PHYSICAL CHEMISTRY OF MILK

Lesson 27. REDOX SYSTEM IN MILK

Module 10. Oxidation reduction potential

Lesson 27
REDOX SYSTEM IN MILK

27.1 Introduction

The oxidation reduction reactions involve transfer of electrons between atoms or molecules. Transfer of oxygen (O) or hydrogen (H) or both also may occur. Oxidation is loss of electrons while reduction is the gain of electrons. In a redox system when half of the system is having oxidation reaction and the other half is having a reduction reaction there will be no flow of electrons either in to the system or go out of the system. In normal milk there are several complicated biological systems with varying composition and concentration. In addition to this microorganism gaining entry in to milk contribute certain redox systems to it depending upon the type of the organisms.

27.2 Redox System in Mlik

A reversible system give a potential at a metal electrode is the intensity factor which is the measured potential. Potential measured for the reversible systems is similar to the potential measured at the hydrogen electrode while determining the hydrogen ion concentration i.e., pH. The quantity factor for the oxidation reduction is the overall concentration of active substances [Ox]⁺ [Red].

Several factors influence this potential in a system. These factors are

· Reversibility of the system

· Standard potential of the system(e0)

· Or the position on the scale of the potential

· Concentration of the active components of this system.

Methylene blue is reduced by freshly drawn milk when it is drawn from udder anaerobically indicating a more negative potential than methylene blue system. Exposure to oxygen will change this potential to be more positive than the mehtylene blue system. Apart from this the chief oxidation and reduction systems present in milk are Ascorbate, lactate and riboflavin.

The ascorbic content of fresh milk is about 0.25 meq.litre^-1 the milk is drawn from the udder all the ascorbate present in milk will be in its reduced form but reversible oxidation to dehydro ascorbate occurs at rates dependent on temperature, copper (Cu) and oxygen (O₂) concentration. In order to inhibit growth of microorganisms milk is held at temperature between 1 to 4^oC, and the reduced form of ascorbate gradually decreases over several days as dehydro ascorbate levels are constant (0.05mEq.Litre.^-1) their ratio remains large until the system disappears. Preventing the contamination with copper (Cu) and by deaeration the ascorbate content can be preserved. The concentration of free riboflavin is only 4µM. it plays an important role in the photo oxidation. It does not contribute significantly either to the O-R potential i.e., Eh or poising action. Milk contains only low concentration of small molecular weight thiols. Thiols of the native proteins are not active in oxidation reduction systems. Due to heat denaturation of proteins unfolding and uncoiling would occur due to which these thiols groups contribute to the O-R potential of milk. The concentration of such activated thiols in milk heated under UHT conditions may attain 0.18 mEq.liter-1.

The redox potential E^o of milk and standard potential E^o of some important systems in milk are plotted against pH is presented in the Fig. 27.1. Individual milk samples in equilibrium with air generally have Eh’s in the range of +0.25 to +0.35 V at 250C at their normal pH of milk i.e 6.6 to 6.7. Not much information is available on the capacity of the oxidation – reduction system in milk. Fresh raw whole milk will reduce 0.6 to 0.8 m mol of ferricyanide to ferrocyanide per liter at 500 ^oC for 20 min.

Besides redox potential which pertain to equilibrium conditions the kinetics of the oxidation reduction reactions is also important. Some reactions are very slow because of a high free energy of activation (ΔG*) which implies that it may take considerable time before equilibrium is reached. Accurate measurement of Eh of milk takes considerable time as ther are several systems in milk which overlap and these reactions are either slowly reversible or even irreversible. Measuring OR potential of milk is complicated due to entry of oxygen during determination. In milk oxidation reduction systems resist change in the potential when the oxidant and reductant are near equilibrium state. This phenomenon is similar to the buffering action in acid base equilibrium. This phenomenon is known as ‘poising’. It is difficult to obtain meaningful data on the poising index though milk has considerable poising capacity.

Fig. 27.1 The redox potential (E⁰) of milk and the standard potential (Eh) of various systems in relation to pH
(Source: Dairy chemistry and physics, Walstra and Jenness,1984)

Concentration of lactate-pyruvate system in milk is negligible. Enzymatic activation of this system would influence the redox system. At higher pH the aldehyde group of lactose is oxidizable to carboxyl group, but this reaction is not a reversible and hence it will not influence the Eh at pH of 6.6.

Redox reactions in milk systems are influenced by heat treatment, by concentration of O₂ and metal ions such as Cu²⁺, by exposure to light and by oxido redcutases of milk and or micro organisms. The redox potential of metal ions may depend closely on their binding to various ligands, including some (Metallo) proteins.