Redox potential of milk

The concentration of dissolved oxygen is the principal factor affecting the redox potential of milk. Milk is essentially free of 02 when secreted but in equilibrium with air, its 02 content is about 0.3 mM. The redox potential of anaerobically drawn milk or milk which has been depleted of dissolved oxygen by microbial growth or by displacement of 02 by other gases is more negative than that of milk containing dissolved 02. 

Figure 11.3 Decrease in the redox potential of milk caused by the growth of Lactococcus lac [is...

Fermentation of lactose during the growth of micro-organisms in milk has a major effect on its redox potential. The decrease in the h of milk caused by the growth of lactic acid bacteria is shown in Figure 11.3. A rapid decrease in h occurs after the available 02 has been consumed by the bacteria. Therefore, the redox potential of cheese and fermented milk products is negative. Reduction of redox indicators (e.g. resazurin or... 

Many hypotheses have been advanced for these differences in oxidative susceptibility, including the oxidation potential of milks, and the action of xanthine oxidase and lactoperoxidase, which is controversial. However, there are no substrates for these enzymes in milk. The action of various metallo proteins in milk may be confused as enzymes. These metallo proteins act as powerful lipid oxidation catalysts in the presence of oxygen and redox systems involving ascorbic acid. 

The primary starter performs several functions in addition to acid production, especially reduction of the redox potential ( h, from about + 250 mV in milk to —150 mV in cheese), and, most importantly, plays a major, probably essential, role in the biochemistry of cheese ripening. Many strains produce bacteriocins which control the growth of contaminating micro-organisms. 

The concentration of ascorbic acid in milk (11.2-17.2mgl-1) is sufficient to influence its redox potential. In freshly drawn milk, all ascorbic acid is in the reduced form but can be oxidized reversibly to dehydroascorbate, which is present as a hydrated hemiketal in aqueous systems. Hydrolysis of the lactone ring of dehydroascorbate, which results in the formation of 2,3-diketogulonic acid, is irreversible (Figure 11.2). 

The Eh of milk is influenced by exposure to light and by a number of processing operations, including those which cause changes in the concentration of 02 in the milk. Addition of metal ions (particularly Cu2+) also influences the redox potential. Heating of milk causes a decrease in its h,... 

Manzocco et a/.483 compared the redox potential and the POA of some potent oxidants with those of some foods (milk, bread). For the POA, hydroxy radicals generated from H202, peroxy radicals from ABAP, and DPPH, as well as milk and bread, were allowed to react with crocin in aqueous solution at 40 °C and the POA was taken as the ratio of the decrease in Acrocin at 5 min to oxidant concentration ... 

Xanthine Oxidase. This molybdoenzyme is readily available from cows milk in gram quantities (28) and is relatively stable, which accounts for the fact that it is by far the most intensively studied molybdoenzyme. Bray and Swann (5) have reviewed comprehensively the earlier literature, and recent papers by Olson et al. (20) summarize combined kinetic and thermodynamic approaches to the states of the prosthetic groups during catalysis. Two molybdenum, four iron-sulfur centers, and two FAD groups are present in each molecule. An important point raised by Edmondson, et al. (29) is that the rates of internal electron transfer among the prosthetic groups appear to be much more rapid than turnover. Olson et al., (20) deduced that the reduction potentials of the two processes Mo(VI) <— Mo(V) <— Mo(IV) were —60 and —31 mv, respectively, relative to the redox potential for one of the iron-sulfur centers (center II) in the molecule. Thus, at equilibrium one can never have more than a small fraction of molybdenum as... 

Although whey protein concentrates possess excellent nutritional and organoleptic properties, they often exhibit only partial solubility and do not function as well as the caseinates for stabilizing aqueous foams and emulsions (19). A number of compositional and processing factors are involved which alter the ability of whey protein concentrates to function in such food formulations. These include pH, redox potential, Ca concentration, heat denaturation, enzymatic modification, residual polyphosphate or other polyvalent ion precipitating agents, residual milk lipids/phospholipids and chemical emulsifiers (22). 

The crystal structure of the four distinct redox cofactors of bovine milk XO is shown in Figure 5.2 as a representative example of XO and XDH. The Mo active site, two 2Fe-2S clusters (FeSI and FeSII) and FAD are apparent and electrons flow from Mo" to FAD (left to right) after (hypo)xanthine oxidation (hydroxylation) and NAD binds at the FAD site to be reduced by FAD. Of the XO/XDH enzymes to be studied to date the Mo " and Mo " redox potentials are consistently low amongst the lowest of all Mo enzymes (-350 mV to -500 mV vs. NHE). The two FAD redox potentials (FADVFADH and FADH/ FADH2) fall in the range (-250 to -400 mV vs. NHE). 

In summary the contribution of LAB to flavor formation is decisive in some fermented foods, for example fermented milks and cheeses fermented only with LAB. However, their direct contribution to flavor formation can be low when LAB are part of a complex microbial community including in particular yeasts or fungi with high enzymatic activities, or when food includes flavoring constituents, such as in some baked foods or kimchi. In all cases, LAB are essential as primary agents of the acidification of the food matrix. They are also associated with other changes such as a decrease of redox potential, which likely influences flavor-related reactions (Urbach 1995). Table 19.2 gives some examples of the flavor-related role of LAB in different fermented foods. 

The rapid change of potential shown in Figure 8.2 occurs only after the dissolved oxygen has been consumed by the bacteria and may be identified by the change in color of certain dyes added to the milk. These dyes are oxidizers of a redox system. Since the time elapsing before these dyes are reduced to the colorless reductant form is roughly proportional to the number of bacteria present, this reduction time is an index of the degree of bacterial contamination. 

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