Pressure induced changes in the gelation of milk protein concentrates

Trends in High Pressure Bioscience and Biotechnology R. Hayashi (editor) 9 2002 Elsevier Science B.V. All rights reserved.

P R E S S U R E I N D U C E D C H A N G E S IN T H E G E L A T I O N PROTEIN CONCENTRATES

445

OF MILK

B.J. Briscoe, P.F. Luckham and K.U. Staeritz Department of Chemical Engineering and Chemical Technology Imperial College of Science, Technology and Medicine Prince Consort Rd, London SW7 2BY, United Kingdom, e-mail: [email protected]

Gelation of aqueous milk protein concentrates made up of spray dried milk was monitored both at atmospheric pressure and at moderate pressures (up to 1000 bar) using rheological methods. A Paar Physica UDS 200 rheometer was used to measure the gelation time at atmospheric pressure as a function of the temperature for a protein concentration of 16% [w/w]. The temperature dependence of the gelation time is well approximated by the Ross-Murphy model which describes the concentration dependence of the gelation time and has been used in the past to describe the concentration dependence of the gelation time for some biological polymers. Using a purpose built Haake High Pressure High Temperature Rheometer, the effects of moderate pressures upon the gelation of milk protein concentrates (13% [w/w] and 16% [w/w] protein) were investigated. It was found that the gelation time reduces significantly (by up to almost an order of magnitude), when pressures of up to 1000 bar were applied. These findings, combined with the work of others indicate, that these pressures bring about two effects: 1) casein micelle dissociation and as a result facilitate reassociation of those smaller micelle fragments, and 2) denaturation of betalactoglobulin. These two mechanisms contribute to different degrees to the accelerated gelation process brought about by elevated pressure in the range investigated. It is not clear if association of whey proteins with caseins occur at moderate pressures around 50~ We have modeled the dependence of the rate of gelation on pressure by a Eyring reaction rate process type equation. This description is not inconsistent with the experimental results obtained.

1. I N T R O D U C T I O N It was at the end of the nineteenth century that the effects of the application of hydrostatic pressure on milk was originally investigated [1]. Hite' s work was instigated from the shortcomings that were being experienced with sterilisation and Pasteurisation Processes. The problem experienced at that time was the rapid temperature jumps associated with those processes. He was able to show that the application of 13.9 kbar, 1390 MPa applied for one hour was able to postpone the souring of milk for around 4 days. Despite this success,

446

however, technical difficulties associated with the process meant that commercially this was a non-viable process. In the developed world, in the last fifteen to twenty years, there has been a reluctance by the consumer to purchase heavily processed foods or foods containing additives. Thus food scientists have revisited the potentially valuable effects of pressure on foods. In the hundred years since Hite's study there have been considerable developments in process technology and the application of high pressures in the engineering of plastics, ceramics, metals, etc., has become routine. Now in Japan, it is possible to purchase high pressure processed foods such as fruit juices and sauces. It has been noted that compared to temperature processed products these foods retain a remarkable degree of perceived "freshness". The reason for this is simple. In a heating process, inevitably some part of the food, that in contact with the heat source is hotter for longer than the food in the middle and so becomes overheated and, to some extent "spoils". The application of a hydrostatic pressure to a fluid is uniform and hence localised "spoiling" is considerably reduced. Let us consider the role of pressure here. From Le Chatelier's principle we recall that when a system is subject to a constraint then it will adapt to remove the influence of the constraint. When applying pressure, that constraint will be volume, thus processes with a negative change in volume (-AV) will be enhanced and those with a positive change in volume (+AV), will be suppressed. The effect of pressure on milk is complex because of the complexity of milk itself. It must be remembered that milk is largely water, which is a hydrogen bonded liquid, whose structure breaks down at high pressures. The inter and intra molecular interactions between the proteins and water are also subject to the influence of pressure. For example, hydrogen bonding, which stabilises the a helix and 13 sheet structures, is influenced by pressure but this effect is probably less than ionic and hydrophobic interactions. For example, the ionisation of charged groups on a protein would involve a volume change, because the water molecules close to the charge reorient. This ordered alignment of water would give a dense layer of water around the charge and hence there would be a net volume decrease. As a result, application of pressure should increase the ionisation of charged groups on biological molecules. There have been many studies probing the effects of high pressures on milk proteins, particularly on the casein micelles. These all show that pressure breaks-down these micelles into smaller aggregates and that this changes the properties of the milk (see for example: [915, 19]). Studies have also been carried out on 13-1actoglobulin (Moiler (1998) [17], Tanaka (1996) [18]) and again some structural changes in the protein are seen to occur. There have also been studies to determine the effect that pressure has upon the physical properties of milk. For example Johnson et al [16] have shown that a more rigid product is produced in milk which has been treated at high pressures compared to untreated milk. However, this study itself was not formed at pressure; only the milk was held at a high pressure and the test performed at ambient pressure. Recently Zhu et al have developed a rheometer that probes the rheological properties of a material whilst it is still at high pressure. In his work, on aqueous polymer solutions, he showed that increasing pressure was rather analogous to increasing temperature, or adding electrolyte, in that it changed the solvency of the water for the polymer from a "good" solvent to a "poor" one. In this study, we report similar experiments on reconstituted skimmed milk, and in particular have investigated the gelation of reconstituted skimmed milk. Preliminary results of our findings have been published elsewhere [8, 5].

