Preservation of Foods by Low Temperature Flashcards
What temperature do most foods begin to freeze?
Refrigeration or cool storage generally refers to storage at temperatures above freezing, while 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.
How long will refrigerated or cool storage preserve most perishable foods for?
Refrigerated or cool storage generally will preserve most perishable foods for days or weeks, while good frozen storage conditions with proper packaging can preserve foods for months or even years.
At what temperature is there no significant growth of spoilage or pathogenic organisms?
Below -9.5°C, there is no significant growth of spoilage or pathogenic organisms, and there is a gradual decrease in numbers of living organisms.
However, freezing and frozen storage do not lead to complete destruction, and rapid growth and multiplication can occur during thawing.
What is the most mild approach to food preservation?
Refrigeration
What are refrigeration temperatures?
Refrigeration/Cool storage of foods falls between -2°C to 16°C. However, in the food industry, the refrigeration temperatures should be in the range of 4°C and below.
The Food and Drug Regulations of Canada state that perishable foods that can support the growth of disease-causing microorganisms must be stored at temperatures of 4°C and below.
Does refrigeration prevent against microorganisms?
A drop of 10°C slows down rate of senescence by 2 to 3 times and microbial growth by 3 to 6 times.
Refrigeration temperatures inhibit the growth of most disease-causing microorganisms but can favour the psychrotrophic microorganisms.
Some spoilage-causing microorganisms, particularly moulds, can grow at temperatures as low as -8°C.
When is refrigeration used in fermented food products?
In addition to being a mild approach to short-term preservation, refrigeration is also sometimes applied to achieve desirable attributes of fermented food products such as cheeses, beef and wine upon cool ripening or aging.
How is the storage life of meats, fish, and dairy products maximized?
Each food commodity has its optimum refrigeration conditions for maximum storage life and retention of quality and nutritional value.
The storage life of meats, fish and dairy products are maximized by temperatures that approach 0°C.
Describe some ways that refrigeration could negatively affect some fruits and vegetables.
Bananas will undergo a colour change in the peel from yellow to black, while sweet potatoes may show decay, pitting and internal discolorations, when stored at temperatures below 13°C.
Apples may become soggy or show internal browning if stored below 1-2°C.
Potatoes and avocados may also become brown if stored below 5 and 7°C, respectively.
What are other factors to be controlled for products in refrigerated storage?
Humidity and gas atmosphere composition.
A very fine humidity balance must be maintained to prevent dehydration of the food while avoiding creation of conditions that are so humid that mould growth and food spoilage are favoured.
Many food products are stored under controlled and modified atmospheres in conjunction with refrigerated storage for the extension of storage life.
What does a controlled atmosphere refer to?
A 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 of the atmosphere around the product is constantly monitored and maintained at preset levels.
“controlled atmosphere storage”- facilities where the atmosphere (CO2, O2 and N2) and humidity are carefully controlled and temperature kept low to slow the rate of respiration and ripening of the apples, thus extending the storage life of the fresh fruit.
What does modified atmosphere refer to?
Modified atmosphere refers to the creation of atmospheric conditions around the product that are different from the normal atmosphere.
In this type of system, food items are placed into a package.
The air in the package is then removed either by drawing a vacuum, then backflushing the package with the desired gas mixture before sealing the package, or simply by flushing the package with the desired gas mixture until the air in the package is replaced by the desired gas mixture (usually a combination ofcarbon dioxide and nitrogen) before sealing the package.
Why does the composition of the atmosphere in a modified atmosphere packed food product change over time?
The changes being governed by metabolic activities of the food, the microorganisms in the food, and the gas permeability of the packaging materials used.
Describe the benefit of vacuum packaged meats and cheeses.
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 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.
Retail cuts of cheese are often packed under vacuum or under gas mixtures to prevent the growth of moulds which are common aerobic spoilage-causing microorganisms of cheeses.
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 inhibition of growth of spoilage-causing microorganisms and to prevent the growth of microorganisms that may be capable of causing illness.
Over prolonged storage, numerous undesirable changes can be observed, in addition to those brought on by microbial spoilage. Give examples.
- staling of bread
- loss of crispness in fruits and vegetables
- change in colour of fresh meat
- loss of flavour and nutrient value (e.g. vitamins)
- oxidative changes
- drip or syneresis from fish
What are low temperature processing and materials designed for?
Low temperature processing and its packaging materials are designed to extend the food’s shelf life by slowing down microbial growth and chemical/enzymatic reactions.
What happens to microbial growth when the food begins to thaw?
Microbial growth and chemical/enzymatic reactions will resume once the food is thawed or exposed to “warmer” temperatures.
What temperature does cool storage refer to?
Refrigeration (cool storage) refers to temperatures between -2°C to16°C . In particular at 4°C.
This provides only a short term shelf-life extension in food as psychrotrophic organisms can still grow.
Below what temperature is there no significant growth of spoilage or pathogenic organisms?
Below -9.5°C
What temperatures does freezing refer to?
Freezing refers to temperatures below the freezing point of water.
In the food industry, a minimum of -18°C is required. Food is preserved by the use of lower temperatures and lower water activity.
What undesirable changes may happen during freezing?
During freezing, several factors (e.g. freezing rate, final storage temperature, etc) must be controlled in order to prevent undesirable changes (oxidation reactions, freezer burn, ice crystal damage, etc)
Packaging materials should assist in preventing these undesirable changes.
What is the basis for preservation by freezing/frozen storage?
The basis for preservation by freezing/frozen storage is that freezing permits longer term storage than refrigerated storage due to:
- lower temperatures used (remember that microorganisms can not grow well at temperatures below -9.5°C)
- 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.
Describe fluidized-bed freezers.
Garden peas and individually quick frozen shrimp or prawns are frozen in fluidized-bed freezers.
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 being 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.
Describe plate freezing.
During 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.
Explain packaging considerations for frozen products.
Packaging materials to be used for frozen foods must be resistant to the transfer of water vapour from the food to the dry environment within the freezing unit.
The packaging material must not shatter in the cold temperatures encountered in frozen storage.
Therefore glass is not a good material for packaging frozen foods because it tends to shatter and it is not flexible.
The packaging material should resist the formation of pinholes during normal handling.
Breaches in the packaging material will promote the development of freezer burn on the exposed areas of the food.
Depending on the food, the packaging material may have to possess barrier properties toward light and/or oxygen.
