Influence of radio frequency treatment on in-shell walnut quality and Staphylococcus aureus ATCC 25923 survival

An accurate approach to predict Salmonella Enteritidis PT 30 survival based on dynamic thermal resistance during hot air assisted radio frequency pasteurization of in-shell walnuts

A significant variation between experimental and predicted log reductions of Salmonella at a constant water activity level has been previously found during a simultaneous hot air assisted radio frequency (HARF) pasteurization and drying of low-moisture foods (LMFs). However, how the dynamic thermal resistance of Salmonella changes with the changing temperature and water activity during HARF is unclear. Thermal inactivation kinetics of Salmonella Enteritidis PT 30 (S. enteritidis PT 30) in walnut shell powder and on in-shell walnuts were investigated under three temperatures and water activities. The dynamic thermal resistance (D-value) of S. enteritidis PT 30 in walnut shell powder and on in-shell walnuts were quantitatively described by modified Bigelow models during simultaneous HARF pasteurization and drying. The survival of S. enteritidis PT 30 was predicted and validated based on the dynamic thermal resistance on in-shell walnuts during HARF pasteurization. The results showed that the D-values of S. enteritidis PT 30 on in-shell walnuts were significantly (P < 0.05) higher than those in walnut shell powder at different temperatures and water activities, indicating a need to determine the thermal resistance parameters based on real-case pasteurization scenarios. The dynamic thermal resistance of S. enteritidis PT 30 in walnut shell powder and on in-shell walnuts first decreased and then increased during the simultaneous HARF pasteurization and drying. The experimental log reduction (2.06 ± 0.08) of S. enteritidis PT 30 on in-shell walnuts was close to the predicted value (2.07) during the simultaneous HARF pasteurization and drying. Predicting microbial survival based on the dynamic thermal resistance of S. enteritidis PT 30 on in-shell walnuts is more accurate and reliable than that based on the static one. The developed model and proposed approach are valuable and crucial for the commercialization of simultaneous RF pasteurization and drying processes of LMFs.

Radiofrequency affects the decrystallization efficiency and physicochemical properties of rape honey via crystal structure modification and inactivating enzyme

Crystallization degrades the physicochemical properties of honey and reduces consumer acceptance. To address this issue, radiofrequency was developed to investigate the decrystallization efficiency and quality impact mechanism of rape honey. The results showed that radiofrequency significantly decreased the number and size of crystals, leading to shortening the decrystallization time to less than 10 min. The response surface optimization methodology further indicated that the highest decrystallization rate (98.72 ± 0.34 %) and lower 5-Hydroxymethylfurfural (2.45 ± 0.12 mg/kg) contents were obtained. Furthermore, radiofrequency changed the honey from a pseudoplastic into a Newtonian fluid efficiently due to the volumetric heating feature. It is worth noting that the inactivation of glucose oxidase reduced the antibacterial capacity, while the increase in total phenolic and flavonoid contents improved the antioxidant capacity of rape honey. In summary, current findings indicated that radiofrequency is a potential alternative decrystallization technology for water baths.

Effect of ultrasound combined with chemical pretreatment as an innovative non-thermal technology on the drying process, quality properties and texture of cherry subjected to radio frequency vacuum drying

To obtain high-quality cherry products, ultrasound (US) combined with five chemical pretreatment techniques were used on cherry prior to radio frequency vacuum drying (RFV), including carboxymethyl cellulose coating (CMC), cellulase (CE), ethanol (EA), isomaltooligosaccharide (IMO), and potassium carbonate + ethyl oleate (PC + AEEO). The effect of different pretreatments (US-CMC, US-CE, US-EA, US-IMO, US-(PC + AEEO)) on the drying characteristics, quality properties, texture, and sensory evaluation of cherries was evaluated. Results showed that the dehydration time and energy consumption were decreased by 4.17 - 20.83 % and 3.22 - 19.34 %, respectively, and the contents of individual sugars, soluble solid, total phenolics (TPC), natural active substances, total flavonoids (TFC), and antioxidant properties (DPPH, ABTS and FRAP) were significantly increased after US combined with five chemical treatments (P < 0.05). Moreover, the pretreatment played important role in improving texture properties and surface color retention in the dried cherries. According to the sensory evaluation analysis, the dehydrated cherries pretreated with US-CMC exhibited the highest overall acceptance, texture, crispness, color, and sweet taste showed lower off-odor, bitter taste and sour taste compared to control and other pretreatments. The findings indicate that US-CMC pretreatment is a promising technique for increasing physicochemical qualities and dehydration rate of samples, which provides a novel strategy to processing of dried cherry.