447

2. E X P E R I M E N T A L M E T H O D S Milk protein concentrates were made up from skimmed milk powder (Premier Baverages, UK), which was produced by spray drying and had undergone a special agglomeration process. In the experiments reported here, the milk was reconstituted at two different protein contents (13%w/w and 16%w/w). In the first step the milk powder was stirred with the help of a magnetic stirrer and in each case (in order to reach the final protein concentration), a Silverson (UK) high shear stirrer was used while no heat was supplied. It was assumed, that the milk proteins in the concentrates are either dissolved or in the case of casein micelles, are well dispersed. In order to reduce drying-out of the sample surface, the sample was covered with a thin layer of liquid paraffin (the tetradecane) during the experiments using the Paar rheometer in ambient air. The Haake High Pressure High Temperature (HPHT) Rheometer (Germany/UK), a purpose built viscometer based upon a Haake RV 100 viscometer adopts the principle of a standard rotational viscometer and uses a special magnetic coupling to transmit the driving torque from the sample which is housed in a pressure chamber [2], [3], [4], [5]. Pressures of up to 1000 bar can be applied to the material under investigation and the instrument measures the material properties while pressure is applied. The Haake HPHT Rheometer is unable to precisely measure the viscoelastic material properties but is able to measure the shear stress in a continuous rotational mode. The maximum measureable shear stress is about 60 Pa and is limited by the coupling strength of the magnets imbedded in the rotor and of the turning magnets on the opposite side of the (stainless steel) diaphragm which separates the driving parts of the rheometer from the parts which are inside the autoclave. The sample feeding and pressure generating unit has previously been modified by Huxley-Bertam Engineering (Cambridge, UK). The pressure vessel or autoclave, which was initially developed for flow operation and process control, was replaced by a special construction, which facilitates the operation in the discontinuous modes. The total volume of the autoclave was about 300 ml. A special pressure tube connected the pressurisation cylinder with the autoclave and valves allowed the regulation of the flow of the sample from the pressurisation cylinder into the autoclave. Inside the autoclave the flow was altered by some deviations of the instrument from the ideal Searle Type cylindrical rotational viscometer. The stator contained some slits in the outer cylinder, thus allowing the sample to enter the geometry laterally during the actual measurements. The temperature control was achieved by heating electrically the whole metal block of the autoclave, into which the cover of the instrument holding the geometry was placed. For the temperature control a Haake temperature controller was used. The commercially available Paar Physica rheometer was operated both in steady state and in oscillatory mode using the Z2 Searle-Type geometry with a sample volume of 100 ml.

3. E X P E R I M E N T A L

RESULTS

AND DISCUSSION

The changes of the dynamic mechanical properties of milk protein concentrates reconstituted at 16% protein content were measured in order to monitor the temperature dependence of the gelation time. In Figure 1 typical courses of the measured values of G' and G" , at a frequency of 1 Hz during gelation at 50~ are represented, monitored at a displacement of 0.04 units. These measurements were carried out at ambient pressure on the Paar Physica

448 UDS 200 rheometer. Initially the storage modulus is close to zero, whilst the loss modulus is larger than the storage modulus, although still small in value. For around 10,000 s little change is observed, after this point both the storage and loss modulus increase, and the rate of increase in the storage modulus is faster than that of the loss modulus, such that at about 19,300 s the cross over point of the moduli occurs. G' reaches its steepest slope at about 28,000 s and reaches a plateau above 40,000 s.

