To begin our new series, "What you need to know about refrigeration", we will be taking a look at one of refrigeration's most important applications, namely the cold storage room. As well as explaining the specific features of cold storage we will also explain on the "how" and the "why". This series is based on a number of complimentary e Learning modules from Danfoss for individual self-tuition on a PC. They are suitable for any type of student, from beginners to refrigeration experts.
"A cold storage room is a refrigerated food storage chamber that prevents perishable food from spoiling." The primary purpose of a cold storage room is to refrigerate fresh, pre-cooled and frozen foods. Products that are kept in cold storage include meat, eggs, dairy products, frozen foods, fish, vegetables, fruit and beverages. When storing these different products it is necessary to consider a number of factors. A product such as meat, for example, is "dead": the processes that cause it to spoil begin as soon as the animal is slaughtered. Products such as fruit and vegetables, on the other hand, are still "alive". Their metabolism does not stop when they are harvested, so it is important to maintain them at the proper humidity and temperature to prevent deterioration. Humidity is also important for meat, although loss of moisture is not so readily discernible just from looking at the product, and results only in a small loss of profit when the meat is subsequently sold by weight rather than rendering it unsaleable. Some dairy products and beverages have to be stored below a certain temperature to prevent chemical reactions and microbial processes that would be detrimental to their quality.
What is "food quality"? The term is used to denote both the physical properties of and an individuals’ sensory perceptions. Sensory perceptions include taste, colour, smell and consistency, Physical properties include types and levels of bacterial contamination, chemical processes and the types and quantities of added preservatives. There are a number of different ways of ensuring food quality. Various conservation and storage methods can be used to preserve food, such as refrigeration, desiccation, curing, preservation in sugar, smoking, canning, irradiating and excluding light. This article is concerned only with the refrigeration of foodstuffs as a way of preserving their quality. On closer examination, there are a number of individual factors that affect the quality of the food in cold storage. For example, microbial processes within the stored product, including the proliferation of microorganisms on and in the product and the growth of bacteria on the surface. Chemical processes such as oxidation or hydrolysis can also affect the product. Many of these processes involve enzymes in the product that act as catalysts and speed up the reaction. Degradation of the connective tissue in beef is one example of such a chemical process, but there are also effects from mechanical processes, including packaging and handling. Transport and shipment of food products are typical mechanical processes that affect their quality. Another point is the moisture in a cold storage room. Such rooms are usually maintained at a high level of humidity to prevent the refrigerated goods from drying out. The air-flow rate is another decisive factor. It is necessary find a suitable mean value for the air-flow rate, not too fast, which would tend to increase dehydration, and not too slow, which would impact on the air distribution and hence the refrigeration process. For certain varieties of fruit, such as apples and pears, the composition of the atmosphere in the cold storage room is even modified to maintain product quality. This is done by increasing the proportion of CO2 in the air, which slows down respiratory and decay processes and consequently increases the storage life. For fruit growers, every day of extra storage life is money in the bank. For refrigeration professionals, the cooling process is the key factor for cold storage chambers. The refrigeration plant must keep the temperature at the desired level. In general, the storage life can be extended by altering the temperature.
The temperature has a considerable effect on the microbial and chemical processes that take place within the food products. Low temperatures slow microbial growth. In general, the growth rate changes exponentially in relation to the temperature, which means that a change of few degrees already shows a marked increase in the growth rate. For many microbes, optimal growing conditions are found at temperatures above 0°C, while others can continue to multiply at lower temperatures, even right down to –12°C. Deep-freezing a product generally kills only 10 to 90% of the microorganisms. In other words, it is not possible to sterilize a product by freezing it. Parasites are a group of microorganisms than can sometimes be killed by deep-freezing. For this purpose it is generally sufficient to store the product at –20°C for a few days (such as a week). Temperature also affects chemical processes. The lower the temperature the slower the reaction, but most chemical reactions cannot be completely halted by lowering the temperature. Such processes continue until the temperature reaches absolute zero (–272.15°C). In some cases (e.g. when storing beef) certain tissue degradation is actually desired. When cooling fresh foods such as meat, it is important not to lower the temperature too quickly, because that might damage the product. In the case of fruit and vegetables, the temperature influences the metabolism and thus also the amount of heat generated by respiratory processes.
Humidity and Air-flow Rate
Humidity also affects food quality. Microorganism growth is also influenced by the moisture content of the ambient air. The moisture content of food products promotes the growth of microorganisms. In a low-humidity atmosphere, moisture will be removed from the product causing it to lose both weight and quality. We therefore need to maintain a certain level of humidity in the ambient air in order to keep food products fresh and prolong their shelf life. In order to continuously provide the proper moisture content we need to maintain a constant temperature. In addition to the moisture content, the air-flow rate is also important for the quality of food products. The air exchange rate (the rate at which the existing air is replaced by fresh) is what ensures good air quality in the cold store, but the flow rate also affects the cooling and dehydration of the product: a higher rate increases the transfer of heat from the product but also its dehydration rate. An increased rate of dehydration means that more water will be removed from the product, leaving a lower moisture content. We therefore have to provide for an appropriate air-flow rate within the refrigeration plant. The product factor (PF) depends on the air-flow rate and the type of food product. In general, higher air-flow rates increase the product factor, but this then also increases the dehydration rate. Some products have a surface that favors dehydration (lettuce, mushrooms) while other products, such as apples are less favorable.
This concludes the first part of our consideration of cold storage rooms, in which we dealt with produce being refrigerated. Construction of a suitable cold storage room, will be dealt with in the next episode in this series. This will concentrate on the practical challenges and requirements for setting up such an environment.