the significance of fat and protein in the sensory response and to evaluate any interaction between fat and protein, a statistical analysis of variance was performed. The results are in Table 5. Analysis shows that changes in both fat and protein could be detected by the panel, but no interaction between fat and protein seemed to occur. In evaluating the significance of the taste responses for milk with different protein con-
811
tents, it should be remembered that the protein itself is rather tasteless, and the observed response is undoubtedly due to constituents that accompany the proteins when it is concentrated by ultrafiltration. To get an idea of which nonprotein constituents may contribute to the sensory discrimination between milks standardized to different protein content, the standardized milk samples were analyzed for lactose, ash, sodium, potassium, calcium, and phosphorus. The results of these estimations are summarized in Fig. 5. The content of lactose is virtually constant within the protein intervals investigated. For sodium and potassium there is only a small variation amounting to about -+5% of the total. The content of calcium and phosphorus on the contrary increase in proportion to protein. This was expected because of protein-bound phosphorus and colloidal calcium and phosphorus. The variation in calcium and phosphorus accounts for about 60% of the variation in total ash. These data seem to indicate that the cause of the sensory differences was either a physical phenomenon such as tactility or some other ash constituents that were not determined.
TABLE 4. Chi squares for differences between paired samples of three types of liquid milk standardized for protein.
Protein content %
Chi square corrected for continuity
Significance
Skim milk
1.5 2.5 3.5 4.5 5.5 6.5
38.25 17.85 .74 .59 .96 3.96
*** *** n.s. n.s. n.s. *
Half-skimmed milk
1.5 2.5 3.5 4.5 5.5 6.5
5.27 1.15 .00 .21 1.35 .06
* n.s. n.s. n.s. n.s. n.s.
Whole milk
1.5 2.5 3.5 4.5 5.5 6.5
2.84 2.30 .21 t .90 .30 .30
n.s. n.s. n.s. n.s. n.s. n.s.
Type of liquid milk
* *
*99.9% probability.
*95% probability.
812
POULSEN
TABLE 5. Analysis of variance of sensory response from fat, protein, and their interaction. Source of variation
Degrees of freedom
Mean square
Treatment Replication Protein (P) Fat (F) PXF Error
17 7 5 2 10 119
3.57"** 1.63 n.s. 6.05*** 7.38*** 1.46 n.s. .82
***99.9% probability.
Layout of Industrial Equipment for Protein-Standardization of Liquid Milk by Ultrafiltration
The problem of laying out of industrial equipment that could standardize protein continuously and under automatic control was turned over to The Danish Dairies Machine
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Works, Kolding, representing the D.D.S., manufacturers of the membrane equipment. They suggested the flow diagram, shown in Fig. 6. Figure 6 shows an arrangement for automatic standardization of both protein and fat in liquid milk. The milk is preheated in the regenerative section of the plate heat exchanger, separated into cream and skim milk and the skim milk is fed to the ultrafiltrationplant. The ultrafiltration-plant is arranged either to increase or decrease protein content by controlled removal of either permeate or retentate. By doing this, the retentate also would be protein-standardized, and this might be advantageous for other purposes. After the protein is standardized, cream is added with a Milko-tester and controller. Finally the standardized milk is homogenized, H.T.S.T.-pasteurized, and cooled. The following outline is given for a 25,000 liter/h unit (Table 6). When the protein content is decreased, skim milk and permeate are mixed in front of the final protein controller. The skim milk connection is shown as a stippled line in Fig. 6. In (2) the flow diagram was published (Fig. 7) for the manufacturing of milk with low fat and enriched protein. The authors were em-
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FIG. 5. Variations in some of the milk constituents with protein as standardized by uhrafihration. Journal of Dairy Science Vol. 61, No. 6, 1978
FIG. 6. Plant for automatic standardization of protein and fat.
SYMPOSIUM: STANDARDIZING MILK FOR PROTEIN CONTENT
813
TABLE 6. Examples of protein standardization in milk by use of ultrafiltration equipment. Increasing the protein content Actual protein % in raw milk Desired protein % in liquid milk Degree of concentration* (3.6-.2)/(3.4-.2) Amount of protein standardized liquid milk (25,000/1.0625) Amount of permeate (25.000--23,529) Membrane area Pt/P0 at 50C
3.4 3.6 1.0625 23,529 kg/h 1,471 kg/h 27 m 2 6.5/2.0 bar
Decreasing the protein content Actual protein % in raw milk Desired protein % in liquid milk Protein % in surplus retentate (adjusted to) Surplus retentate containing 8.0% protein I25,000 × (3.8-3.6)1/(8.0-3.6) Concentration degree* (8.0-.2)/(3.8-.2) Amount of protein standardized liquid milk (25,000-1,136) Amount of milk through the U.F. unit (1,136 × 2.1667) Membrane area Px/P0 at 50C
3.8 3.6 8.0 1,136 kg/h 2.1667 23,864 kg/h 2,461 kg/h 34 m 2 6.5/2.0 bar
*Corrected for nitrogen components in permeate (% N × 6.38 app. equal to .2).
