Summary of paper on acid cheese firmness
This is a summary of
Anema, S.G. (2008). Effect of Milk Solids Concentration on the gels formed by acidification of heated pH-adjusted milk. Food Chemistry, 108, 110-118.
This study found that the firmness of “acid gels” produced from milk could be manipulated by changing either the pH or the concentration of the milk before heat treatment, and that the amount of whey protein probably plays a key role in that change.
I (Nathan Faulhaber) was completely unfamiliar with the field of rheology before reading this paper, so I had to learn some new things. Following are a few definitions that I learned along the way. I figured that I might as well list them here for quick reference, in case they’re ever useful to anyone:
STRESS: Stress is the force that is applied to a substance being tested. The breaking stress of a gel is the force that is needed to cause the continuous phase to rupture.
STRAIN: Strain is the amount of distortion that the stress produces, or, if you prefer:
“A strain is a measure of deformation representing the displacement between particles in the body relative to a reference length.” From: https://en.wikipedia.org/wiki/Deformation_(mechanics) - Strain
G′: Often called the “storage modulus,” G′, is the elastic energy, or “rebound energy” contained in a sample after it is deformed. NOTE THAT G' STANDS FOR SOMETHING DIFFERENT IN RHEOLOGY THAN IT DOES IN CHEMISTRY!!!
Anyway, here’s the summary:
This study found that the firmness of “acid gels” produced by the acidification of skim milk could be manipulated by controlling either the pH or the concentration of the milk during heat treatment. Increasing the pH produced firmer gels, and decreasing the pH produced gels that were less firm.
Quantitatively, the paper says on its first page that:
“As the pH of the milk was increased from pH 6.5 to pH 7.1 prior to heat treatment and acidification, the G′of the acid gels progressively increased so that the firmness (based on storage modulus, G′) at pH 7.1 was approximately twice that observed at pH 6.5 (Anema, Lee, Lowe, & Klostermeyer, 2004; Guyomarc’h et al., 2007; Rodriguez del Angel & Dalgleish, 2006 ).”
Firmer gels could also be produced by increasing the milk concentration prior to heat treatment, or, conversely, gels that were less firm could be made from milk samples that were less concentrated.
The pH of the milk samples was adjusted by the “slow addition” of either 3M HCl or 3M NaOH. They were then heat-treated at 80°C for 30 minutes. After heat-treatment, the samples were re-adjusted back to their original pH, again using 3M HCl or NaOH.
They were then acidified at 30°C in order to produce gels, by adding glucono-δ-lactone (GDL). The pH necessary to produce gels was about 4.2.
Notably, similar effects of pH alteration were observed at all milk concentrations, suggesting a similar (or identical) mechanism of action. The paper states on page 114 of the journal:
“When the breaking stress was plotted as a percentage change relative to that observed for acid gels prepared from milks heated at the natural pH, the curves for all four milk concentrations fell close to a single curve (Fig. 2C), indicating that a change in pH has a similar relative effect on the breaking stress. This was similar to the effect observed for the final G′ (Fig. 1C) and, as a consequence, the breaking stress and final G′ were correlated (not shown).”
Firmness (based on the storage modulus, G′) was measured using a rheometer. Measurements were taken both at 30°C, to simulate room temperature, and at 5°C, to simulate refrigerated temperature.
It was found that, among samples that were heat treated at the same pH, the amount of increase in firmness due to reduced temperature (30°C vs. 5°C) was greater for the 25% total solid (TS) milk than for the 10% TS milk. The author theorizes that this was due to hydrophobic bonds being temperature sensitive.
As for a mechanism of action for the correlation between pH and firmness, there is evidence that the change was due to the amount of “non-sedimentable” whey protein in the milk (Figures 3 and 4). The paper states on journal pages 110-111 that:
“As the pH of the milk is increased from about pH 6.5 to pH 7.1 before heating, κ-casein progressively dissociates from the casein micelles so that, at pH 6.5, the majority of the κ-casein is associated with the casein micelles whereas, at pH 7.1, about 60–70% of the κ-casein is found in the milk serum (Anema, 2007; Donato & Dalgleish, 2006 ). As the denatured whey proteins interact with the casein micelles via disulfide bonding with the κ-casein, this dissociation of κ-casein probably explains why the association of the whey proteins with the casein micelles is pH-dependent (Anema, 2007 )... The milk samples with a greater level of non-sedimentable denatured whey proteins produce acid gels with a higher G′ than those where most of the denatured whey proteins are associated with the casein micelles (Anema et al., 2004; Rodriguez del Angel & Dalgleish, 2006 ).”
The levels of non-sedimentable κ-casein and whey protein in the aqueous phase were determined by both SDS and native PAGE, followed by laser densitometry.
I tried many different things, but this figure would just not upload, for some reason. Sorry.