Abstract
Radio frequency (RF) heating was applied to stirred yoghurt after culturing in order to enhance the shelf-life and thereby meet industrial demands in countries where the distribution cold chain cannot be implicitly guaranteed. In parallel, a convectional (CV) heating process was also tested. In order to meet consumers’ expectations with regard to texture and sensory properties, the yoghurts were heated to different temperatures (58, 65 and 72 °C). This second part of our feasibility study focused on the changes in microstructure and texture caused by post-fermentative heat treatment. It was shown that there were always microstructural changes with additional heat treatment. Compared to the dense and compact casein network in the stirred reference yoghurt, network contractions and further protein aggregation were observed after heat treatment, while at the same time larger pore geometries were detected. The changes in microstructure as well as other physical and sensorial texture properties (syneresis, hardness, cohesiveness, gumminess, apparent viscosity, G’, G’’, homogeneity) were in good agreement with the temperature and time of the heat treatment (thermal stress). The RF heated products were found to be very similar to the stirred reference yoghurt, showing potential for further industrial development such as novel heating strategies to obtain products with prolonged shelf-life.
Highlights
Yoghurt is a cultured milk product, typically obtained by fermentation with Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus salivarius subsp. thermophilus
Gentle radio frequency (RF) heating was applied to stirred yoghurt, a matrix that represents an acidic casein gel with a very sensitive structure
As already described in the first part of this study [12], it was not possible to establish a stable heating regime for RF 72 °C products since in some cases a significant overheating followed by strong contraction of the yoghurt curd and whey separation was observed
Summary
Yoghurt is a cultured milk product, typically obtained by fermentation with Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus salivarius subsp. thermophilus. Barua & Hampton [6] and Keefer & Murray [7] aimed to produce sterile natural yoghurt having the taste and texture of natural yoghurt In these studies, and in contrast to the approach described in a patent of Egli & Egli [8], a mix of additives was added in a single step to the product after fermentation, resulting in less complex and less time-consuming processing to obtain sterile yoghurt. The aroma and taste profiles were found to be very similar to the reference yoghurt, overall supporting the potential to prolong the yoghurts’ shelf-life Based on these observations, this second part of the study investigates the effect of gentle post-fermentative heat treatment on yoghurt microstructure and texture. A trained sensory panel performed sensory texture profile analysis (STPA) during the whole storage period
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