Question 5 of 14: Protein Denaturation Temperature and Rancidity Control

Question 5: Protein Denaturation Temperature and Rancidity Control (12.5 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Given. Fig. 2 plots mean droplet diameter $d_{32}$ (μm) of a 20 wt% corn-oil-in-water emulsion, stabilised by 2 wt% whey protein isolate (WPI) at pH 7, against temperature (30–90°C) for five NaCl concentrations (0, 25, 50, 75, 100 mM). Every curve is essentially flat at $d_{32}\approx0.6$ μm from 30°C up to about 70°C, then rises sharply, peaking near 80°C (the peak height increasing with salt concentration, up to about 1.6 μm at 100 mM NaCl) before easing slightly toward 90°C.

Find. An estimate of the whey protein isolate's denaturation temperature from the graph's shape.

(a) While the emulsion is stable and the whey protein layer at the oil–water interface stays folded, the droplet size stays essentially unchanged with temperature. Once heating carries the interfacial protein past its denaturation temperature, it unfolds, exposing buried hydrophobic and free thiol groups that promote inter-droplet bridging/flocculation and some coalescence — which is exactly what a rise in the measured $d_{32}$ represents. The onset of that rise, common to all five curves, is therefore the graph's estimate of the denaturation temperature: $\boxed{T_d \approx 70\ {}^{\circ}\text{C}}$, consistent with the well-documented denaturation range of the major whey protein, $\beta$-lactoglobulin ($\approx$70–78°C). The fact that the size of the post-denaturation rise (but not its onset temperature) grows with NaCl concentration is also consistent with this reading: added salt screens electrostatic repulsion between already-unfolded protein-coated droplets, promoting more aggregation once denaturation has occurred, without changing the temperature at which unfolding itself begins.

(b) High-fat foods go rancid mainly through free-radical autoxidation of unsaturated fatty acids (and, more slowly, through hydrolytic rancidity from free fatty acid release). Preventing it therefore means attacking every stage of that chain reaction and its catalysts: (i) exclude oxygen — vacuum or modified-atmosphere (nitrogen/CO$_2$-flushed) packaging, oxygen-barrier films, and oxygen scavenger sachets remove the co-reactant the chain reaction needs; (ii) control light — opaque or UV-blocking packaging, since light (particularly with a photosensitiser present) accelerates initiation; (iii) control temperature — refrigerated/frozen storage slows the oxidation rate directly; (iv) add antioxidants — chain-breaking antioxidants (tocopherols, BHA/BHT, rosemary extract) intercept the free-radical chain propagation step; (v) add metal chelators — citric acid or EDTA sequester trace pro-oxidant metals (Fe, Cu) that would otherwise catalyse initiation; and (vi) manage water activity — oxidation is actually fastest at both very low and very high $a_w$, so holding the product near the monolayer $a_w$ ($\approx0.2$–0.3) rather than assuming "drier is always safer" minimises the rate.