A review of food safety in low-moisture foods with current and potential dry-cleaning methods

Food is one of the basic needs of human life. With the increasing population, the production and supply of safe and quality foods are critical. Foods can be classified into different categories including low moisture, intermediate moisture, and high moisture content. Historically, low-moisture foods have been considered safe for human consumption due to the limited amount of moisture for microbial activity. Recalls of these foods due to pathogens such as Salmonella and undeclared allergens have brought attention to the need for improved cleaning and sanitization in dry food manufacturing facilities. In the food industry, cleaning and sanitation activities are the most efficient methods to prevent microbial contamination; however, water is most often required to deliver cleaning and sanitation agents. A well-written and properly implemented sanitation standard operating procedure can take care of microbial and allergen cross-contamination. Nevertheless, there are unique challenges to cleaning and sanitation processes for low-moisture food manufacturing facilities. The introduction of moisture into a low-moisture food environment increases the likelihood of cross-contamination by microbial pathogens. Hence, the use of water during cleaning and sanitation of dry food manufacturing facilities should be limited. However, much less research has been done on these dry methods compared to wet sanitation methods. This review discusses recent foodborne outbreaks and recalls associated with low-moisture foods the accepted methods for cleaning and sanitation in dry food manufacturing facilities and the limitations of these methods. The potential for air impingement as a dry-cleaning method is also detailed.

Inactivation of Foodborne Pathogens on Inshell Walnuts by UV-C Radiation

ABSTRACT

Inshell walnuts can be contaminated with pathogens through direct contact or cross-contamination during harvesting and postharvest hulling, drying, or storage. This study aimed to assess the efficacy of UV-C radiation in inactivating foodborne pathogens on inshell walnut surfaces. Intact inshell walnut surfaces were inoculated separately with Salmonella,Escherichia coli O157:H7, Listeria monocytogenes, and Staphylococcus aureus and then were subjected to UV-C radiation at doses of 29.4, 147.0, 294.0, 588.0, and 882.0 mJ/cm2. UV-C radiation inactivated the inoculated pathogens in a dose-dependent manner, and a tailing effect was observed for the inactivation of pathogens. UV-C radiation at 29.4 and 882.0 mJ/cm2 reduced the populations of Salmonella Enteritidis PT 30, Salmonella Typhimurium, E. coli O157:H7, L. monocytogenes, and S. aureus on inshell walnut surfaces by 0.82 to 1.25 and 1.76 to 2.41 log CFU per walnut, respectively. Scanning electron photomicrographs showed pathogenic bacterial cells in the cracks and crevices of the inshell walnut surface, and the shielding of microorganisms by the cracks and crevices may have contributed to the tailing effect observed during UV-C inactivation. No significant changes (P > 0.05) were found in walnut lipid oxidation following UV-C radiation at doses up to 882.0 mJ/cm2. Together, the results indicate that UV-C radiation could be a potential technology for reducing the populations of various foodborne pathogens on inshell walnut surfaces while maintaining the quality of walnuts.

HIGHLIGHTS

Temperature and Moisture Dependent Dielectric and Thermal Properties of Walnut Components Associated with Radio Frequency and Microwave Pasteurization

To provide necessary information for further pasteurization experiments and computer simulations based on radio frequency (RF) and microwave (MW) energy, dielectric and thermal properties of walnut components were measured at frequencies between 10 and 3000 MHz, temperatures between 20 and 80 °C, and moisture contents of whole walnuts between 8.04% and 20.01% on a dry basis (d.b.). Results demonstrated that dielectric constants and loss factors of walnut kernels and shells decreased dramatically with raised frequency within the RF range from 10 to 300 MHz, but then reduced slightly within the MW range from 300 to 3000 MHz. Dielectric constant, loss factor, specific heat capacity, and thermal conductivity increased with raised temperature and moisture content. Dielectric loss factors of kernels were greater than those of shells, leading to a higher RF or MW heating rate. Penetration depth of electromagnetic waves in walnut components was found to be greater at lower frequencies, temperatures, and moisture contents. The established regression models with experimental results could predict both dielectric and thermal properties with large coefficients of determination (R2 > 0.966). Therefore, this study offered essential data and effective guidance in developing and optimizing RF and MW pasteurization techniques for walnuts using both experiments and mathematical simulations.

