| dc.description.abstract | During the last few years the trend in food trade has moved rapidly from frozen to fresh products. Problems with temperature control in the processing and the distribution of fresh foods have at the same time been well documented. Temperature recordings and observations in the Norwegian chill chain were performed in order to examine if this information has lead to improvements in the chill chain for fresh foods and also get an overview of the situation. As earlier, the results were depressing. Very inadequate temperatures were found in the retail part of the chain. The temperature conditions in the retail open multi-floor cabinets were especially unsatisfactory. Inadequate temperatures were also found during the loading and unloading of products, in the period between product delivery (at the shops) and location of the products in refrigerated equipment, on the way between shops and consumers’ kitchens and during storage in domestic refrigerators. The knowledge of heat transfer processes and the influence of temperature on food quality were very limited throughout the whole chill chain. The surveys showed that in order to improve this situation, all chill chain participants, consumers included, must be educated. It is also incredibly important that fresh foods are properly chilled before they go into distribution. Overloaded storerooms, trailers and counters and poor airflow conditions should be avoided so that the refrigeration system runs economically and effectively. Simple working rules for the chill chain participants were outlined.
In modified atmosphere packaging (MAP) a gas layer between product and packaging surface reduces the heat flows in chilling and warming processes. To determine the heat transfer resistance due to the insulation layer in most modified atmosphere packages (MA-packages), chilling and warming experiments were performed with modified atmosphere packaged (MA-packed) fish and meat products. For comparison some of the products were also vacuum-packed. Specific heat transfer parameters and coefficients for the products were calculated. The results showed much slower chilling and warming rates for the MAP products compared to the vacuum-packed ones. The challenge is how to maintain a low temperature for MAP products through the chill chain. The growth of some pathogenic bacteria could also be promoted in MAP products if the produce is not properly chilled.
In order to determine the chilling and warming times of a small fish paté product that was packaged in small containers without air, chilling and warming experiments were performed and product specific heat transfer parameters were calculated. The heat transfer parameters were furthermore used in new chilling/warming time estimates at other process conditions. With an ambient temperature of –10 °C for example, it would take from 0.7 (5 m/s) to 2.0 (~0 m/s) hours to chill the products from 80°C to 2°C. With an ambient temperature of 18 °C (~0 m/s), it would only take 0.4 hours to warm the products from 2 °C to 7 °C, a temperature where many pathogen bacteria can grow and produce toxins. In order to minimize the energy consumption during chilling of the fish product, a one-stage chilling process performed in a chilling tunnel was compared with a two-stage chilling process, where the first chilling stage was done at 23 °C, and the second stage was in a chilling tunnel. The two-stage chilling doubled the chilling time, but energy was saved. The heat removed from the products, packaging included, was estimated to be 0.08 kWh/kg and 0.04 kWh/kg for the one-stage and the two-stage processes respectively.
With superchilling, the product temperature is lowered somewhat below the freezing point of the product before it goes into distribution. MA-packed sirloin steaks were superchilled in order to extend the shelf life of the products and check for biochemical quality changes due to superchilling. Quality changes of superchilled and chilled MA-packed sirloin steaks were studied during storage for 9 and 16 days at constant and varying temperature conditions. Storage at varying temperatures was disadvantageous for the biochemical quality of the products, whereas superchilled and ordinary chilled products obtained similar biochemical quality. The bacterial growth was generally high and increased during storage. Varying storage temperature gave very high plate counts and superchilling gave somewhat lower plate counts than ordinary chilling. Similar quality changes were also studied for superchilled and chilled MA-packed salmon fillets. The changes were studied during storage for 17 days. The biochemical quality of the superchilled products was as good as of the ordinary chilled salmon fillets. Superchilling gave lower plate counts than ordinary chilling. Superchilling is a promising process that should be developed and optimized further.
This work has clearly shown that there is an unsatisfactory temperature situation and a need for education in the Norwegian food industry. The quality of fresh foods would be greatly improved if the temperatures in the chill chain were lowered to around –1°C. Superchilling could be introduced to ensure such low temperatures through the chill chain for fresh foods. | nb_NO |
