REFRIGERATION AND FREEZING OF FOODS
Refrigeration and freezing of perishable food products is an important and fascinating application area of heat transfer and thermodynamics. Refrigeration slows down the chemical and biological processes in foods and the accompanying deterioration and the loss of quality. The storage life of fresh perishable foods such as meats, fish, fruits, and vegetables can be extended by several days by cooling, and by several weeks or months by freezing. There are many considerations in the design and selection of proper refrigeration and heat transfer mechanisms, and this chapter demonstrates the importance of having a broad base and a good understanding of the processes involved when designing heat transfer equipment. For example, fruits and vegetables continue to respire and generate heat during storage; most foods freeze over a range of temperatures instead of a single temperature; the quality of frozen foods is greatly affected by the rate of freezing; the velocity of refrigerated air affects the rate of moisture loss from the products addition to the rate of heat transfer, and so forth. We start this chapter with an overview of microorganisms that are responsible for the spoilage of foods since the primary function of refrigeration is to retard the growth rate of microorganisms. Then we present the general considerations in the refrigeration and freezing of foods and describe the different methods of freezing. In the following sections we describe the distinctive features and refrigeration needs of fresh fruits and vegetables, meats, and other food products. Next we consider the heat transfer mechanisms in refrigerated storage rooms, Finally, we discuss the transportation of refrigerated foods since most refrigerated foods spend part of their life in transit in refrigerated trucks, railroad cars, ships, and even airplanes.
1 CONTROL OF MICROORGANISMS IN FOODS
Microorganisms such as bacteria, yeasts, molds, and viruses are widely encountered in air,water, soil, living organisms, and unprocessed food items, and cause off-flavors and odors, slime production, changes in the texture and appearances, and the eventual spoilage of foods. Holding perishable foods atwarmtemperatures is the primary cause of spoilage, and the prevention of food spoilage and theprematuredegradation of quality due to microorganisms is the largest application area of refrigeration. The first step in controlling microorganisms is to understand what they are and the factors that affect their transmission, growth, and destruction.Of the various kinds of microorganisms, bacteria are the prime cause for the spoilage of foods, especially moist foods. Dry and acidic foods create an undesirable environment for the growth of bacteria, but not for the growth of yeasts and molds. Molds are also encountered on moist surfaces, cheese, and spoiled foods. Specific viruses are encountered in certain animals and humans, and poor sanitation practices such as keeping processed foods in the same area as the uncooked ones and being careless about handwashing can cause the contamination of food products.
When contamination occurs, the microorganisms start to adapt to the new environmental conditions. This initial slow or no-growth period is called the lag phase and the shelf life of a food item is directly proportional to the length of this phase (Fig. 17–1). The adaptation period is followed by an exponential growth period during which the population of microorganisms can double two or more times every hour under favorable conditions unless drastic sanitation measures are taken. The depletion of nutrients and the accumulation of toxins slow down the growth and start the death period.
The rate of growth of microorganisms in a food item depends on the characteristics of the food itself such as the chemical structure, pH level, presence of in inhibitors and competing microorganisms, and water activity as well as the environmental conditions such as the temperature and relative humidity of the environment and the air motion (Fig. 17–2).
Microorganisms need food to grow and multiply, and their nutritional needs are readily provided by the carbohydrates, proteins, minerals, and vitamins in a food. Different types of microorganisms have different nutritional needs, and the types of nutrients in a food determine the types of microorganisms thatmay dwell on them. The preservatives added to the food may also inhibit the growth of certain microorganisms. Different kinds of microorganisms that exist compete for the same food supply, and thus composition of microorganisms in a food at any time depends on the initial make-up of the microorganisms.
All living organisms need water to grow, and microorganisms cannot grow in foods that are not sufficiently moist. Microbiological growth in refrigerated foods such as fresh fruits, vegetables, and meats starts at the exposed surfaces where contamination is most likely to occur. A fresh meat package left in a room will spoil quickly, as you may have noticed. A meat carcass hung in a controlled environment, on the other hand, will age healthily as a result of dehydration on the outer surface, which inhibits microbiological growth there and protects the carcass.
Microorganism growth in a food item is governed by the combined effects effects of the characteristics of the food and the environmental factors.
We cannot do much about the characteristics of the food, but we certainly can alter the environmental conditions to more desirable levels through heating, cooling, ventilating, humidification, dehumidification, and control of the oxygen levels. The growth rate of microorganisms in foods is a strong function of temperature, and temperature control is the single most effective mechanism for controlling the growth rate.
Microorganisms grow best at “warm” temperatures, usually between 20 and 60 C. The growth rate declines at high temperatures, and death occurs at still higher temperatures, usually above 70 C for most microorganisms. Cooling is an effective and practical way of reducing the growth rate of microorganisms and thus extending the shelf life of perishable foods. A temperature of 4 C or lower is considered to be a safe refrigeration temperature. Sometimes a small increase in refrigeration temperature may cause a large increase in the growth rate, and a considerable decrease in shelf-life of the food (Fig. 17–3). The growth rate of some microorganisms, for example, doubles for each 3 C rise in temperature.
