A shift from the isobaric to the isochoric thermodynamic state can reduce energy consumption and augment temperature stability in frozen food storage

Could 'Isochoric Freezing' Revolutionise Food Preservation?

The present article responds to the food engineering community's growing interest in an emerging and lauded approach to food preservation, popularised by its developers as 'isochoric freezing'. A strong campaign in the scientific literature and mass media has recently promoted this technique as a universal replacement for traditional food freezing and the frozen supply chain by highlighting a number of alleged advantages of 'isochoric freezing'. Some of these claims therefore require a more neutral and critical assessment against the background of the today's state of the art in food freezing technologies. Hence, this article spotlights several concerns regarding the plausibility, energy expenditure, resource efficiency, process rate, throughput and safety of 'isochoric freezing', as well as the correct use of food refrigeration terminology. The aspects considered are intended to make food scientists, technologists and engineers more aware of the real capabilities and the application perspectives of this still immature mode of refrigerated food processing.

Assessing the impact of isochoric freezing as a preservation method on the quality attributes of orange juice

Isochoric (constant volume) freezing is a novel food preservation technology that has demonstrated the ability to preserve food products at subfreezing temperatures in an unfrozen state, thereby avoiding the detrimental effects of ice formation. It minimizes the quality loss of fresh fruits and juices, increases their nutrient content, and reduces microbial counts. Orange juice (OJ) samples were subjected to conventional freezing (CF) and isochoric freezing (IF) for 7 days and then stored at 4°C for an additional 7 days. We evaluated the microbiological and physicochemical quality of CF and IF OJ before and after storage. The IF was performed at three different conditions: -5°C/73 MPa, -10°C/93 MPa, and -15°C/143 MPa. The results indicate that the total aerobic count of OJ remained below the detection limit after heat treatment, 7 days of CF and 7 days of IF. Yeast and mold counts increased in fresh and CF OJ after 7 days of storage at 4°C, whereas IF OJ remained below the detection limit. Less color difference was observed in IF (-15°C/143 MPa) OJ compared to heat-treated and CF OJ. Heat treatment inactivated 42% of pectin methylesterase (PME), whereas 7-day long IF increased PME activity up to 150%. Additionally, IF (-15°C/143 MPa) OJ showed reduced pulp sedimentation, which can be advantageous, as sedimentation in juices has been a recognized technological issue in the juice industry. Ascorbic acid level was significantly higher in IF (-15°C/143 MPa) OJ compared to fresh and CF OJ after storage.

Recent developments in freezing of fruits and vegetables: Striving for controlled ice nucleation and crystallization with enhanced freezing rates

Fruits and vegetables are rich in essential nutrients such as minerals, vitamins, and antioxidants; however, they have short shelf life. Freezing is a superior method of preservation compared to other techniques with respect to nutrient retention and maintenance of sensory attributes. However, several physical and textural quality changes associated with freezing and thawing pose a serious problem to the quality of frozen products. Some of the disadvantages associated with the currently employed methods for freezing fruits and vegetables include low rates of heat exchange in blast freezers, shape limitation in plate freezers, high cost of operation in cryogenic freezing, and freezing solution dilution in immersion freezing. Therefore, novel freezing technologies have been developed to achieve controlled ice nucleation and crystallization, enhanced freezing rate, decreased phase transition time, and maintained temperature stability. This review discusses some of the most recent approaches employed in freezing and points to their adoption for maintaining the quality of fruits and vegetables with extended storage.

