Module 7 Flashcards
Compare cool versus freezing storage.
- Refrigeration or cool storage generally refers to storage at temperatures above freezing - preserves perishable foods for days/weeks.
- Freezing and frozen storage occur at temperatures below freezing. However, whereas pure water freezes at 0°C, most foods do not begin to freeze until a temperature of -2°C or lower is reached - preserve foods for months or even years.
At what temperature is there no signifcant growth of spoilage or pathogenic organisms?
Below -9.5°C
However, freezing and frozen storage do not lead to complete destruction, and rapid growth and multiplication can occur during thawing.
What is refrigeration?
- One of the mildest approaches to food preservation, falls between -2°C to 16°C, but in the food industry should be in the range of 4 and below.
- A drop of 10C slows the rate of senescence by 2 to 3 times and microbial growth by 3 to 6 times
- Refrigeration temperatures inhibit most disease-causing microbes, but can favour psychrotrophic microorganisms
- Some spoilage-causing microorganisms, particularly moulds, can grow at temperatures as low as -8°C.
- Short term extension to storage life of foods
What are the optimal conditions for refrigerated storage of foods?
- Depends on the food product
- Maintenance of controlled temperatures is very important to ensure maximum storage life and to prevent chill injury.
- Humidity and gas atmosphere composition is often controlled in refrigerated storage as well.
Define: controlled atmosphere.
- Refers to a condition in which the atmosphere surrounding a food product is different from that of the normal atmosphere, and the composition is constantly monitored and maintained at preset levels.
Define: modified atmosphere.
- Refers to the creation of atmospheric conditions around the product that is different from the normal atmosphere. In this type of system, food items are placed into a package. The air in the package is either removed by drawing a vacuum, then backflushing the package with the desired gas mixture before sealing, or simply by flushing the package with the mixture until air is replaced.
- The combination of the atmosphere in a modified atmosphere packaged food product changes over itme, the changes being governed by the metabolic activities of the microbes in the food, and the gas permeability of the packaging materials used.
Keep in mind the fact that although food products packed under modified atmospheres have a longer storage life, those products must be kept in refrigerated storage in order to maximize the benefits of inhibiting the growth of spoilage-causing and pathogenic microorganisms.
Describe vacuum packaged food.
Vacuum packaged processed or cured meats are another example of MAP, except in this case once the vacuum is applied the product is packaged. There is no backflushing with a gas mixture prior to sealing the package. These products have a much longer storage life than those stored in the air. Removal of oxygen from these products through the vacuum process suppresses the growth of the aerobic spoilage-causing bacteria. This leads to the extension of the storage life of these products.
Keep in mind the fact that although food products packed under modified atmospheres have a longer storage life, those products must be kept in refrigerated storage in order to maximize the benefits of inhibiting the growth of spoilage-causing and pathogenic microorganisms.
Over prolonged storage, numerous undesirable changes can be observed, in addition to those brought on by microbial spoilage, which include [6]:
- staling of bread
- loss of crispness in fruits and vegetables
- change in colour of fresh meat
- loss in colour of fresh meat
- loss of flavour and nutrient value (e.g., vitamins)
- oxidative changes
- drip or syneresis from fish
Freezing provides a longer shelf-life for food. Compare commercial and house-hold freezers.
- Commercial freezing requires a minimum of -18°C.
- Household freezers are usually run from -12 to -14°C. Newer models can reach -18°C
What is the basis for preservation by freezing/frozen storage?
Freezing permits long term storage due to:
1. lower temperatures (microbes do not grow well at temperatures below -9.5C)
2. lower water activity (by freezing the ‘free’ water present in the food)
Both factors slow down chemical and enzymatic reactions as well as microbial growth.
Freezing kills microorganisms.
True or False?
False.
Freezing slows/stops microbial growth. However, storage of food at freezing temperatures does not kill all microorganisms and in fact, many disease-causing and spoilage-causing microorganisms can survive in frozen foods for many years (e.g. Listeria monocytogenes). Once the food is thawed, the surviving microorganisms can resume their growth and function, causing disease or spoilage if the proper conditions for microbial growth prevail.
Freezing does not kill microorganisms.
True or False?
True.
Freezing slows/stops microbial growth. However, storage of food at freezing temperatures does not kill all microorganisms and in fact, many disease-causing and spoilage-causing microorganisms can survive in frozen foods for many years (e.g. Listeria monocytogenes). Once the food is thawed, the surviving microorganisms can resume their growth and function, causing disease or spoilage if the proper conditions for microbial growth prevail.
Are frozen or thermally processed foods more nutritional?
Frozen foods are generally of higher nutritional and aesthetic quality than thermally processed foods. The faster the rate of freezing, the better the retention of quality, both from sensory and nutritional perspectives.
Define: freezing point.
The temperature at which ice crystals are in equilibrium with air-saturated water at 1 atmospheric pressure. Solutes in water will depress the freezing point.
Why is the freezing point of food below the freezing point of water?
Because foods contain solutes dissolved in the aqueous (water) phase. The dissolved solutes have the net effect of lowering the freezing point of foods by several degrees Celsius.
Once the water starts to crystallize, there is an abrupt rise in temperature due to the evolution of the latent heat of fusion or crystallization. Only after all the water has frozen (crystallized) will the temperature approach the temperature of the freezing environment.
How much water remains unfrozen in frozen storage?
- Depends on temperature as well as the food product itself.
- For example, for beef, 70% of the water remains in the unfrozen state at -4.0C, compared to 3% and only trace amounts at -9.0 and -18C, respectively.