Gelation time, 16% protein, 50 C, displ. = 0.04, tg = 19300s 25

G' 20

G"

~' lO 5 0

oo

20000

30000

40000

50000

time[s]

Figure 1

Dynamic mechanical properties of 16% protein concentrates as a function of the elapsed time

In Figure 2 the dependence of the gelation time on the temperature, determined by the crossover-point of the loss and storage moduli is represented. The chosen frequency of oscillation was 1 Hz and the displacement was 0.04 units. The data were obtained from measurements similar to those presented in figure 1. Based upon a model developed by Ross-Murphy [6] a non linear regression was carried out according to the following equation

logtg=k-nlog(~cc-1) The dotted lines in figure 2 correspond to this model. The full lines correspond to the RossMurphy model without logarithmic conversion, i.e. tg --

The regression is based on the Levenberg-Marquardt method, a commonly used algorithm for non-linear regression. The Ross-Murphy model [6] represents the behaviour of the gelation

449 process as a function of the temperature for the current substance rather well. In this equation the analogy with the critical behaviour near a thermal phase transition was pointed out by Ross-Murphy [7].

n o n - l i n e a r tiffing, 16% protein, Ross-Murphy equation (1991)

"' log. conv 50000

~

"~'

v~thout

3OOOO

0

35

Figure 2

45

55 temperature [ C]

65

75

Dependence of the gelationtime on temperaturefor 16% protein content at ambient pressure

The gelation time as measured in the steady state shear measurements on the Paar Physica rheometer and on the Haake instrument, was defined as the steepest part of the shear stress-time curve. [8, 5]. In a previous paper [8] the two different measures of the gelation time were compared and found to be similar: In order to directly compare the gelation time measured with the Haake instrument with the gelation time measured with the Paar Physica rheometer in the oscillatory mode, a strain of ], = 0.1 was chosen as this induces an amount of shear disruption and hence gel time delay, comparable to that produced by the Haake system [8, 51. Figures 3 and 4 show the shear stress plotted as a function of time for various pressures at two milk protein concentrations, 13% and 16% respectively. It can be seen from both graphs that increasing the pressure decreases the gelation time. (The gelation time is now taken to be the steepest slope of the shear stress time curves). This effect is rather large, a factor of over 7 for the more dilute case. The question is, why does this occur? As was mentioned earlier, pressure will encourage any process which results in the reduction in volume of the system. Gelation is such a process as any particles, casein micelles, in the milk are attracted to each other in a gelation process. It has been determined by many workers [9-15, 19] that increased pressure encourages the casein micelle to fragment into smaller units, which would gel then more easily [5, 8]. Studies carried out on ]3-1actoglobulin at 500 bar [17, 18] prove, that at moderate pressures these globular proteins partly denature. We conclude, that acceleration of gelation is mainly due to micelle fragmentation and to a lesser extend to a limited denaturation of [3-1actoglobulin.

450

Haake, g e l a t l o n time, 13% p r o t e i n , 50 C, s t r a i n rate = 1131s 25 ,

20 ,.,,

1 !

g.

/

' ' 1 5 ;

f

1000 bar

to

/

r 1 bar

5 ~.

-

,,•

0 0

10000

20000

30000

40000

50000

60000

70000

time [ s ]

Figure 3

Dependence of the gelation time on pressure: Stress versus time for aggl. rec.milk (13% protein) at 50~ for three different pressures. Haake HPHT rheometer ( ~ = 113 /s). The gelation time is defined as the maximum slope and is at 1 bar : tg = 53,300s (+ 200s), at 600 bar: tg = 29,000s (+ 200s), at l O00bar: tg = 7,500s (+ 200s).

Haake, gelation time, 16% protein, strain rate = 113/s, T=50C 50 45 40 35

~"

30

~

2s

Q 20 ~

15

10 5 0 0

10000

20000

30000

40000

50000

60000

time [ s ]

Figure 4

Dependence of the gelation time on pressure: Stress versus time for aggl. rec.milk (16% protein) at 50~ for two different pressures. Haake HPHT rheometer ( ~ = 113 /s). The gelation time is defined as the maximum slope and is at 1 bar : tg = 23,500 s (+ 200s), at 1000bar: tg -- 8,900 s (+ 200s).

451 We shall now speculate upon the effects of pressure on the rate of gelation. As a simple first step the kinetic approach introduced simultaneously by Eyring [20] and Evans and Polanyi [21 ] will be adopted. According to Evans et al [21] tile specific rate constant kp can be written as:

kp = k o exp(- PAV*RT)

where k0 is the rate constant at zero pressure, kp the rate constant under pressure P and AV*is the pressure (as opposed to thermal) activation volume, which is the difference between the partial molar volume of the activated complex minus that of the reactants. Taking the rate of gelation kt (proportional to 1/t~) as the specific rate constant kp, we find for our system, that the experimental results are reasonably consistant with the above equation. Figure 5 shows the fitting of the experimental results for cp = 13 % protein and a temperature of T = 50~ Note that during the experiments the temperature fluctated slightly (AT~ + 2~ due to the large surface area of the pressurization vessel. It appears, that the aggregation of milk proteins under moderate pressures is a process, that can be treated to a first approximation by this first order kinetic approach. Further work will need to be performed to determine whether this approach is truly and generally applicable, or whether one only observes an effect of pressure on the gelation rate once the casein micelles have begun to fragment, which according to Schmidt [10], is around 100 bar.