Effects of Isochoric Freezing on the Quality Characteristics of Raw Bovine Milk

This study investigated the effects of isochoric freezing (IF) on the shelf-life and quality of raw bovine milk over a 5-week period. The results were compared with conventional refrigeration (RF) and refrigeration after pasteurization (HTST). The IF treatment process entailed storing liquid raw milk in isochoric chambers in thermodynamic equilibrium at -5 °C/77 MPa and -10 °C/96 MPa. Several parameters were analyzed, including microbiology count, physicochemical properties, indigenous enzyme activity, protein content, volatile organic compounds profile, and lipid degradation. Both raw and pasteurized milk experienced increases in the microbial level past the acceptable threshold (≥5.5 log CFU/mL) after 2 weeks and 5 weeks, respectively, leading to the deterioration of other parameters during storage. In comparison, microbiology count decreased significantly during storage for both IF treatment conditions but was more pronounced for the higher pressure (96 MPa) treatment, leading to undetectable levels of microorganism after 5 weeks. IF treatment maintained stable pH, titratable acidity, viscosity, lipid oxidation, volatile profiles, total protein content, and lactoperoxidase activity throughout the storage period. Color was preserved during IF treatment at -5 °C/77 MPa; however, color was impacted during IF treatment at -10 °C/96 MPa. Protein structures were also modified during pressurized storage in both IF treatments. Overall, the study demonstrated that isochoric freezing could significantly increase the shelf-life of milk by reducing microbiology activity, whilst maintaining its nutritional content. These results underscore the potential role of isochoric freezing as a valuable tool in eliminating pathogens while maintaining quality characteristics similar to raw milk over long storage periods.

Preservation of black grapes by isochoric freezing

Fruits are perishable. It's crucial to have an efficient preservation technique that may extend storage duration while maintaining the physical quality and nutritional values to avoid wastage. The majority of long-term storage solutions for fruits use refrigeration. In this study, we evaluate the potential of isochoric freezing as an alternative method of preservation for black grapes (Vitis vinifera L.). We compare the properties of black grapes preserved for 7 days in trehalose solution at -4 °C in isochoric conditions (average pressure 34.2 MPa) with those of fresh black grapes and with grapes preserved isobarically in four conditions (room temperature, in the fridge, in the freezer, and in a plastic bag filled with trehalose solution). The results indicate that grapes preserved by isochoric freezing at temperatures below the freezing point of water do not lose weight; on the contrary, they resulted in a very small (2%) weight gain. Freezing under isochoric conditions did not result in significant changes in terms of macroscopic appearance, colour, firmness, °Brix values, or pH. We consider that isochoric freezing has the potential to be used as a preservation method for grapes while maintaining physicochemical parameters similar to those of fresh fruits.

Novel isochoric impregnation to develop high-quality and nutritionally fortified plant materials (apples and sweet potatoes)

Isochoric impregnation was explored as a novel pressure-assisted infusion technique to fortify plant materials with bioactive compounds. Apple and potato cylinders were impregnated with a sucrose solution containing 4% ascorbic acid (AA) while freezing under isochoric conditions. Isochoric impregnation resulted in greater infusion of AA compared to infusion at atmospheric pressure, which demonstrated the feasibility of this impregnation technology. Processing temperatures (-3°C and -5°C) and processing times (1, 3, and 5 h) significantly affected the AA infusion. The AA content values ranged from 446 to 516 mg/100 g for apples and 322 to 831 mg/100 g for sweet potatoes under isochoric conditions. For both plant materials, isochoric impregnation at -3°C did not cause major changes in texture and microstructure of the biological tissues. These results indicated that isochoric impregnation of solid foods could be a feasible technology for infusion of bioactive compounds without significantly altering their matrix. PRACTICAL APPLICATION: The findings of this study showed that the use of isochoric impregnation as a fortification technique is a promising way to develop fresh-like and value-added products with improved nutrition during preservation at subfreezing temperatures.

Effects of Isochoric Freezing Conditions on Cut Potato Quality

Isochoric freezing is a pressure freezing technique that could be used to retain the beneficial effects of food storage at temperatures below their freezing point without ice damage. In this study, potato cylinders were frozen in an isochoric system and examined using full factorial combinations of three processing procedures (immersed in water, vacuum-packed and immersed in ascorbic acid solution), four freezing temperatures/pressures (−3 °C/37 MPa, −6 °C/71 MPa, −9 °C/101 MPa and −15 °C/156 MPa) and two average compression rates (less than 0.02 and more than 0.16 MPa/s). The effects of process variables on critical quality attributes of frozen potatoes after thawing were investigated, including mass change, volume change, water holding capacity, color and texture. Processing procedure and freezing temperature/pressure were found to be highly significant factors, whereas the significance of the compression rate was lower. For the processing procedures, immersion in an isotonic solution of 5% ascorbic acid best preserved quality attributes. At the highest pressure level of 156 MPa and low compression rate of 0.02 MPa/s, potato samples immersed in ascorbic acid retained their color, 98.5% mass and 84% elasticity modulus value. These samples also showed a 1% increase in volume and 13% increase in maximum stress due to pressure-induced hardening.