Describe freezing and thawing curves.
- removal of heat from the product (sensible heat)
- freezing of water (liquid) into ice crystals (solid) (the latent heat of fusion or crystallization)
- further cooling to the surrounding temperature
Describe changes in that occur in foods during freezing, storage, and thawing that can be both chemical and physical in nature.
Various chemical, enzymatic and physical changes are promoted as a result of the concentration of components (concentration effects) in the unfrozen water phase within the frozen foods.
For example:
- Chemical changes such as oxidative rancidity or oxidation of flavour components, pigments and vitamins.
- Enzymatic reactions such as enzymatic browning or lipolytic rancidity.
- Meats become tougher due to protein denaturation by chemical effects and cell breakage by ice crystals
What is the objective in freezing foods?
- To promote the formation of tiny ice crystals rather than the formation of fewer large ice crystsals that cause cellular damage.
- Ice crystal damage can lead to the loss of water from the food product once it is thawed.
- The drip that is found in thawed strawberries or beef is due in part to ice crystal damage to the cells, leading to leakage of cellular fluids into extracellular spaces, and to the loss of water-holding capacity of food components as a result of concentration effects.
- Emulsions and other dispersions are destabilized by the growth of numerous small ice crystals to larger, less numerous but more damaging ice crystals; such growth in ice crystal size is usually caused by temperature fluctuations.
- You may have noted shrinkage and development of graininess in ice cream stored in the frost-free freezer section of your refrigerator. This is due to the partial melting of the ice cream during temperature fluctuations that result from the defrost cycles. This leads to foam destabilization and crystallization of lactose as a consequence of concentration effects.
What is freezer burn?
Other undesirable changes include the formation of package ice and freeze dehydration which is popularly called freezer burn and can produce unsightly food surfaces and loss of nutrients.
“Freezer burn” is a misnomer since the food does not “burn” in the freezer but rather takes on an appearance of having been burnt because of the moisture loss that occurs during this freeze dehydration.
How can we minimize changes in food during freezing, frozen storage, and thawing?
- blanching
- proper temperature control for freezing and frozen storage
- appropriate packaging
What are the 4 most common factors affecting the quality of frozen foods?
- rate of freezing
- final storage temperature
- stability of storage temperature
- rate of thawing
Rapid freezing rates are desirable since the formation of many small ice crystals is favoured. Freezing rates vary depending on [5]
- Food composition - some food components such as proteins and fats act as insulators, slowing down freezing.
- Temperature difference - the greater the difference between the food and the refrigerant, the faster the freezing rate
- Product thickness/geometry and heat transfer rate - the thinner the food piece or greater the heat transfer rate, the faster the freezing rate
- Air velocity - the greater the velocity of refrigerated air or circulating refrigerant, the faster the freezing rate
- Degree of contact - the more contact between the food and the cooling medium, the faster the freezing rate.
The final temperature of frozen foods during storage is dictated by:
- Texture changes
- Chemical reactions
Why is the stability of the storage temperature in frozen foods important?
It is important to note that as the temperature of the frozen food increases, the amount of unfrozen water increases. Thus small fluctuations in storage temperature can cause melting of small ice crystals with subsequent refreezing of the liquid water onto other small ice crystals as the temperature drops, leading to the formation of fewer but larger ice crystals which can produce negative changes in the food quality.
Garden peas and corn kernels are frozen individually at the vegetable processing plant. When the package is picked up the contents should feel like a bag of marbles. However, often when we purchase frozen peas in the retail store, those peas/corns are presented as a solid block.
Why does this change in physical appearance occur?
- The intermittent thawing and refreezing that takes place in the freezer display cabinets that are commonly used in retail stores are the main cause.
- As the peas partially defrost, the layer of water on the outside of each pea melts and as the temperature of peas then decreases again, the melted layer of water on adjoining peas freezes, thus causing the peas to stick together.
- One way of telling whether frozen foods such as garden peas have been handled properly throughout the distribution and retail chains is to determine whether the peas are frozen together in a large mass.
- If they are, temperature abuse has occurred at some point in the handling systems.
Describe individually quick frozen foods.
- In this type of air freezing, solid particles ranging in size from peas to strawberries are being exposed through a movement of the cold air (-20 to -34°C) at high velocity as they pass along a conveyor belt.
- This will impart a vibratory motion to food particles, accelerating the freezing rate.
- The cold air is forced upward through the bed lifts and suspends the food particles, thus fluidization occurs. In this way, a rapid freezing rate is accomplished and an IQF (individually quick frozen) product is produced.
- In other words, food items are frozen as individual pieces and are not stuck together.
- IQF products are packaged after freezing.
Describe plate freezing.
- Food products are placed in contact with a metal surface which is cooled by a cold brine, or a vaporizer refrigerant such as ammonia.
- The packaged food either rests on, slides against or is pressed between the cold metal plates.
- These plates maintain firm contact with two major surfaces of packages to facilitate heat transfer and prevent bulging of the packages during the freezing process.
- Fish sticks and frozen fish fillets are commonly frozen in plate contact freezers
Describe cryogenic freezing.
- Cryogenic freezing is accomplished with cryogenic liquids, with liquid nitrogen being the most commonly used.
- This is a very rapid freezing method in which un-packaged or thinly packaged foods are exposed to extremely cold freezant.
- In contrast to the liquid immersion freezing, heat removal is accomplished during a change of state by the freezant.
- Products such as TV dinners, preformed hamburger patties and other high-value food products are frozen in cryogenic freezers because of the excellent retention of quality imparted by the rapid rate of freezing and small ice crystal formation.
