Buffering curves of ideal whey fraction obtained from a cascade mebrane separation process

Abstract

Skimmed milk was fractionated via a cascade system: Graphic plotting of milcrofiltration (MF); ultrafifltration (UF); nanofiltration (NF); and reserve osmosis (RO). The buffering curves of each fraction were studied over the pH range 4–7. Depending on their composition, the individual permeate streams showed different buffering capacity values and pH ranges where the buffering occurred. The concentration of active buffering substances in the permeates decreased (in mmol/L) from ~26.6 (MF) to ~17.4 (UF) to 1.39 (NF) to 0.07 (RO). Contributions to the total buffering capacity for MF permeate, which represents the serum phase of milk, were ~37% from whey proteins and ~63% from milk salts (especially citrates, phosphates and carbonates) including lactose and water.

Introduction

The buffering capacity of milk ando f milk constituents is important for most processing steps in dairy technology and especially for the raw milk quality and its intentional heating and adicification during the manufacturing of dairy products. This study describes the individual contribution of the differentt mil serum fractions, which can be obtained by membrane filtration techniques, to the overall buffering effect of milk.

The mass action law of Guldberg and Waage, the foundational law of chemical equilibrium (De Levie, 2002), allowed Van Slyke (1922) to derive and propose the term buffer value (β) as a quantative unit for measuring the effect of a buffer substance. Acid-base equilibria in milk have been the subject of chapters in many books (among others: Roeder (1954); Jenness and Patton (1959); Walstra and Jenness (1984); Singh et al. (1997) and of journal articles (e.g. Buchanan and Peterson, 1927; Whittier, 1929; Wiley, 1935a; Kirchmeier, 1980; Lucey et al., 1993; Salaün et al., 2005; Wolfschoon Pombo et al., 2012; Wolfschoon Pombo and Wolfschoon Ribeiro, 2014c), which was initially probably due to the importance of buffering in the milk acidifiction process. The mathematical understanding of the buffer value was developed and clearly shown by Donald Van Slyke (1922) and in the presente article it suffices to mention that buffer value is understood as the ratio in the change in the number of moles of acid (or base) needed to change the pH of a system by a (defined) pH unit,

                                    β  =  dB   or β =  –dA           (1)

                                            dpH            dpH       

where B and A are the number of equivalents per litre of the strong base or acid, respectively. For a simple buffer, the differential ratio was given by van Slyke as:

                                    β  =   2.3C(K[H+])           (2)

                                              ([K + [H+])2          

where C is the total buffer concentration and K the dissociation constant of the conjugate acidic–basic species. Graphic plotting of this ratio against pH would show where the maximum buffering occurs.

The latest comprehensive review of the buffering capacity of dairy products was made by Salaün et al. (2005). Many attemps have been made to explain the buffering by the serum phase of milk using either rennet, acid or sweetwhey (Dolby; McDowall, 1935; Wiley, 1935b; Boulet; Rose, 1954; Hill et al., 1985). However, the effect of rennet and of acidification on milk coagulation leads to a serum phase which does not truly reflect the native state of the individual components.

Membrane separation processes (MF = microfiltration; UF = ultrafifltration; NF = nanofiltration; and RO = reverse osmosis) change the partition of milk components between the retentate and permeate streams. It has been shown that, for example, the buffering of MF skim milk concentrates increases proportionally to the concentration fator and especially due to its casein content (e.g. Salaün et al., 2005; Wolfschoon Pombo et al., 2012). In UF retentates, the increase is due to the concentration of not only casein but also whey proteins (e.g. Brulé et al., 1974; Covacevich; Kosikowski, 1979; Srilaorkul et al., 1989; St-Gelais et al., 1992) and calcium (Li; Corredig, 2014). The buffering curves of the milk fractions obteined from different membrane separation processes, apart from the MF and UF retentates, have not been determined. About 100 years ago, Lucius van Slyke (Van Slyke; Bosworth, 1915), using a porous earthenware filter and with a pressure of about 2.9 bar, separated and analysed the native serum phase of milk, and although its composition is well known today, only minor attention has been given to its buffering action. It is the purpose of the present study to fractionate skim milk MF permeate (the so-called ideal whey. i.e. the milk water phase) in cascade and to determine the buffering curves of permeates and retentates obteined from the membrane processes.

Conclusions

The presente study analysed the buffering of all the permeate streams derived from skim milk using a cascade filtration processes (MF, UF, NF and RO), which changed the composition of the milk serum phase according to the permeability of the particular membranes. This allowed a better understanding of the partition and contribution of the individual milk components to the changing buffering capacity. The was manifested in the MF permeate, whereas the buffer effect for the water phase without the whey proteins, that is from the milk salts (especially citrates and phosphates), lactose, and NPN was depicted by the curve of the UF permeate. The difference between both curves (MF permeate minus UF permeate) represented the buffer contribution of the whey proteins alone. The NF permeate showed the low buffering of the monovalent salts that might be associated with the little phosphate avaiable. The remaining buffering effect was basically arose due to the water and was noticeable in the NF and RO permeates.

It was shown that the different streams had different buffering capacity values and pH ranges where buffering occurs. This knowledge is important for any process steps which involve pH changes such as heating and acidification. In bacterial fermentations, buffering capacity directly affects fermentation time, amount of acid generated and hence flavour perception.

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 [120] POMBO, A. F. W; SPIEGEL, T. L.; HERNANDEZ-ZENIL, E. Buffering curves of ideal whey fraction obtained from a cascade mebrane separation process. International Journal of Dairy Technology, v. 69, p. 1-10, 2016.