Experimental analysis and computer simulation of the initial stages of cheddar cheesemaking
Abstract
Detailed experimental analyses of the cheesemaking system before running the whey from the curd, were carried out. These were complete simultaneous sets of analyses during each batch and included measurements of individual bacterial concentrations in the curd and whey, titratable acidities of the curd and whey, syneresis and vat temperature. It was found that there were linear correlations between calcium, phosphate and lactate concentration, titratable acidity and buffer index of the whey. Similar correlations were found in rennet whey collected from acidified milk under equilibrium conditions. It was shown that a large percentage of the titratable acidity of the whey measured during cheese making was the result of bacterial growth in. the whey. The temperature dependence of whey pH between 10 and 40°C was -0.055 pH units per 10°C rise in temperature.
A logical model of the cheesemaking system before running was developed and constructed as a digital computer program. This consisted of subprograms to simulate bacterial growth, syneresis and diffusion in a sphere. It was found during the experimental analyses that the concentration of bacteria in the whey increased sharply in the first 15 minutes after cutting the coagulum but stopped increasing for a further 10 minutes before increasing again less rapidly. The model proposed to describe this behaviour was that the bacteria are initially free to move out of the curd but when the average pore size in the curd becomes too small the bacteria are filtered from the exuding whey.
The curd particles were represented in the model by a single sphere which was divided into spherical shell elements. Syneresis was described by a second order rate equation. Lactate inhibition of bacterial growth was described using enzyme kinetics. Titratable acidity was considered as a single diffusible species and diffusion between adjacent elements was calculated using Fick’s first law of diffusion.
The simulation program was used in three ways: Firstly it was successful in fitting curves to the experimental data and these were used to derive relationships between various experimental results. Secondly it was used to determine parameters which were not measured experimentally, some of which would be very difficult to measure. Thirdly the model was used to make a series of predictions based on a cheesemaking control experiment, by varying one parameter at a time to determine individual cause and effect relationships and their mechanisms. The whey acidity 115 minutes after cutting was found, by experiment and simulation, to cons of approximately the following proportion: 25-30% of the acid produced by bacteria in the whey; 40-45% of the acid lost from the curd as a result of syneresis; and 25-30% as a result of acid diffusing from the curd.
The curd acidity at a particular whey acidity was found to vary considerably when cheesemaking parameters were altered. It was found that no combination of time after cutting and whey acidity could be used to predict curd acidity. Diffusion of lactose into the curd before running was found to be negligible because of the outward movement of whey during syneresis.
The model showed that diffusion of acidity (and presumably lactate concentration) from the surface of the curd particles caused the specific bacterial growth rate to be increased near the surface. Thus the bacterial concentration, rate of bacterial growth and rate of acid production were higher while the acidity was lower near the surface than it was at the centre of the curd particles.
