The effects of pressure and dissolved carbon dioxide on the viscosity of milk concentrate

Abstract

The combined effects of dissolved supercritical carbon dioxide and pressure on the viscosity of milk concentrate were evaluated. The experiment used an AR-G2 strain-controlled rheometer, (TA Instruments, Newcastle, DE, USA) equipped with a Peltier temperature controlling system and a pressure cell. Samples of milk concentrate obtained from evaporation of fresh milk, provided by a milk powder manufacturing plant were used for most measurements, being collected and analysed in different days. All experiments with samples from the industry were conducted at 1400 psia and 40˚C with a constant shear rate of 60s-1. The apparent viscosities were recorded by Rheology Advantage software. The determinations were carried out in triplicates using samples of milk concentrate obtained from fresh milk. This study used samples collected from the Fonterra milk powder manufacturing plant of Te Rapa, Hamilton, New Zealand. Skim and whole milk concentrate were used in this study. For the purpose of comparison, the effect of nitrogen was also determined. Particular samples of skim milk concentrate showed age thickening occurring over the subsequent hours following collection from the manufacturing plant. Some of the readings showed oscillatory results at atmospheric pressure under supercritical carbon dioxide and nitrogen. Even with unexpected oscillation and age thickening occurring, the effect of the supercritical carbon dioxide over the viscosity of the milk concentrate can be considered significant. A consistent increase in viscosity was observed as a result of the experiment with carbon dioxide. One of the original goals of this research was to prove the hypothesis that supercritical carbon dioxide could be used for reduction of the viscosity of skim milk concentrate. The results obtained by this study proved that, at the provided conditions, supercritical carbon dioxide would actually increase the viscosity of skim milk concentrate. The increase in viscosity observed after 40 min under supercritical carbon dioxide pressurisation was, on average, 3.2 times. A similar effect was also identified for the experiment with whole milk concentrate, an average increase of 4.1 times the original viscosity. No significant change to viscosity was observed in the control group study with nitrogen during experiment at the same conditions.