Pressure-related cooling and freezing techniques for the food industry: fundamentals and applications

Cooling and freezing are two widely used methods for food preservation. Conventional cooling and freezing techniques are usually with low efficiency and prone to damage foodstuffs. In order to increase cooling and freezing efficiencies and ensure better food quality, many efforts have been performed. As effective solutions, pressure-related techniques such as vacuum cooling (VC), vacuum film cooling (VFC), vacuum spray cooling (VSC), pressure shift freezing (PSF) and isochoric freezing (ICF) have attracted a lot of interests. The current review intends to provide an overview of pressure-related cooling and freezing techniques for the food industry. In the review, the fundamentals including principles, experimental systems, thermodynamic and kinetic mechanisms and their relevant mathematical models are presented, latest applications of these techniques in the food industry are summarized, and future trends concerning technological development and industrialization are highlighted. Pressure plays an important role in improving the cooling and freezing processes and ensuring food qualities, and mathematical modeling is an effective tool for understanding the thermodynamic and kinetic mechanisms of these processes. However, the latest researches showed that despite many merits of these pressure-related processes, limitations still exist in applying some of the techniques in the food industry. For achieving technological development and industrialization of the pressure-related processes, further researches should focus on improving model performance, integrating multiple technologies, and cost control.

Effect of isochoric freezing on quality aspects of minimally processed potatoes

The enhanced interest in greater convenience foods has recently led to the expansion of minimally processed potato products. This study investigated the effects of isochoric freezing on pre-peeled potato cubes, including quality attributes (microstructure, texture, and color), nutritional value (ascorbic acid (AA) content, total phenolic content, and antioxidant capacity), and polyphenol oxidase activity. Isochoric freezing (-3 °C/30 MPa) was compared with isobaric freezing (-3 °C/0.1 MPa) and individual quick freezing followed by frozen storage at -20 °C for 4 weeks. The isochoric sample had lower drip loss and volume shrinkage as well as better preserved texture and microstructure than the other samples. All freezing methods caused an increase in total phenolic content and antioxidant capacity, but a decrease in AA content. Also, all freezing methods caused browning of the thawed potatoes, but isochoric freezing delayed its onset for more than 1 week. PRACTICAL APPLICATION: Results showed that isochoric freezing of pre-peeled and cut potatoes caused less freeze damage than isobaric and individual quick freezing, which might find application in the commercial preservation of minimally processed food products.

Antibiotic Susceptibility, Biofilm-Forming Ability, and Incidence of Class 1 Integron of Salmonella spp., Escherichia coli, and Staphylococcus aureus Isolated from Various Foods in a School Canteen in China

In recent years, the food poisoning incidents from school canteens have aroused widespread concern in China. Microbial contamination to the foods is the main factor responsible for these food poisoning events. In this study, identification of microbial pathogens including Salmonella spp., Escherichia coli, and Staphylococcus aureus in samples (frozen pork, fresh pork, fresh chicken, and different fresh vegetables) of a school canteen in China during 2017 to 2018 was performed. The antibiotic susceptibility pattern, class 1 integron, and biofilm formation ability of the isolated pathogens were also investigated. In total, 96 strains were isolated (32 Salmonella spp., 32 E. coli, and 32 S. aureus). The antibiogram study results demonstrated that 61.5% strains were found resistant to at least one type of antibiotics, and 17.7% were resistant to three or more antibiotics. In addition, 31.3% strains possessed class 1 integron. Among the integron-positive isolates, 38.9% Salmonella spp. and 87.5% E. coli contained ∼800 or/and 1500 bp size gene cassette within the integrons. However, four S. aureus strains possessing class 1 integron without gene cassette were found. Although none of the isolated strains were found strong biofilm producer, 44.8% were found to have weak or moderate biofilm formation ability. Despite biofilm formation ability or not, the Salmonella spp. containing positive class 1 integron showed significant resistance to cefazolin and gentamicin. In addition, class 1 integron-positive E. coli isolates having the biofilm formation ability hardly showed sensitive to four antibiotics, such as amikacin, amoxicillin-clavulanate, cefazolin, and gentamicin. Therefore, it is necessary to reduce the prevalence of antibiotic resistance gene cassettes containing antibiotic resistance genes by the prudent use of antibiotics in livestock farms, and the improvement of food processing and storage environment.