ployed by the Swedish c o m p a n y Alfa-Laval, who kindly have m a d e it available for this presentation. Figure 7 shows t h e l a y o u t of a batch process designed for p r o d u c t i o n of 25,000 kg per day o f liquid milk with an i n c r e m e n t in protein
c o n t e n t o f a b o u t 1%. The u h r a f i l t r a t i o n unit is the D O R R - O L I V E R t y p e with a m e m b r a n e area of 20 m 2 described. Skim milk (4 C) is fed by the p u m p (P.U. 1) through the plate heat exchanger, where the t e m p e r a t u r e is raised to 30 C before entering the u h r a f i h r a t i o n unit.
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RETENTATE
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! 1
FIG. 7. Batch process layout for protein enrichment of liquid milk. Journal of Dairy Science Vol. 61, No. 6, 1978
814
POULSEN
The capacity is adjusted by the regulation valve (RV 1). In the ultrafiltration unit, the skim milk is circulated by the pump (P.U. 2) at a capacity of 100 mS/h. This gives a pressure difference across the membrane of approximately 1.8 bar. The permeate is sent to tank T 2. The surplus flow of skim milk returns through the plate cooler to the process tank. The pump P.U. 3 acts as a booster pump. When the desired concentration is reached, the filtering process is stopped and the filtration unit is emptied into the process tank. After fat standardizing, the product is pumped by P.U. 1 to homogenizing and HTST heat treatment. The permeate is removed continuously by pump P.U. 4. The whole process is claimed to be temperature, pressure, and flow controlled, and it has safety alarm for critical pressure, temperature, o r flow.
The entire installation is designed for C.I.P.
and experiments have shown that the protein content can be adjusted within a broad range without detectable organoleptic consequences. Units for ultrafihration have improved rapidly over recent years in construction, materials, and reliability and the units are coming closer to traditional and empirical sanitary requirements for dairy equipment in general. Further improvement, especially of the controlling methods and controlling instruments, is desirable. But there is no doubt that the manufacturers of dairy equipment in the near future will be able to present a fully reliable, easily operated, and automatically controlled commercial scale equipment for that purpose. Political considerations may create the greatest obstacles. It is a challenge to all dairy scientists and technologists to fill the technological gap so that an o p t i m u m utilization of the vast possibilities of the protein standardization will be possible when the legal aspects are clarified.
SUMMARY
Lack of uniformity in the milk supply has caused much trouble to the dairy industry. However, no dairymen would want alI milk to have the same composition. No composition would be o p t i m u m for all puposes. What the dairyman needs is suitable equipment and sufficient knowledge to adjust the composition of milk for various purposes and the permission to do so. The permission should be founded on approved standards of !dentity and controlled by a requirement for precise labeling. It would be highly advantageous if standards of identity for standardized liquid milk for consumption could be uniform world-wide. All the vital aspects involved need careful discussion in a gathering of experts who cover the whole range of dairy interests. It seems obvious that close cooperation between the I.D.F. and the A.D.S.A. would create the most ideal basis for preparing appropriate action. Ultrafiltration has proved feasible for standardizing the protein content in liquid milk,
Journal of Dairy Science Vol. 61, No. 6, 1978
REFERENCES
1 The EEC regulation for liquid milk and cream. Council Regulation (EEC), 1411/71 of June 29, 1971. Code of principles concerning milk and milk products and associated standards. FAO/WHO (5th ed.) Rome, 1966. 2 Gerhold, Hayer, and GUtter. 1975. Eiweissangereicherte Milchprodukte durch Ultrafiltration ernahrungsphysiologische hochwertige Nahrungsmittel. Deutsche Milchwirtschaft 26:265. 3 Pangbom, R. M. and W. L. Dunkley. 1964a. Sensory discrimination of fat and solids-not-fat in milk. J. Dairy Sci. 47:719. 4 Pangbom, R. M. and W. L. Dunkley. 1964b. Difference-preference evaluation of milk by trained judges. J. Dairy Sci. 47:1414. 5 Pangbom, R. M. and W. L. Dunkley. 1966. Sensory discrimination of milk salts, lactose, nondialyzable constituents, and algingum in milk. J. Dairy Sci. 49:1. 6 Wahid-UI-Hamid, S. S. and L. J. Manus. 1960. Effect of changing the fat and nonfat solids of milk. J. Dairy Sci. 43:1430. 7 Stull, J. W. and J. S. HiUman. 1960. Relation between composition and consumer acceptance of milk beverages. J. Dairy Sci. 43:945.