Microbiological Design and Validation of Thermal and High Pressure Processing of Acidified Carrots and Assessment of Product Quality

Abstract

Modification of pH and combined use of novel processing methods may be a good strategy to improve the quality of canned vegetables. In this study, selected thermal (TP) and high pressure-assisted thermal (HP-T) processing methods were validated for citric acid-infused carrots (pH ≤ 4.5) using Bacillus licheniformis spores. Previously established thermal inactivation kinetics data were used to setup the target process times (to achieve 7-log kill of B. licheniformis). The microbial spores were inoculated at the center of simulated carrot alginate beads and subjected to different processing methods. Delivered process lethalities, evaluated by the microbial count/re-count method and measured time–temperature data, were equal to or higher than the targeted values. No survivors were found after the treatments, demonstrating the adequacy of the processes. Texture, color and β-carotene retention in processed carrots, evaluated and compared with those processed under conventional canning, showed higher texture retention (P < 0.05) in the modified processing methods. Residual hardness values of carrots were 86% with HP-T, 70% with ohmic heating and 8% with conventionally canned product. The same trend was observed with chewiness value. However, processing methods showed no differences (P > 0.05) with respect to color change. In terms of β-carotene, carrots subjected to a relatively more severe heat treatment (water immersion mode in static retort) showed better β-carotene extractability than samples from HP-T. Practical Applications Conventional thermal processing of “low acid” foods (pH > 4.6) experiences significant quality loss due to the long thermal processing times required to inactivate spores of the key pathogen Clostridium botulinum. Alternative thermal processing techniques have evolved to shorten the processing times by enhancing heating rate to the product through process modifications or using thin profile packages. Even though quality retention can be enhanced through these modifications, thermal damage to quality is still indispensable since the sensitively microbial destruction to heat remain unaltered. Product acidification moves the “low acid” foods to “acid” category, thereby shifting the process regulation from the high temperature sterilization to the lower temperature pasteurization conditions. Savings in energy and reduction in process time immediately become obvious. Novel acidification methods are necessary for making the process efficient. Bacillus licheniformis is important and a suitable microorganism for validating thermal processing of acidified foods. The combined use of acid infusion methods and alternative thermal processing technologies could play a significant role to improve quality of acidified foods as compared to current practices.