Effect of heating rates on thermal destruction kinetics of Escherichia coli ATCC25922 in mashed potato and the associated changes in product color

Understanding retort processing: A review

Retort processing is a food preservation technique to address the challenge posed by Clostridium botulinum for commercial sterility of a food product to get microbiologically safe and stable products by heating. This review aims to explore the journey of retort processing, starting from its early use in single-batch canned foods and progressing to its contemporary applications with different types of containers and heating mediums. Additionally, it will delve into the adaptability of retort equipment, including its ability to operate in stationary and various agitation states, as well as its flexibility in processing speed for both single-batch and continuous operations.

Enhanced heat tolerance of freeze-dried Enterococcus faecium NRRL B-2354 as valid Salmonella surrogate in low-moisture foods

In microbial studies of low-moisture foods (LMFs, water activity less than 0.85), freeze-dried bacteria benefit us to inoculate LMFs without introducing extra water or altering food physiochemical properties. However, the freeze-drying process would bring unavoidable damage to bacterial cells and results in less-resistant inoculum that are unlikely to be qualified in microbial studies. Herein, we enhanced bacterial heat tolerance by subjecting the cells to mild heat (42-50 °C) to counteract the reduced heat tolerance and survivability of freeze-dried bacteria. Enterococcus faecium NRRL B-2354 (E. faecium), a Salmonella surrogate in LMFs, was used as the target microorganism because it was widely accepted in microbial validation of thermal pasteurizing LMFs. Three types of LMFs (peanut powder, protein powder, and onion powder) were used as LMFs models to validate the freeze-dried E. faecium in comparison with Salmonella enterica Enteritidis PT 30 (S. Enteritidis) prepared by the traditional aqueous method. The heat tolerance (D65℃ value) of E. faecium increased at all treatments and peaked (+31.48 ± 0.13%) at temperature-time combinations of 45 °C-60 min and 50 °C-5 min. Survivability of freeze-dried inoculum and its heat tolerance retained well within 50 d storage. The freeze-dried E. faecium was prepared in this study brought equal or higher heat tolerance (D85℃ or D75℃) than S. Enteritidis in tested LMFs models. For instance, the D85℃ of freeze-dried E. faecium (heat-treated at 50 °C for 5 min) and S. Enteritidis in whole egg powder are 35.56 ± 1.52 min and 28.41 ± 0.41 min, respectively. The freeze-dried E. faecium with enhanced heat tolerance appears to be a suitable Salmonella surrogate for dry-inoculating LMFs. Our protocol also enables industry-scale production of freeze-dried inoculum by broth-cultivation method combined with mild-heat treatment.

Heat penetration and quality analysis of retort processed vegetables for home meal replacement foods

Heat penetration characteristics of different vegetable products were investigated during retort processing. A custom-developed variable temperature retort-sterilizer allowed us to test the following target retort-temperatures; 120, 130, 140, and 150 °C, combined with the following holding times: 1, 3, 5, 7 min. Radish showed the highest heating rate (9.56 ± 0.21 °C/min) among the tested vegetables, including radish, carrot, and potato. Textural qualities of retort-processed vegetables showed a close correlation with thermal dose. Hardness of potato was 3.07 ± 0.07 N after retort processing at 120 °C for 7 min, with a thermal dose of 127 ± 7 k °C s. Better hardness (3.72 ± 0.06 N) was obtained after retort processing at 150 °C for 3 min, with a thermal dose of 122 ± 6 k °C s. The data reported herein indicate that retort temperature should be appropriately controlled for different vegetable products based on their specific heat-penetration characteristics.

Supplementary Information

The online version contains supplementary material available at 10.1007/s10068-023-01247-8.

Insight into the Interaction of Malondialdehyde with Rabbit Meat Myofibrillar Protein: Fluorescence Quenching and Protein Oxidation

This research explored the effects of oxidative modification caused by different malondialdehyde (MDA) concentrations on rabbit meat myofibrillar protein (MP) structural characteristics and the interactions between MDA and MP. The fluorescence intensity of MDA-MP adducts, and surface hydrophobicity increased, whereas the intrinsic fluorescence intensity and free-amine content of MPs decreased as MDA concentration and incubation time increased. The carbonyl content was 2.06 nmol/mg for native MPs, while the carbonyl contents increased to 5.17, 5.57, 7.01, 11.37, 13.78, and 23.24 nmol/mg for MP treated with 0.25 to 8 mM MDA, respectively. When the MP was treated with 0.25 mM MDA, the sulfhydryl content and the α-helix content decreased to 43.78 nmol/mg and 38.46%, while when MDA concentration increased to 8 mM, the contents for sulfhydryl and α-helix decreased to 25.70 nmol/mg and 15.32%. Furthermore, the denaturation temperature and ΔH decreased with the increase in MDA concentration, and the peaks disappeared when the MDA concentration reached 8 mM. Those results indicate MDA modification resulted in structural destruction, thermal stability reduction, and protein aggregation. Besides, the first-order kinetics and Stern-Volmer equation fitting results imply that the quenching mechanism of MP by MDA may be mainly driven by dynamic quenching.

