Physical Chemistry of Milk

Lesson 24. BUFFERING CAPACITY AND BUFFERING INDEX

Module 9. Buffers

Lesson 24

BUFFERING CAPACITY AND BUFFERING INDEX

24.1 Introduction

Constituents in any biological fluid retain their native state so long the pH of the solution is maintained unchanged. For effective utilization and to perform the designated functions by that fluid the constituent exist in different forms e.g., in milk the fat exists as an emulsion while the major protein casein exists in the form of micelles. Maintenance of uniform pH becomes possible when buffers are present in that solution. Milk also has some normal constituents which are having buffering action which prevents the changes due to variations in the milk such as microbial population in it or atmospheric changes environment in the pH of milk within a given pH range.

The buffering capacity of milk products is an important physico-chemical characteristic that corresponds to the ability of the product to be acidified or alkalinized. The parameters of this value depend on several compositional factors including small constituents (inorganic phosphate, citrate, organic acids) and milk proteins (caseins and whey proteins). Natural and induced variations in the composition of milk affect this physico-chemical parameter. Thus, some processing treatments and physico-chemical changes, such as heat treatment, membrane separation technology, high-pressure treatment and salt addition, results in a buffering capacity specific to the transformed product (heated milk, retentate, fermented milk). In cheese manufacture, the preparation of cheese milk and the cheese making parameters have major influences on the buffering capacity of the curd at mould removal and during ripening. Cheese quality is therefore affected during ripening, via enzymatic activities and microbial growth.

The maximum buffer index of skim milk, standardized milk (4.5% fat) and whole milk are 0.0290, 0.0283 and 0.0270 for buffalo and 0.0333, 0.0283 and 0.0277 for cow respectively. Both milks exhibit a maximum buffering at pH 5.3 to 5.4 and show the same buffer intensity curve between pH 4.5 and 10.0. Sodium citrate and pyrophosphate are favourable stabilizers shifting the maximum buffering of milk towards the normal pH of the milk. These additives cause an increase in milk pH and higher rate of dispersion.

24.2 Buffering Capacity

In milk aggregation or dispersion of protein micelles and fat globules is dependent up on its buffering capacity, pH, acidity and electrical conductivity. Determination of these properties and range of variation will help in assessing the technological properties of milk. The natural range of variation of this property in milk and milk products will determine several technological properties of milk. Stability of milk suspension is mainly controlled by the acid base equilibrium in milk. Substances which liberate protons (acid) and of those that combine with them (Bases) are responsible for maintaining this stability of milk constituents. The ionized and ionizable constituents of milk are in a state of rather delicate physical balance. The treatments such as heating or changing in milk compositional qualities alter the state of dispersion of proteins and salts which is reflected in protons. Buffers resist changes in pH. The Buffering Capacity increases as the molar concentration (molarity) of the buffer salt/acid solution increases. The closer the buffered pH is to the pKa, the greater the Buffering Capacity. Buffering Capacity is expressed as the molarity of Sodium Hydroxide required to increase pH by 1.0 unit/ litre of solution.

β = dn/dpH

Where n is number of equivalents of added strong base. Note that addition of dn moles of acid will change pH by exactly the same value but in opposite direction.

24.3 The Buffering Index

This can be defined as the differential ratio of the increase in the amount of strong acid or strong base added, to pH variation. The field of application of both notions is different: the buffer capacity is used in the quantitative chemical analysis and the buffer index in studying biological systems.

24.4 Constituent Contributing to the Buffering Capacity of Milk

Milk contains many acidic and basic groups. Because of the presence of acidic and basic groups the pH changes much less than when titrating with water: these groups have a buffering action. The buffering capacity of solution can be expressed in the buffer index which is the molar quantity of acid or base needed to change the pH of 1 litre of solution by one unit; it is thus the derivative of the titration curve. The buffering index of an ionizable group is given by the following formula

Where C is the total molar concentration of the group (dissociated plus associated). Again, it concerns intrinsic constants, but the above equation also holds for monoprotic acids if Ka is the stoichiometric dissociation constant. This equation implies that the buffering capacity is maximal (0.58 c) for the pH = pK; it is reduced to 33% of the maximal values for _pH- _pK= ±2

The buffering groups of milk are given in Table 24.1.

Table 24.1 Buffering groups in milk

The ionizable groups of the proteins, the phosphates, and the citrates mainly determine the acidity and the buffering capacity of milk. There will be variation in the titration curves due to the considerable variation in the concentration of these substances. Some of the ionizable groups in a protein may even be unavailable for titration, as they are buried in the hydrophobic interior of the molecules. Ionizable groups of the whey proteins and casein molecules. are not exposed The presence of Ca²⁺ ions strongly modifies the ionization of di and triprotic acids such as phosphoric acid and citric acids they will behave as if they are titrated by Ca(OH)₂ instead of NaOH or HCl. Lowering of the pH of milk will make calcium phosphate soluble because it is not soluble in normal pH of milk whereas colloidal phosphate will increase when the pH is increased. The Magnesium citrate and protein affect the composition and quantity of colloidal phosphate. Consequently the buffering index of various groups is not strictly additive.