Modelling and ice crystallization/recrystallization of foods in superchilling technology

Abstract

The superchilling process is defined as a method of preserving food by partial ice-crystallization. The product quality of superchilled food is very promising, and has nearly the same sensorial attributes and nutritional value as the original product. However, more research is required to make the technology more suitable to the preservation of food. The optimal degree of superchilling and information on the development of the ice crystals during the superchilling process and storage are needed because of their large influence on the quality of the final superchilled food.

The main objective of this thesis was modelling and studying of ice crystallization/recrystallization of food during the superchilling process and storage. In order to fulfil the objective the following research activities have been carried out: A one-dimensional model for predicting partial freezing time necessary to achieve an optimal degree of superchilling in foods was developed. The degree of superchilling is the amount of free water frozen (5 -30%) inside the food and is among the most important parameters which influence the quality of superchilled product. The study of the ice crystallization/recrystallization of food in superchilling technology was studied based on the superchilling rate, and the state of food muscle. The relationship between the development of ice crystals in salmon and quality parameters during the superchilled storage was also studied. The final research activity in this thesis was to study differences in the superchilling storage methods, shell freezing and non- shell freezing.

The developed model was sufficient to study the thermal behaviour of food, and had the advantage that it was simple, very fast and detailed enough to estimate the superchilling time and behaviour of food. The model was validated experimentally using salmon, and there was good agreement between the numerical and experimental results. Further study to quantify the model using other food products is recommended.

The characteristics of ice crystals have a large influence on the quality of the final superchilled food. At a high superchilling rate (227 W/m2.K, -30 ℃ and 2.1 min), smaller and well distributed ice crystals within and outside the cell were formed compared to a slow superchilling rate (153 W/m2.K, - 20 ℃ and 4.2 min), where larger and extra-cellular ice crystals were formed. The state of muscle also has an influence on the characteristics of ice crystals. In pre-rigor muscle, the ice crystals were formed inside the cells regardless of the superchilling rate. However, at a slow superchilling rate the ice crystal size was larger than at a high superchilling rate. The formation of the ice crystals inside the cells, regardless of the superchilling rates, is the most important factor for reducing the damage of food muscles and hence maintaining the quality.

New information was discovered in this work on the development of ice crystals during the superchilling process and storage of salmon. There was a significant increase in ice crystal size between the superchilling process (day 0) and superchilled storage (after 1 day of storage). The ice crystals formed in the surface layer were 4 times larger after only 1 day of storage than those formed at day 0. Prior to temperature equalisation, ice crystals growth progressed from the surface to the centre of the superchilled food. Different layers with different sizes of ice crystals within the superchilled salmon were also observed. This was due to thermal behaviour within the superchilled sample, and because we have both ice at the surface and water at the centre, the diffusion process should occur. The recrystallization at this time (between day 0 and 1) is unavoidable however, after temperature equalization (after 1 day of storage) and control of temperature during storage there was no significant growth of ice crystals for the entire storage time.

The development of ice crystals in red salmon muscle was also studied during the superchilling process and storage. The size of the ice crystals formed in the red salmon muscle was smaller than those formed in the white salmon muscle. In addition, the ice crystals formed in the pre-rigor red muscle was smaller than that formed in the post-rigor red salmon muscle. These findings are significant for the industry because small ice crystals indicate better quality.

Quality changes have been studied with a focus on physical measurements, water holding capacity (WHC) and drip loss. The disappearance of liquid water is a major factor affecting the protein changes during superchilled storage. It was observed that the drip loss was lower in superchilled salmon compared to conventional chilled salmon, and frozen salmon between 1 and 14 days of storage. No significant differences were found in WHC and drip loss between 1 and 14 days of storage in superchilled salmon.

The two superchilling storage methods showed differences in the development of ice crystals within the superchilled salmon. In non-shell frozen samples, the ice crystals were mainly formed in the extracellular spaces. Fine and well distributed ice crystals were formed in both the intracellular and extracellular spaces in shell frozen samples.

Generally, the results found in this study have given more information in the superchilling area. The developed model which can be scaled-up to the industrial level, together with information on the development of the ice crystals, which has a large influence on the quality of the final superchilled food are useful for the industry in estimating the refrigeration requirements for a superchilling system and designing the necessary equipment. In addition, the quality study revealed that the superchilling is practicable if the product is partially freezing fast, with an optimal degree of superchilling (5 - 30 %), good packaging and a strict control of the temperature during superchilled storage.