FNH 200 - Preservation by low temperatures

Below ___ (temperature), there is no significant growth of spoilage or pathogenic organisms and there is a gradual decrease in numbers of living organisms.

- 9.5

Describe air blast freezing

- moderately fast freeze because of vigorous circulation of cold air - the product is placed on trays or mesh belts and passed slowly through an insulated tunnel - may range from -18 to -34 oC - operate at lower temperatures than still air freezers and rely on movement of the cold air at high velocity over the food in order to achieve rapid removal of heat and to maximize the freezing rate

Describe air freezing and name three kinds

- oldest and most common 1. still-air "sharp" freezer - eg. of still air freezers or low air velocity systems: freezer section of household refrigerator 2. air blast freezer 3. fluidized bed freezer (IQF)

Why is there "graininess" in ice cream stored in your freezer?

- partial melting of the ice cream during temperature fluctuations that result from defrost cycles --> leads to foam destabilization and crystallization of lactose as a consequence of CONCENTRATION EFFECTS

Controlled atmosphere

- refers to a condition in which 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. Recall: controlled atmosphere storage facilities (Lesson 1 ) - a non-cheical process where the atmosphere (CO2, N2, O2), temperature (kept low) and humidity are carefully controlled to slow the rate of respiration and ripening of apples and extending the storage life

Modified atmosphere

- refers to the creation of atmospheric conditions around the product that are different from the normal atmosphere. - MAP: commonly used with some refrigerated products to extend shelf life

What are the three transitions that occur during freezing and thawing curves?

- removal of heat from the product (sensible heat) - freezing - freezing of water (liquid) into ice crystals (solid) (latent heat of fusion or crystallization) - further cooling to the surrounding temperature

What are the packaging requirements for frozen foods?

- resistant to transfer of water vapour from the food to the dry environment within the freezing unit - must not shatter in the cold temperatures encountered in the frozen storage - resist formation of pinholes during normal handling -- breaches in the packaging material will promote the development of freezer burn on the exposed areas of food - depending on the food, the packaging material may have to possess barrier properties toward light and/or oxygen

Freezer burn (and why it occurs)

- the formation of package ice and freeze dehydration - looks burnt because of the moisture loss that occurs during this freeze dehydration

Describe fluidized bed freezer

- used for individually quick frozen shrimp or prawns, garden peas - solid food particles are exposed through a movement of cold air at high velocity as they pass along a conveyor belt --> this imparts a vibratory motion to food particles, accelerating the freezing rate - cold air being forced upward through the bed lifts and suspends the food particles, thus fluidization occurs = rapid freezing rate is accomplished and and IQF product is produced (not stuck together)

Describe slush freezers (aka scraped surface heat exchangers)

- used only for fluid food products - for ice cream, rotator aids in promoting rapid freezing and development of small crystals and incorporation of air bubbles into the freezing mix which results in the formation of a solid foam

Commercial freezing requires a minimum temperature of ___.

-18 oC

What is the common final storage temperature?

-18 oC is the storage temperature where optimum storage life is usually obtained

Some spoilage-causing microorganisms, particularly moulds, can grow at temperatures as low as ___.

-8oC

What are the 4 factors affecting freezing rate?

1. Temperature difference; the greater the temperature difference between the food and the refrigerant, the faster the freezing rate. 2. Product thickness/geometry and heat transfer rate: the thinner the food piece or greater the heat transfer rate, the faster the freezing rate 3. Air velocity: the greater the velocity of refrigerated air or circulating refrigerant, the faster the freezing rate 4. Degree of contact: the more contact between the food and the cooling medium, the faster the freezing rate

What are the three basic methods of freezing foods?

1. air freezing 2. indirect contact freezing 3. immersion and cyrogenic freezing

What are three ways to minimize changes in food during freezing, frozen storage and thawing?

1. blanching 2. proper temperature control for freezing and frozen storage 3. appropriate packaging

The basis for preservation by freezing/frozen storage is that freezing permits longer term storage than refrigerated storage due to (2 reasons):

1. lower temperatures used (recall that microorganisms can not grow well at temperatures below -9.5oC) 2. lower water activity (by freezing the "free" water preset in the food) - both factors slow down chemical and enzymatic reactions and microbial growth

The changes of the atmosphere composition in a modified atmosphere depends on three things:

1. metabolic activities of the food, 2. microorganisms in the food, 3. gas permeability of the packaging material used

What are the 4 most common factors affecting the quality of frozen foods?

1. rate of freezing 2. final storage temperature 3. stability of storage temperature 4. rate of thawing

Refrigeration/cool storage of foods falls between ___ to ___. However in the food industry, the refrigeration temperatures should be in the range of ___ and below.

