Supercooling of Perishable Foods for Extended Shelf Life: An Investigation of Quality

Abstract

The use of cold storage is the most popular method to preserve highly perishable foods, such as beef and fish. However, at refrigeration temperatures the shelf life of these foods are limited, and spoilage leads to massive food waste. Moreover, freezing significantly affects the food’s properties. Ice crystallization and growth during freezing will cause irreversible textural damage to the food through volumetric expansion, moisture migration by induced osmotic pressure gradients, and freeze concentration of solutes leading to protein denaturation. Although freezing will preserve perishable foods for months, these disruptive changes decrease the consumer’s perception of the food’s quality. Therefore, the development and testing of new and improved cold storage technologies is a worth-while pursuit. In this study, a previously developed technique that used a combination of pulsed electric fields (PEF) and oscillating magnetic fields (OMF) to preserve a food at temperatures below its equilibrium freezing point without internal ice formation occurring was improved upon and subsequently tested. The process of maintaining a food product unfrozen below its equilibrium freezing temperature is known as supercooling. Beef and tuna were tested with the combination PEF and OMF supercooling procedure. These foods were chosen because they are considerably valued by consumers, highly perishable, and have fairly uniform compositions. In the first part of this study, beef was preserved in its supercooled state (-4ºC) for 2 weeks. Its quality aspects were tested and compared with those of refrigerated (4ºC) and frozen (-10ºC) control samples periodically throughout the 2 week storage. It was found that significant color changes (p < 0.05) occurred in the refrigerated samples that did not occur when the beef was supercooled. The color changes were an indication of major spoilage. Furthermore, cell damage caused by ice crystals in the frozen beef was observed in both optical and transmission electron micrographs, whereas ice damage was not seen in the micrographs of the supercooled samples. The cellular damage in the frozen samples led to significantly higher amounts of drip loss (p < 0.05) and significantly lower Warner-Bratzler shear values (p < 0.05) than in both the refrigerated and supercooled samples at each time point tested over the 2 week storage. It was concluded that the combination PEF and OMF supercooling procedure was suitable for improved preservation over refrigeration of beef without the damaging effects of ice formation and growth during freezing. The second part of this study focused on testing the use of electrochemical impedance spectroscopy (EIS) in investigating physiochemical changes in sashimi grade ahi tuna after 8 days of PEF and OMF supercooled storage at -3ºC. The EIS parameter Py was used in comparing refrigerated, supercooled, and frozen tuna samples. A decrease in Py correlates with a decrease in overall impedance. After all treatments, a significant decrease in Py was observed (p < 0.05). However, a more pronounced difference was found between the frozen-thawed samples (36.30 ± 0.63%) and all other samples (>44%). This considerable decrease in impedance indicated the loss of capacitance which characterizes the destruction of cells. The supercooled samples did not display a drastic decrease in impedance, and, from that, it could further be concluded that no cellular damage due to ice formation and growth occurred. EIS proved its ability to distinguish between samples that had and had not been frozen-thawed, however further investigations of electrode design must be done to optimize its use for detecting the onset of spoilage. Instead, the K value (a ratio of ATP-breakdown products) was used to assess the extent of spoilage in the tuna samples. A higher K value indicates a greater extent of spoilage. The K values of the frozen, supercooled, and refrigerated samples after 8 days of treatment were 19.89 ± 0.19, 26.00 ± 0.04, and 74.29 ± 0.73%, respectively. As with the beef, the combination PEF and OMF supercooling procedure was suitable for improved preservation over refrigeration of tuna without the damaging effects of ice formation and growth during freezing. This overall study produced a substantial extension of the supercooled state of perishable foods (beef and tuna) using a combination of PEF and OMF. Also, the overall results demonstrated the preservation ability of this supercooling technique to be advantageous over refrigeration without causing a significant decrease in textural integrity of the product, which was observed with freezing. Apart for proteinaceous foods (beef and fish), the quality of other perishable foods under cold storage would be greatly improved using this supercooling technology. Furthermore, as supercooling was found to have a sizable improvement over refrigeration on a perishable product’s shelf life, the implementation of supercooling areas in household and commercial refrigeration units would help in diminishing food waste. Currently, a commercially viable supercooling unit for all perishable food items is being developed and fabricated. The fabrication of such a unit includes circuitry integration of control logics as well as complete assimilation with a commercial freezer. The buildup of this technology will provide a meaningful improvement on cold storage of perishable foods, and will have significant impact on the refrigeration market as a whole.