Chilled and frozen storage
Chilled and frozen storage
Friday, 10 April 2009
Chilled and frozen storage has never been a major process engineering research area at Langford although the facilities and expertise have been widely used in the microbiological, shelf life and packaging studies of other groups on site.
Many of the studies have concentrated on improving industrial and commercial profitability by reducing weight loss during chilled (Winstanley & Malton, 1984) or frozen (Cutting & Malton, 1974) storage or both (Cutting, 1974; Malton & James, 1984). These studies clearly showed the importance of maintaining low temperatures and low air velocities over unwrapped meat during chilled storage. High relative humidities were also very important in minimising weight loss, but very high humidity, >90%, promoted bacteria growth. In commercial storage, -1°C, 90% RH and 0.3 ms-1, were found to “represent near ideal conditions for minimal weight loss” (Malton & James, 1984). Storage temperature and air velocity were also the critical factors in the frozen storage of stockinet covered meat. Average weight losses from lamb carcasses were 0.2, 0.4 and 1.0% per month approximately at -30, -20 and -10°C respectively (Cutting & Malton, 1974). The studies also showed the advantage of the old brine pipe-cooled stores over the modern forced air cooled equivalents. Weight losses in the former were less than 0.05% per month at temperatures of -10 and -18°C. With no differences reported between the temperatures.
In the late 1980s MAFF provided funding to examine long term frozen storage of foods at Langford and facilities, including a -80°C and controlled temperature cycling chambers, established. Typically, the frozen storage life of food, at temperatures below –20°C, range from 6 months to many years. Conventional storage life trials using taste panels have therefore to be carried out over long time periods. Initial studies looked at speeding up the process by using higher storage temperatures or chemical methods of testing (Evans, Bratchel & James, 1988). However, no suitable method was found. A comprehensive review was also carried out on the frozen storage of meat and meat products (Evans & James, 1990) and this review was updated as part of an EU Concerted action project (Archer et al., 1998). The conclusions of this work were that much of the underlying data had been gathered from practical experience backed up by a relatively small number of controlled scientific experiments. Much of the scientific data dated back to the time when meat was either stored unwrapped or in wrapping materials that are no longer used. It was thus not surprising, when consideration was made of the changes in packaging and handling methods over the last century, that there is a considerable scatter in published data on storage lives for similar products.
From an industrial point of view we concluded that there were two main questions that needed to be answered.
•What magnitude of temperature fluctuation within a frozen storage environment can be tolerated without any detrimental effect on the storage life of the stored meat?
•How is the storage life of a meat product produced from a previously frozen raw material affected by the length of time the raw material has been stored?
Investigations were set up in the late 1980s to provide the required information, but with the Government’s closure of IFR-BL in 1990, the questions still remain unanswered.
Much of the recent work at FRPERC on chilled and frozen storage has been on the performance of cold-stores. A considerable amount of this work has been confidential for commercial operators, but there have been two separate defra LINK research projects looking at different aspects of cold-store use.
In one project a predictive program, ‘ColdRoom’, has been developed to allow cold-storage operators, contractors, and manufacturers to specify and design cold rooms to keep food at optimum temperatures under actual working conditions, and users to rapidly predict the effect of operating conditions and loading patterns on performance and identify how they can avoid unacceptable food temperatures (Ketteringham & James, 2005).
A separate project has looked at air entrainment through cold-store entrances. In cold-stores energy is wasted due to air infiltration into the room during loading and unloading and other instances when the barrier between the cold and warm environments is removed. Air infiltration is also the main source of frost on evaporators and can lead to accidents caused by ice. As part of this project much work was carried out on the measurement (using systems such as Laser Doppler Anemometry (LDA) with our colleges in the Department of Aeronautical Engineering at the University of Bristol) and prediction (using Computational Fluid Dynamics (CFD) modelling) of air movement through the doorways of refrigerated rooms (Foster et al., 2002; 2003). The effectiveness of an air curtain fitted over the entrance to a cold store to prevent this infiltration has been measured and predicted using both 2-D analytical and 3-D CFD models (Foster et al., 2006; 2007). This work has shown the importance of correctly setting up air curtains to give optimal effectiveness.
Other work has looked at bacterial contamination of refrigeration systems in chill rooms. An extensive survey of chilled rooms in 15 UK food plants revealed high levels of bacteria in all the plants surveyed. These included plants processing raw meat and salads, Chinese ready meals, dairy products, slicing and packing of cooked meats and catering establishments (Evans et al., 2004). The work demonstrated that bacteria were present on evaporator cooling coils in all factory cold rooms visited. Although evaporator-cleaning procedures were carried out in some factories as part of routine maintenance these were not shown to be effective at maintaining low levels of bacteria on evaporators. To maintain evaporator hygiene it is suggested that more regular cleaning procedures, possibly by means of automated cleansing systems, should be considered. Laboratory experiments showed that no bacterial growth occurred in tests on a refrigerator coil that reproduced the operating conditions measured in industry (James et al., 1998). The difference being that in the experimental situation a biofilm of organic debris was not allowed to accumulate.
Recent research at FRPERC has demonstrated that some vegetables can be stored at temperatures significantly below their freezing point without freezing occurring. This work was initiated following an opinion sought regarding the freezing of garlic bulbs (Allium sativum L.). A company had reputably been importing ‘frozen’ garlic bulbs at a temperature of -6°C, however the UK Customs questioned whether it was ‘frozen’. An independent report by another research organisation had made an assumption that freezing would cause clear and obvious changes to the colour and structure of the cells in the garlic cloves. They did not find this in garlic cloves that they inspected, hence they concluded that the samples had not been previously frozen. However, a short experimental study carried out by us indicated that the degree of freezing, and potentially damage, within a clove of garlic was not only a function of temperature but was also influenced by the rate of temperature reduction and possibly by the length of storage time. The study also found unusual supercooling characteristics that indicated that garlic cloves may not always be in a frozen state at a temperature of -6°C. This initiated further studies into the supercooling characteristics of garlic (James, Seignemartin & James, 2009). This work shows that peeled garlic cloves demonstrate significant supercooling during freezing under standard conditions and can be stored at temperatures well below their freezing point (-2.7°C) without freezing. The nucleation point or ‘metastable limit temperature’ (the point at which ice crystal nucleation is initiated) of peeled garlic cloves was found to be between -7.7°C and -14.6°C. Peeled garlic cloves were stored under static air conditions at temperatures between -6°C and -9°C for up to 69 h without freezing, and unpeeled whole garlic bulbs and cloves were stored for one week at -6°C without freezing. Further, as yet unpublished work, has shown significant supercooling to occur in a wide variety of other vegetables (such as shallots, cauliflower and peppers).