Increased heat tolerance and transcriptome analysis of Salmonella enterica Enteritidis PT 30 heat-shocked at 42 ℃

In this study, we compared the heat tolerance parameter (D_65℃) values of Salmonella enterica serovar Enteritidis PT 30 (S. Enteritidis ) heat adapted at different degrees (at 42 ℃ for 20-180 min) and cultivated using two methods. The treated group with the highest D_65℃ value (LP-42 ℃-60 min) and the untreated groups (Control-TSB and Control-TSA) were subjected to transcriptome analysis. Heat-adaptation increased the D_65℃ values of S. Enteritidis by 24.5-60.8%. The D_65℃ values of the LP-42 ℃-60 min group (1.85 ± 0.13 min, 7.7% higher) was comparable to that of the Control-TSA. A total of 483 up- and 443 downregulated genes of S. enteritidis were identified in the LP-42 ℃-60 min group (log₂fold change > 1, adjusted p-value < 0.05). Among these genes, 5 co-expressed and 15 differentially expressed genes in the LP-42 ℃-60 min and Control-TSA grops possibly contributed to the high D_65℃ values of S. Enteritidis . The Rpo regulon was involved in the heat adaptation of S. Enteritidis , as evidenced by the significant upregulation of rpoS, rpoN, and rpoE. KEGG enrichment pathways, such as biosynthesis of secondary metabolites, tricarboxylic acid, and ribosomes were identified and mapped to reveal the molecular mechanisms of S. enteritidis during heat adaptation. This study quantified the enhanced heat tolerance of S. Enteritidis heat adapted at different degrees of heat-adaptation. The results of this study may serve as a basis for elucidating the molecular mechanisms underlying the enhanced heat tolerance at the transcriptome level.

Thermal Inactivation Kinetics and Radio Frequency Control of Aspergillus in Almond Kernels

Mold infections in almonds are a safety issue during post-harvest, storage and consumption, leading to health problems for consumers and causing economic losses. The aim of this study was to isolate mold from infected almond kernels and identify it by whole genome sequence (WGS). Then, the more heat resistant mold was selected and the thermal inactivation kinetics of this mold influenced by temperature and water activity (a_w) was developed. Hot air-assisted radio frequency (RF) heating was used to validate pasteurization efficacy based on the thermal inactivation kinetics of this target mold. The results showed that the two types of molds were Penicillium and Aspergillus identified by WGS. The selected Aspergillus had higher heat resistance than the Penicillium in the almond kernels. Inactivation data for the target Aspergillus fitted the Weibull model better than the first-order kinetic model. The population changes of the target Aspergillus under the given conditions could be predicted from Mafart’s modified Bigelow model. The RF treatment was effectively used for inactivating Aspergillus in almond kernels based on Mafart’s modified Bigelow model and the cumulative lethal time model.

A Comparison of Three Methods for Determining Thermal Inactivation Kinetics: A Case Study on Salmonella enterica in Whole Milk Powder

ABSTRACT

Different methods for determining the thermal inactivation kinetics of microorganisms can result in discrepancies in thermal resistance values. In this study, thermal resistance of Salmonella in whole milk powder was determined with three methods: thermal death time (TDT) disk in water bath, pouches in water bath, and the TDT Sandwich system. Samples from three production lots of whole milk powder were inoculated with a five-strain Salmonella cocktail and equilibrated to a water activity of 0.20. The samples were then subjected to three isothermal treatments at 75, 80, or 85°C. Samples were removed at six time points and cultures were enumerated for survivors. The inactivation data were fitted to two consolidated models: two primary models (log linear and Weibull) and one secondary model (Bigelow). Normality testing indicated that all the model parameters were normally distributed. None of the model parameters for both consolidated models were significantly different (α = 0.05). The amount of inactivation during the come-up time was also not significantly different among the methods (α = 0.05). However, the TDT Sandwich resulted in less inactivation during the come-up time and overall less variation in model parameters. The survivor data from all three methods were combined and fitted to both consolidated models. The Weibull had a lower root mean square error and a better fit, according to the corrected Akaike's information criterion. The three thermal treatment methods produced results that were not significantly different; thus, the methods are interchangeable, at least for Salmonella in whole milk powder. Comparisons with more methods, other microorganisms, and larger varieties of food products using the same framework presented in this study could provide guidance for standardizing thermal inactivation kinetics studies for microorganisms in foods.

HIGHLIGHTS