rate of gelation, 13 % protein, 50 C, shear rate = 1131s

1.00E-03

i

.00E-04

'~ 1.00E41S

1.00E-06 -100

100

300

500

700

900

1100

pressure [ bar ]

Figure 5

Rate of gelation vs pressure. Fitting according to the Eyring reaction rate process type equation

It is sensible, to also derive an equation for the combined influences of temperature and pressure on the gelation time. The temperature dependence on gelation time at atmospheric pressure is governed by unfolding of globular proteins, as the caseins are very heat resistant. At moderate pressures we concluded, that the aggregation is mainly due to casein micelle dissociation which, as a result, facilitated gelation. The nature of the dependence of the

452 gelation time on temperature at moderate pressures may be different, as association of caseins with whey proteins may occur. Further experiments will need to show, whether gelation caused by the action of moderate pressures is a combined effect of heat and pressure (and involves a combined effect on caseins and whey proteins), or whether moderate pressures alone may bring about gelation at temperatures, where whey protein denaturation does not occur at atmospheric pressures.

REFERENCES [1] [21 [3] [4] [5] [6] [7]

[8]

[9] [10] [ll] [12] [13] [14] [15] [16] [17]

[18] [19]

[20] [21]

Hite, B.H., Bull. West Virginia Univ. A gric. Exp. Statn. 58(1899)15-35. Briscoe, B.J.; Luckham, P.F.; Zhu, S., Macromolecules 29 (1996) 6208-6211. Briscoe, B.J.; Luckham, P.F.; Zhu, S., Proc.Roy.Soc., A455(1999)737-756. Zhu, S.: "Rheology of Polymer Solutions at High Temperature and High Pressure". In: PhD-Thesis (1997), Imperial College, London. Staeritz, K.U.; Briscoe, B.J.; Luckham, P.F., High Pressure Research, Vol. 19(2000)55-60. Ross-Murphy, S.B., Food Polymers, Gels and Colloids - E.Dickinson (ed.), Cambridge: Royal Society of Chemistry (1991) 357-368. Ross-Murphy, S.B.; Tobitani, A.: "The Gel-Time- Theory and Practice". In: The Wiley Polymer Networks Group Review Series, Volume one, K. te Nijenhuis and W.J.Mijs (editors) (1998)39-49. Briscoe, B.J.; Luckham, P.F.; Staeritz, K.U.: "Dynamic and Steady State Measurements of Gelation Time of Milk Protein Concentrates at Moderate Pressures" In: Proceedings of the 2"d International Symposium on Food Rheology and Structure, March 12-16, 2000, Zuerich - P. Fischer, J.Marti, E.J. Windhab (eds.), 317-320. Payens, T.A.J.; Heremans, K., Biopolymers 8(1969)335-345. Schmidt, D.G.; Buchheim, W., Milchwissenschaft 25(1970)10, 596-600. Buchheim, W.; Prokopek, D., Dtsch. Milchwirtschaft 43(1992) 1374-1376. Shibauchi, Y.; Yamamoto, H.; Sagara, Y., Conformationai Change of Casein Micelles by High Pressure Treatment, in: High Pressure and Biotechnology, C. Balny; R.Hayashi; K. Heremans; P.Mason (eds.) Colloque INSERM/John Libbey Eurotext Ltd. (1992) 239-242. Lopez-Fandifio, R.; Ramos, M.; Olano, A., J.Dairy Res. 65(1998)69-78. Desobry-Banon, S.; Richard, F.; Hardy, J., J. DaiO, Sci. 77(1994)3267-3274. Ohmiya, K.; Fukami, K.; Shimizu, S.; Gekko, K., J. FoodSci. 52(1987)1, 84-87. Johnston, P.E.; Austin, B.A.;, Murphy, R.J., Milchwissenschafi 4(1993)48,206-209. Moller, R.E., Stapelfeldt, H., Skibsted, H.L., J.Agric.Food Chem. 46(1998)425-430. Tanaka, N.; Kunugi, S.; J.Biol.Macromol. 18(1996)33-39. Schmidt, D.G.; Payens, T.A.J., J.Coll.InterfSci. 39(1972)3,655-662. Eyring, H., J.Chem.Phys. 3(1935)107-115. Evans, M.G.; Polanyi, M., Trans. Faraday Soc. 3 I,(1935)875-894.