Another factor that affects microbiological growth and transmission is the relative humidity of the environment, which is a measure of the water content of the air. High humidity in cold rooms should be avoided since condensation that forms on the walls and ceiling creates the proper environment for mold growth and buildups. The drip of contaminated condensate onto food products in the room poses a potential health hazard.
Different microorganisms react differently to the presence of oxygen in the environment. Some microorganisms such as molds require oxygen for growth, while some others cannot grow in the presence of oxygen. Some grow best in low-oxygen environments, while others grow in environments regardless of the amount of oxygen. Therefore, the growth of certain microorganisms can be controlled by controlling the amount of oxygen in the environment. For example, vacuum packaging inhibits the growth of micro-organisms that require oxygen. Also, the storage life of some fruits can be extended by reducing the oxygen level in the storage room.
Microorganisms in food products can be controlled by (1) preventing contamination by following strict sanitation practices, (2) inhibiting growth by altering the environmental conditions, and (3) destroying the organisms by heat treatment or chemicals. The best way to minimize contamination in food processing areas is to use fine air filters in ventilation systems to capture the dust particles that transport the bacteria in the air. Of course, the filters must remain dry since microorganisms can grow in wet filters. Also, the ventilation system must maintain a positive pressure in the food processing areas to prevent any airborne contaminants from entering inside by infiltration. The elimination of condensation on the walls and the ceiling of the facility and the diversion of plumbing condensation drip pans of refrigerators to the drain system are two other preventive measures against contamination. Drip systems must be cleaned regularly to prevent microbiological growth in them. Also, any contact between raw and cooked food products should be minimized, and cooked products must be stored in rooms with positive pressures. Frozen foods must be kept at 18 C or below, and utmost care should be exercised when food products are packaged after they are frozen to avoid contamination during packaging.
The growth of microorganisms is best controlled by keeping the temperature and relative humidity of the environment in the desirable range. Keeping the relative humidity below 60 percent, for example, prevents the growth of all microorganisms on the surfaces. Microorganisms can be destroyed by heating the food product to high temperatures (usually above 70 C), by treating them with chemicals, or by exposing them to ultraviolet light or solar radiation.
Distinction should be made between survival and growth of microorganisms. A particular microorganism that may not grow at some low temperature may be able to survive at that temperature for a very long time (Fig. 17–4). Therefore, freezing is not an effective way of killing microorganisms. In fact, some microorganism cultures are preserved by freezing them at very low temperatures. The rate of freezing is also an important consideration in the refrigeration of foods since some microorganisms adapt to low temperatures and grow at those temperatures when the cooling rate is very low.
2 REFRIGERATION AND FREEZING OF FOODS
The storage life of fresh perishable foods such as meats, fish, vegetables, and fruits can be extended by several days by storing them at temperatures just above freezing, usually between 1 and 4 C. The storage life of foods can be extended by several months by freezing and storing them at subfreezing temperatures, usually between 18 and 35 C, depending on the particular food (Fig. 17–5).
Refrigeration slows down the chemical and biological processes in foods, and the accompanying deterioration and loss of quality and nutrients. Sweet corn, for example, may lose half of its initial sugar content in one day at 21 C, but only 5 percent of it at 0 C. Fresh asparagus may lose 50 percent of its vitamin C content in one day at 20 C, but in 12 days at 0 C. Refrigeration also extends the shelf life of products. The first appearance of unsightly yellowing of broccoli, for example, may be delayed by three or more days by refrigeration.
Early attempts to freeze food items resulted in poor-quality products because of the large ice crystals that formed. It was determined that the rate of freezing has a major effect on the size of ice crystals and the quality, texture, and nutritional and sensory properties of many foods. During slow freezing, ice crystals can grow to a large size, where as during fast freezing a large number of ice crystals start forming at once and are much smaller in size. Large ice crystals are not desirable since they can puncture the walls the cells, causing a degradation of texture and a loss of natural juices during thawing. A crust forms rapidly on the outer layer of the product and seals in the juices, aromatics, and flavoring agents. The product quality is also affected adversely by temperature fluctuations of the storage room.
The ordinary refrigeration of foods involves cooling only without any phase change. The freezing of foods, on the other hand, involves three stages: cooling to the freezing point (removing the sensible heat), freezing (removing the latent heat), and further cooling to the desired subfreezing temperature (removing the sensible heat of frozen food), as shown in Figure 17–6.
Fresh fruits and vegetables are live products, and thus they continue giving off heat that adds to the refrigeration load of the cold storage room. The storage life of fruits and vegetables can be extended greatly by removing the field heat and cooling as soon after harvesting as possible. The optimum storage temperature of most fruits and vegetables is about 0.5 to 1 C above their freezing point. But this is not the case for some fruits and vegetables such as bananas and cucumbers that experience undesirable physiological changes, when exposed to low (but still above-freezing) temperatures, usually between 0 and 10 C. The resulting tissue damage is called the chilling injury and is characterized by internal discoloration, soft scald, skin blemishes, soggy breakdown, and failure to ripen. The severeness of the chilling injury depends on both the temperature and the length of storage at that temperature. The lower the temperature, the greater the damage in a given time. Therefore, products susceptible to chilling injury must be stored at higher temperatures. A list of vegetables susceptible to chilling injury and the lowest safe storage temperature are given in Table 17–1.