16 to -2 oC; 4oC

The Food and Drug Regulations of Canada state that perishable foods can support the growth of disease-causing microorganisms must be stored at temperatures of ___ and below.

4oC

Why is the freezing point of foods somewhat below 0oC (the freezing point of pure 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.

What are the rates of deterioration of frozen foods are governed by? (2)

Chemical composition and physical structure of the foods

Concentration effects

Concentration of components in the unfrozen water phase within the frozen foods promotes various chemical, enzymatic and physical changes: - chemical changes such as oxidative rancidity or oxidation of flavour components, pigments, and vitamins (recall: oxidative rancidity: reaction of oxygen with fats, esp. high unsaturated, fatty acids) - enzymatic reactions such as enzymatic browning or lipolytic rancidity (recall: lipolytic rancidity: induced in foods upon release of free fatty acids by very high temps or lipase enzymes) - meats become tougher due to protein denaturation by chemical effects and cell breakage by ice crystals

What are the optimal conditions for refrigerated storage of foods (i.e. what factors should be considered).

Each food commodity has its optimum refrigeration conditions for maximum storage life and retention of quality and nutritional value - TEMPERATURE: maintenance of CONTROLLED temperatures are very important to ensure maximum storage life of the foods and to prevent chill injury to certain foods - HUMIDITY: a 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 - GAS ATMOSPHERE COMPOSITION: many food products are stored under controlled and modified atmospheres in conjunction with refrigerated storage for the extension of storage life

What is destabilized by the growth of numerous small crystals to larger, less numerous but more damaging ice crystals?

Emulsions and other dispersions - growth of large ice crystals are usually caused by temperature fluctuations

True or false: Refrigeration storage of foods can improve the quality of a food item that is low of quality when placed into cold storage

False

True or false: changes that occur in foods during freezing, storage and thawing are only physical in nature

False - they can be both chemical and physical in nature

True or false: water begins to freeze at 0oC

False, although the freezing point of PURE water is 0oC, water does not begin to freeze until it is supercooled to several degrees below 0oC.

True or False: Freezing and frozen completely kill spoilage and pathogenic organisms

False, freezing and frozen storage do not lead to complete destruction and rapid growth and multiplication can occur during thawing.

True/False: freezing refers to temperatures at freezing point of water?

False, it refers to temperatures below freezing point of water. In food industry, a minimum of -18oC is required

True or false: during freezing, physical, chemical and enzymatic changes don't occur

False, they may occur - especially if freezing and frozen storage conditions are not optimal

True or false: food products packed under modified atmospheres have a longer storage life and are okay to be kept at room temperature

False, they need to 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

Describe indirect contact freezing and name the 4 types

Food is placed on belts or trays and a refrigerant circulates through a wall beside the foods. As the food comes into "contact" with the cold wall, it quickly cools down and freezers. - single plate - double plate - pressure plate - slush freezer

Freezing point

Freezing point is 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.

Modified atmosphere packaging (MAP)

In this type of system, food items are placed into a package. The air in the package is 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 of carbon dioxide and nitrogen) before sealing the package.

Once water starts to crystallize, why is there an abrupt rise in temperature?

It is because of the evolution of the latent heat of fusion or crystallization = you get a plateau, as the increasing temp counters the decreasing temp of the surrounding - only after all the water has frozen (crystallized) will the temperature approach the temperature of the freezing environment

True/False: composition of the atmosphere in a modified atmosphere packed food stays constant

No, it changes over time

What is the objective in freezing foods? (hint: formation of ___)

Promote the formation of tiny ice crystals rather than the formation of fewer but larger ice crystals that can cause cellular damage. Ice crystal damage can lead to loss of water from the food product once it is thawed.

What are the pros and cons of indirect contact freezing?

Pros: - economical - minimal dehydration - minimal package bulging Cons: - slow freezing process - products must be of uniform thickness

What are the pros and cons of fluidized bed (IQF) freezing?

Pros: - has more efficient heat transfer, increased rate of freezing Cons: - non-uniform products can not be fluidized (IQF) easily

What are the pros and cons of air freezing?

Pros: - can freeze various sizes and shapes of food - economical Cons: - possible excess dehydration (freezer burn) - undesirable bulging of packages (by expansion of product) may occur

What are the pros and cons of immersion/cryogenic freezing?

Pros: - rapid freezing process - almost no dehydration - oxygen is excluded, decreasing oxidative spoilage - individual freezing pieces have less freezing damage Cons: - difficult to find suitable freezants - operating costs is high

Psychrotrophic

Psychrotrophs have adapted to living and multiplying in cold temperatures. They are the major cause of spoilage in refrigerated foods

Why does drip formation occur?

