Chilling, chilling time, chilling methods, hydrate slurry, CO2 hydrate slurry
James et al. (2006) have reviewed previous studies related to the chilling of fresh products. The goal of chilling is always to limit the growth of pathogenic and food spoilage microorganisms. It has been proved that chilling reduces the number of both types of microorganisms on the surface of fresh products. In Europe dry air is the preferred chilling method. This method is believed to have clear microbiological advantages but James et al. (2006) show that the published data do not support this belief and direct contact chilling methods might even have larger microbiological advantages. However, in a continuous production line, chlorination or other additive is needed to prevent accumulation of micro-organisms. This is not acceptable in Europe. The rate of chilling has some influence on the taste, texture and appearance of the fresh product. Very rapid chilling will generally lead to robust structure while very slow chilling can produce soft texture. Pederson (1979) has compared the costs of different chilling methods in Denmark. When only energy costs were considered, the cost of a countercurrent water immersion chilling system was one fifth that of an air chilling method. Slush ice is often preferred in immersion systems since cooling takes place without freezing damage of the product while the heat removal takes place at a low temperature. For 70% slush ice content of the slurry, the cooling time is lowest while with 35% slush ice the highest weight gain is attained (around 5%). When air is used, short chilling times require low air temperatures and high air velocities. James et al. (2006) report for a specific product a chilling time (38 to 4 oC) 150 minutes with 0.75 m/s and 1 oC, 60 minutes with 4.1 m/s and -7 oC, and 17 minutes with -40 oC. Short chilling times impose low air temperatures and so low evaporating temperatures and high energy consumption of the primary refrigeration cycle. In this paper the immersion of fresh products in melting carbon dioxide hydrate crystals, produced at +8 oC and 30 bar, is proposed for rapid chilling of fresh products. First the chilling time of a specific fresh product from 30 to 4 oC making use of a â€œshock freezer (2 m/s air velocity, air at -10 oC) and applying a slurry of CO2 hydrate crystals is compared. Also the effect of these two methods on the weight loss of the product is compared. Then the chilling of chicken products making use of the proposed method is experimentally verified. Also the microbiologic contamination of samples chilled using high speed dry air, immersion in melting water and immersion in CO2 hydrate slurry are compared. Finally the economic advantages and practical applicability of the proposed method are evaluated.