Chilling injurydiffers from freezing injury,which is causedbyprolonged exposure of the fruits and vegetables to subfreezing temperatures and thus the actual freezing at the affected areas.Thefreezing injury is characterizedbyrubbery texture, browning, bruising, and drying due to rapid moisture loss. The freezing points of fruits and vegetables do not differ bymuch, but their susceptibility to freezing injury differs greatly. Some vegetables are frozen and thawed several times with no significant damage, but others such as tomatoes suffer severe tissue injury and are ruined after one freezing. Products near the refrigerator coils or at the bottom layers of refrigerator cars and trucks are most susceptible to freezing injury. To avoid freezing injury, the rail cars or trucks should be heated during transportation in sub-freezing weather, and adequate air circulation must be provided in cold storage rooms. Damage also occurs during thawing if it is done too fast. It is recommended that thawing be done at 4 C.
Dehydration, or moisture loss, causes a product to shrivel or wrinkle and lose quality. Therefore, proper measures must be taken during cold storage of food items to minimize moisture loss, which also represents a direct loss of the salable amount. A fruit or vegetable that loses 5 percent moisture, for example, will weigh 5 percent less and will probably be sold at a lower unit price because of loss of quality.
The loss of moisture from fresh fruits and vegetables is also called transpiration. The amount of moisture lost from a fruit or vegetable per unit mass of the fruit or vegetable per unit time is called the transpiration rate. The transpiration rate varies with the environmental conditions such as the temperature, relative humidity, and air motion. Also, the transpiration rate is different for different fruits and vegetables. The tendency of a fruit or vegetable to transpire is characterized by the transpiration coefficient, which is the rate of transpiration per unit environmental vapor pressure deficit. The transpiration coefficient of apples, for example, is 58 ng/s · Pa · kg, whereas it is 1648 ng/s · Pa · kg for carrots and 8750 ng/s · Pa · kg for lettuce. This explains why the lettuce dehydrates quickly while the apples in the same environment maintain their fresh appearance for days.
Moisture loss can be minimized by (1) keeping the storage temperature of food as lowas possible, (2) keeping the relative humidity of the storage room as high as possible, and (3) avoiding high air velocities (Fig. 17–7). However, air must be circulated continuously throughout the refrigerated storage room to keep it at a uniform temperature. To maintain high quality and product consistency, temperature swings of more than 1 C above or below the desired temperature in the storage room must be avoided. Air motion also minimizes the growth of molds on the surfaces of the wrapped or unwrapped food items and on the walls and ceiling of the storage room.
Waxing reduces moisture loss and thus slows down shriveling and maintains crispness in some products such as cucumbers, mature green tomatoes, peppers, and turnips. Waxing also gives the products an attractive glossy appearance. But a wax coating that is too thick may actually increase decay, especially when no fungicides are used.
Refrigeration is not necessary for all food items. For example, canned foods that are heat processed can be stored at room temperature for a few months without any noticeable change in flavor, color, texture, and nutritional value. Refrigeration should be considered for the storage of canned foods longer than two or three months to preserve quality and to avoid corrosion of the cans. Dry foods can last a long time, often more than a year, without refrigeration if they are protected against high temperatures and humidities. Dry foods that have been vacuum packed in water vapor–proof containers can maintain high quality and nutritional value for a long time. Honey can be stored at room temperature for about a year before any noticeable darkening or loss of flavor occurs. Cold storage below 10 C will extend the life of honey for several years. Storage of honey between 10 and 18 C is highly undesirable as it causes granulation.
The use of refrigeration is not limited to food items. It is commonly used in chemical and process industries to separate gases and solutions, to remove the heat of reaction, and to control pressure by maintaining low temperature. It is also used commonly in the beverage industry, in medicine, and even in the storage of furs and garments. Furs and wool products are commonly stored at 1 to 4 C to protect them against insect damage.
During cooling or freezing, heat is removed from the food usually by the combined mechanisms of convection, radiation, and evaporation, and the rate of heat transfer between the food and the surrounding medium at any time can be expressed as (Fig. 17–8)
h = average heat transfer coefficient for combined convection, radiation,
and evaporation, W/m2 · C
As = exposed surface area of the food, m2
Tsurface = surface temperature of the food, C
Tambient = temperature of the refrigerated fluid (air, brine, etc.) away from
the food surface, C
The heat transfer coefficient h is not a property of the food or refrigerated fluid. Its value depends on the shape of the food, the surface roughness, the type of cooling fluid, the velocity of the fluid, and the flow regime. The heat transfer coefficient is usually determined experimentally and is expressed in terms of the Reynolds and Prandtl numbers. Some experimentally determined values of the heat transfer coefficient are given in Table 17–2. The values of h include convection as well as other effects such as radiation and evaporative cooling.