Rupture of cell walls and membranes during freezing and thawing can lead to formation of drip when the product is thawed. - due in part to ice crystal damage to cells, leading to the leakage of cellular fluids into extracellular spaces, and to the loss of water-holding capacity of the food components as a result of concentration effects.

How are large ice crystals formed?

Small fluctuations in storage temperature can cause melting of small ice crystals with subsequent refreezing of the liquid water on to 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. i.e. the intermittent thawing and freezing when stored = can be because of "temperature abuse"

The final temperature for storage of frozen foods is dictated by a number of factors. Name 2.

Texture changes, chemical reactions

How much water remains unfrozen?

This depends both on the temperature and food product iself.

True or False: Refrigeration is one of the mildest approaches to food preservation

True

True or false: frozen foods are generally of higher nutritional and aesthetic quality than thermally processed food.

True

True or false: refrigeration is also sometimes applied to achieve desirable attributes of fermented food products such as cheeses, beef and wine upon cool ripening or aging

True

True or false: when freezing of food is properly done, it can preserve the quality of the food without causing major changes in appearance, texture and flavour

True

True or false: Freezing slows/stops microbial growth

True - however storage of food at freezing temperatures does not kill all microorganisms and many disease causing and spoilage causing microorganisms can survive in frozen foods for many years (eg. 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

True or false: water is frozen out as pure water

True. Until all of the water is frozen, there is an ever-increasing concentration of water-soluble solutes in the unfrozen phase. In fact, there is a considerable proportion of unfrozen water at temperatures below 0oC.

What are examples of MAP?

Vacuum packaged processed or cured meats - except in this case after a vacuum is drawn once the product is put in the package, 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 the aerobic spoilage-causing bacteria = extend storage life Example: cheese are often vacuum-packed or under gas mixtures to prevent the growth of moulds which are common aerobic spoilage-causing microorganisms of cheeses

Refrigeration or cool storage generally refers to storage at temperatures ___ freezing, while freezing and frozen storage occur at temperatures ___ freezing.

above; below

Describe cryogenic freezing

accomplished with cryogenic liquids, with liquid nitrogen being the most commonly used - a very rapid freezing method in which un-packaged or thinly packaged foods are exposed to extremely cold freezant - in comparison to liquid immersion freezing, heat removal is accomplished during a CHANGE OF STATE by the FREEZANT (it boiling) - accomplishes an excellent retention of quality imparted by the rapid rate of freezing and small ice crystal formation

Refrigerated or cool storage generally will preserve most perishable foods for ___ while good frozen storages with proper packaging can preserve foods for ___.

days or weeks; months or years

A drop of 10oC ___ (increases or decreases) rate of senescence (biological aging) by 2-3 times and microbial growth by 3-6 times

decreases

Refrigeration temperatures inhibit the growth of most ___ (spoilage or disease-causing) microorganisms but can favour the psychrotrophic microorganisms

disease-causing

A ___ (slower/faster) thawing rate, allows for achievement for maximum quality retention

faster

A ___ (slower/faster) the freezer rate promotes the formation of many small ice crystals

faster

The ___ (slower or faster) the freezing rate, the better the retention of sensory and nutritional quality

faster

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.

Some spoilage organisms can grow at temperatures below 0oC providing that there is ___ ___ available.

frozen wwater

Ice is a ___ (bad/good) conductor of heat

good - has a high thermal diffusivity = the temperature of a frozen food rapidly approaches the melting point of ice. But, after the rapid temperature increase, subsequent increases in temperature occur very slowly because of the need to supply the latent heat of fusion for the conversion of water from the crystalline state to the liquid state at 0oC.

Describe immersion freezing

immersion of packaged or unpackaged food products directly in a non-toxic refrigerant fluid. ice cream popsicles are freezed this way

As the temperature of frozen foods increases, the amount of unfrozen water ___ (decreases or increases).

increases

Pure water freezes at 0oC, but most foods have a freezing temperature ___ (lower or higher) than that

lower; most foods do not have begin to freeze until a temperature of -2oC or lower is reached

Some ___ pathogens can grow, although slowly, at refrigerator temperatures.

psychrotrophic

Refrigeration storage temperatures provide for ___ term extensions of storage life of foods

short; this is because psychrotrophic organisms can still grow

Latent heat

the quantity of heat required to change the state or condition under which a substance exists, without changing its temperature

If you buy an IQF product - and you find that it is present in a solid block, what is likely to have occurred?

undergone partial thawing and refreezing during storage, distribution, retailing or on the way home from the retail store to your home

Cyrogenic

very low temperatures

How is a solid foam created?

when gas bubbles are dispersed within a solid phase (eg. icecream)