Application of Emerging Technologies for Low Acid Liquid Foods

Abstract

The demand for minimally processed foods has increased in the last few years and gains high acceptability among consumers as it has better nutritional value than highly processed foods. Ultra high temperature (UHT) treatment is carried out to inactivate all types of spores (psychrotrophic, mesophilic, thermophilic) in addition to vegetative cells. However, it has negative impact on vitamins, proteins, colour and flavour. Pasteurization on the other hand is considered as minimally processed but ensues limited shelf life under refrigeration, challenging the transport of pasteurized product to distance markets. Emerging technologies have great potential to process low acid liquid foods at lower temperatures and may lower the energy consumption with better quality compared to conventional technologies. This thesis evaluated the use of ultrasound, ultraviolet and ultra high pressure homogenization with an objective to lower the temperature during processing of low acid liquid foods. Firstly, combination of ultrasound and thermal treatment was investigated to lower the temperature during sterilization. The effect of pre-treatment with ultrasonication (20 kHz, 750 W) on decimal reduction time (D values) of B. subtilis spores ATCC 6633 was evaluated in three different suspending media (water, whole milk and rice porridge) and were compared with thermal treatment. In this study, energy delivered by ultrasound was used to provide the heating needed, which fulfils 55% of the thermal energy requirement in addition to the effect caused by cavitation. The reduction in D values through the use of combined technology is minimal unless excessive ultrasonication is used, which is commercially not viable. Therefore, the combination was not feasible in terms of energy consumption and only gave marginal benefit towards spore inactivation. In the second part of this study, Ultraviolet (UV) treatment followed by heat treatment was investigated for inactivation of B. subtilis and G. stearothermophilus spores. It was shown that UV treatment in combination with heat is effective in inactivation of these spores in skim milk in comparison to whole cow milk and sheep milk. Therefore, this combined treatment could be a good alternative to sterilization of skim milk at lower temperatures compared to UHT treatment (135°C, 3 sec). Moreover, another study investigated the use of UV treatment on commercially pasteurized trim milk to inactivate psychrotrophic spores and thereby extend its shelf life. Microbial study together with physicochemical properties showed that pasteurization followed by UV treatment can enhance the shelf life of trim milk considerably. There are concerns with the usage of UV treatment as it can result in adverse effects on the quality of milk due to photo oxidative changes. However, photo oxidation in milk that include complex reactions requires oxygen in the media. Limiting dissolved oxygen content within milk can minimize oxidative damage and thus, can result in better product quality. Nitrogen purging is one of the effective methods for removing dissolved oxygen from liquids. Therefore, this study evaluates the effects of nitrogen purging (prior to UV treatment) on milk quality. It was found that nitrogen purged UV treated milk caused minimal changes to physicochemical properties by limiting the oxidative changes in milk arises from light and oxygen. The benefits of UV were highest in liquids with low absorption coefficient. Whey is the greenish liquid obtained as a by-product during cheese or casein manufacturing. Currently, whey is preserved through thermal pasteurization with treatment temperature that varies depending on the shelf life required. The effect of UV treatment on inactivation of natural microbial load in crude whey and inactivation of inoculated E. coli ATCC 25922 in reconstituted whey was studied. The work reported here indicates that UV treatment of whey results in a microbiologically safe product while retaining its physicochemical characteristics and could be an energy efficient alternative to the commonly used thermal pasteurization. Although, microbial inactivation and physicochemical properties reveal that UV can lower the temperature in comparison to conventional processing, the results suggest that this technology is limited to liquids with low total solid contents. Therefore, liquids with high total solids were analysed with ultra high pressure homogenization (UHPH). The pressure applied during conventional homogenization ranges from 30 MPa to 50 MPa; generally, it has little or no impact on spores. Previously, it was shown that pressure higher than 100 MPa has damaging effects on spores. Therefore, UHPH was investigated for spore inactivation through its combined application of temperature and pressure (higher than 100 MPa) and for its ability to incorporate into a continuous process. In this study, antimicrobial efficacy against B. subtilis spores was analysed in whole milk and sheep milk that have high solid contents and compared with inactivation in Skim milk. This thesis investigated a science based approach for a better understanding of the key principles of the selected emerging technologies, combination of technologies with heat, and associated benefits/drawbacks. The outcome of this work will help in meeting the challenges for process and product development for targeted food processes delving desirable food properties while maintaining their safety.