A calorimetric study of the thermal denaturation of whey proteins in simulated milk ultrafiltrate

Abstract

SUMMARY. Differential scanning calorimetry (DSC) was used to study thermal transitions of the following whey proteins and enzymes in milk ultrafiltrate solution: /Mactoglobulin, a-lactalbumin, serum albumin, y-globulin, apo-and Fe-lactoferrin, lysozyme, ribonuclease, a-chymotrypsin and xanthine oxidase. Denaturation enthalpies (A-Ho), denaturation temperatures (T D ) and the half width of the denaturation peaks in DSC thermograms (A? 1 !)) were determined and the degree of renaturation was estimated by rescanning previously denatured samples. A fair correlation between the results obtained by DSC and other more classical methods was found in general. However, for some proteins (a-lactalbumin, lysozyme, ribonuclease and xanthine oxidase), which have so far been considered relatively thermostable, calorimetry reveals conformational changes starting at temperatures as low as about 45 °C. In these cases thermostability observed after heat treatment of milk should be interpreted in terms of renaturation and not of high temperatures of denaturation.Calorimetric techniques have been found useful for studying the effect of heat on proteins in aqueous solutions (Privalov, 1974; Sturtevant, 1974). The published calorimetric investigations of protein denaturation, however, either do not include milk proteins or were not carried out in solutions resembling the natural milk medium. It was thus considered useful to examine the thermal transitions of milk proteins in milk ultrafiltrate by differential scanning calorimetry (DSC). This should give some insight into the structure and stability of proteins in milk as well as the changes in the properties of milk and milk products during heat treatment.The present communication summarizes the results of a calorimetric investigation of the heat-induced transitions and reactions observed in milk ultrafiltrate of the following proteins: /

Effects of Heating Conditions on Physicochemical Properties of Skim Milk Powder during Production Process

“…Most α-lactalbumin molecules recover their initial native structure after thermal treatment (Bernal and Jelen, 1985;Barel et al, 1972). Because of such thermal reversibility, α-lactoalbumin is thought to be more heat resistant than β-lactoglobulin (Ruegg et al, 1977). α-Lactalbumin has four intramolecular disulfide bonds, but no sulfhydryl groups.…”

Rennet-Induced Aggregation and Curd Formation from Skimmed Milk Powders Prepared under Different Sterilizing Conditions

“…This discrepancy might have been due to the compact conformation oflactalbumin with four intramolecular disulfide bonds, enabling it to move faster in electrophoretic gels than flexible or extended casein molecules. 28,29) Hence it is reasonable to label the peptide band betweenlactalbumin and -lactoglobulin as para--casein. This peptide was clearly observed in the electrophoretic patterns, and the intensity of this band was nearly constant from 5 to 120 min, particularly in type L and N skimmed milk powders (C and D), illustrating the cleavage of -casein by rennet ( Table 2).…”

Factors Affecting Measurement of Undenatured Whey Protein Nitrogen in Dried Whey by a Modified Harland-Ashworth Test

“…Denaturation of a-la was measured on polyacrylamide gels by Hiller and Lyster (61) and found to be a first order reaction. Conformational changes of a-la were observed by differential scanning calorimetry of temperatures as low as 45C, which explains milk thermostability at that temperature in terms of renaturation and instability of milk at higher temperatures in terms of denaturation (118). Casein interactions with a-la were more evident at pH 4.5 than at pH 6.7 (101).…”

“…A relationship between functional characteristics of whey in model food systems and the degree of protein denaturati on was pa.rti cul arly obvious in samples with less than 20% and over 80% denaturation of the total whey protein (49,54). Denatured whey protein has limited applications in food because of poor solubility (15,46,49,69,104,114,116,118). The presence of high denatured whey protein concentrationsproduces poor whipability, poor foaming stability and low overrun.…”

“…The presence of high denatured whey protein concentrationsproduces poor whipability, poor foaming stability and low overrun. On the other hand, the removal of denatured whey protein greatly improves whippabilityand foaming properties of whey (70,118).…”