Postharvest processing of tree nuts: Current status and future prospects-A comprehensive review

Tree nuts are important economic crops and are consumed as healthy snacks worldwide. In recent years, the increasing needs for more efficient and effective postharvest processing technologies have been driven by the growing production, higher quality standards, stricter food safety requirements, development of new harvesting methods, and demand to achieve energy saving and carbon neutralization. Among all, the technologies related to drying, disinfection, and disinfestation and downstream processes, such as blanching, kernel peeling, and roasting, are the most important processes influencing the quality and safety of the products. These processes make up the largest contribution to the energy consumptions and environmental impacts stemming from tree nut production. Although many studies have been conducted to improve the processing efficiency and sustainability, and preserve the product quality and safety, information from these studies is fragmented and a centralized review highlighting the important technology advancements of postharvest processing of tree nuts would benefit the industry. In this comprehensive review, almonds, walnuts, and pistachios are selected as the representative crops of tree nuts. Current statuses, recent advances, and ongoing challenges in the scientific research as well as in the industrial processing practices of these tree nuts are summarized. Some new perspectives and applications of tree nut processing waste and by-products (such as the hulls and shells) are also discussed. In addition, future trends and research needs are highlighted. The material presented here will help both stakeholders and scientists to better understand postharvest tree nut processing and provide technological recommendations to improve the efficiency and sustainability, product quality and safety, and competitiveness of the industry.

Recent developments for controlling microbial contamination of nuts

In recent years, the consumption of nuts has shown an increasing trend worldwide. Nuts are an essential part of several countries' economies as an excellent source of nutrients and bioactive compounds. They are contaminated by environmental factors, improper harvesting practices, inadequate packaging procedures, improper storage, and transportation. The longer storage time also leads to the greater chances of contamination from pathogenic fungi. Nuts are infected with Aspergillus species, Penicillium species, Escherichia coli, Salmonella, and Listeria monocytogenes. Therefore, nuts are associated with a high risk of pathogens and mycotoxins, which demand the urgency of using techniques for enhancing microbial safety and shelf-life stability. Many techniques such as ozone, cold plasma, irradiation, radiofrequency have been explored for the decontamination of nuts. These techniques have different efficiencies for reducing the contamination depending on processing parameters, type of pathogen, and conditions of food material. This review provides insight into decontamination technologies for reducing microbial contamination from nuts.

Radio frequency heating improves water retention of pork myofibrillar protein gel: An analysis from water distribution and structure

This study was to explore effects of hot air assisted or not assisted radio frequency (RF, 27.12 MHz, 1.4 kW) heating with different electrode gaps (100 mm, 120 mm, and 140 mm) on the water-holding capacity (WHC) of myofibrillar protein (MP) gel and to understand the underlying mechanism through chemical forces, water distribution, and structure. The results showed that the MP gels heated by RF (100 mm) had the highest WHC and uniform gel network structure. As for RF with 100 mm electrode gap, the increased ionic and hydrogen bonds might be conducive to the WHC compared to water bath heating, which was verified by Low-field nuclear magnetic resonance results that the free water converted into the immobilized water. Raman spectroscopy results revealed that RF (100 mm) induced the self-assembly of β-sheet to α-helix, which conduced to the stable and ordered gel network structure.

Pasteurization of flavored shredded pork using Zno nanoparticles combined with radio frequency pasteurization technology

Effects of ZnO nanoparticles combined radio frequency (ZNCRF) pasteurization on the survival of microorganism, flavor and taste of Flavored shredded pork were compared with conventional high pressure steam (HPS) sterilization. The results showed that ZNCRF pasteurization was better than HPS sterilization in terms of flavor and taste parameters and at the same time met the pasteurization requirement. GC-MS and NMR measurements were performed to explore changes in volatile compounds and status and distribution of water within the sample. The ZNCRF pasteurization when carried out for 30 min and the HPS sterilization reduced the relative contents of aldehydes by 18.8% and 19.7%, respectively, while the ZNCRF pasteurization within 20 min had less effect on aldehydes. Both ZNCRF pasteurization and HPS sterilization destroyed the vacuolar membrane of the samples caused the loss of water from the cytoplasm (T₂₃). This work shows that ZNCRF when applied for 20 min is a mild pasteurization method that can be applied to improve the quality of Flavored shredded pork.

Effect of Physical Structures of Food Matrices on Heat Resistance of Enterococcus faecium NRRL-2356 in Wheat Kernels, Flour and Dough

Nonpathogenic surrogate microorganisms, with a similar or slightly higher thermal resistance of the target pathogens, are usually recommended for validating practical pasteurization processes. The aim of this study was to explore a surrogate microorganism in wheat products by comparing the thermal resistance of three common bacteria in wheat kernels and flour. The most heat-resistant Enterococcus faecium NRRL-2356 rather than Salmonella cocktail and Escherichia coli ATCC 25922 was determined when heating at different temperature-time combinations at a fixed heating rate of 5 °C/min in a heating block system. The most heat-resistant pathogen was selected to investigate the influences of physical structures of food matrices. The results indicated that the heat resistance of E. faecium was influenced by physical structures of food matrices and reduced at wheat kernel structural conditions. The inactivation of E. faecium was better fitted in the Weibull distribution model for wheat dough structural conditions while in first-order kinetics for wheat kernel and flour structural conditions due to the changes of physical structures during heating. A better pasteurization effect could be achieved in wheat kernel structure in this study, which may provide technical support for thermal inactivation of pathogens in wheat-based food processing.

Computational modelling of survival of Aspergillus flavus in peanut kernels during hot air-assisted radio frequency pasteurization

In recent years, radio frequency (RF) heating is getting popular as an alternative pasteurization method for agricultural commodities and low moisture foods. Computer simulation is an effective way to help understand RF interactions with food components and predict temperature distributions among food samples after RF treatments. In this study, a computer model based on Joule heating and thermal inactivation kinetic of A. flavus was established to predict both temperature distribution and microbial reduction among peanut kernels after RF processing. For the process validation, three 2-g peanut samples inoculated with 40 μL A. flavus were placed at three representative locations among 2.17 kg peanut kernels and subjected to various processing conditions in a 27.12 MHz, 6 kW RF heating unit together with hot air system. Results showed that the average difference of the sample temperature and microbial reduction between simulation and experiment was small with RMSE values of 0.009 °C and 0.012 °C, and 0.31 log CFU/g and 0.42 log CFU/g for peanut moisture contents of 7.56% and 12.02% w. b., respectively. Nonuniform RF heating resulted in the least lethality of A. flavus at the cold spot. The validated computer model was further used to estimate microbial reduction distributions at other target temperatures based on predicted temperature profiles. This computer model may help design the RF pasteurization protocols for peanut kernels without extensive experiments in food industry.

A comprehensive review on recent developments of radio frequency treatment for pasteurizing agricultural products

Recent pathogen incidents have forced food industry to seek for alternative processes in postharvest pasteurization of agricultural commodities. Radio frequency (RF) heating has been used as one alternative treatment to replace chemical fumigation and other conventional thermal methods since it is relatively easy to apply and leaves no chemical residues. RF technology transfers electromagnetic energy into large bulk volume of the products to provide a fast and volumetric heating. There are two types of RF technology commonly applied in lab and industry to generate the heat energy: free running oscillator and 50-Ω systems. Several reviews have been published to introduce the application of RF heating in food processing. However, few reviews have a comprehensive summary of RF treatment for pasteurizing agricultural products. The objective of this review was to introduce the developments in the RF pasteurization of agricultural commodities and to present future directions of the RF heating applications. While the recent developments in the RF pasteurization were presented, thermal death kinetics of targeted pathogens as influenced by water activity, pathogen species and heating rates, non-thermal effects of RF heating, combining RF heating with other technologies for pasteurization, RF heating uniformity improvements using computer simulation and development of practical RF pasteurization processes were also focused. This review is expected to provide a comprehensive understanding of RF pasteurization for agricultural products and promote the industrial-scale applications of RF technology with possible process protocol optimization purposes.