Properties of refrigerants

n    Low boiling point and high latent heat of vaporization

n    Dense vapor

n    Low toxicity

n    Non flammable

n    Low miscibility with oil

n    Low cost

Ammonia

n    Has good heat transfer properties

n    Not miscible with oil

n    But

n    Toxic

n    Flammable

n    Corrosive to copper pipes

Carbon dioxide

n    Non toxic

n    Non flammable

n    But

n    Higher cost

 

Halogens (freons)

n    Non toxic

n    Non flammable

n    Have good heat transfer properties

n    Lower cost

n    But

n    Miscible with oil

n    Widely used despite miscibility

Chilling medium

n    Air

n    Uses forced convection

n    Increases rate of heat transfer

n    Water

n    Direct immersion on chilled water (hydrocooling)

n    Metal

n    Products cooled by contact with chilled metal surface

 

Cryogenic cooling

n    Absorbs heat directly from the product

n    Uses a cryogen as the refrigerant

n    Changes phase by absorption of heat

n    Differs from mech. Refrigeration

n   Changes phase due to change in pressure

n    Most common cryogens are

n   Solid carbon dioxide

n   Liquid carbon

Cook chill systems

n    Used to replace warm holding of foods

n    Cook chill system

n    Foods are prepared normally

n    Chilled to 3oC within 30 min

n    Completely chilled within 90 min

n    Stored at 0 3oC

n    Have a shelf life of 5 days

Cook chill systems

n    Cook pasteurize chill system

n    Hot food is packaged in a flexible container

n    A partial vacuum is formed to remove oxygen

n    Package is heat sealed

n    Pasteurized to a min. of 80oC for 10 min

n    Cooled to 3oC

n    Shelf life of 2 3 weeks

Effects of Refrigeration on eating habits

n    Increased availability of perishable products

n    Increased variety of products available through out the country

Respiration

n    Metabolic activity continues in plant tissue after harvest

n    Aerobic respiration involves

n    Metabolism of CHO and organic acids in the presence of atmospheric oxygen

n    Produces

n   CO2 , water and heat

n   Small amount of organic volatiles and other substances

Respiration

n    Heat of respiration is sufficient to influence refrigeration requirement

n    Rate of quality loss is associated with the rate of respiration

 

Respiration Rates

n    Not all fruits and vegetables respire at the same rate

n    Climacteric respiration

n    Irregularity in rate of respiration

n    Limited to certain fruits

Respiration of climacteric and non-climacteric fruits

Examples

n     Climacteric

n    Apples

n    Avocado

n    Mango

n    Banana

n    Orange

n    Papaya

n    Peach

n    Plum

n     Non climacteric

n    Cherry

n    Cucumber

n    Fig

n    Grape

n    Grapefruit

n    Lemon

n    Melon

n    Strawberry

Controlled and Modified Atmosphere storage

Controlled and Modified Atmosphere storage

n    Air is composed of:

n    78% nitrogen

n    21% oxygen

n    Remainder is CO2 and other gasses

n    By increasing [CO2] or decreasing [O2] can increase shelf life

CAS

n    Useful for crops that

n    ripen after harvest

n    Deteriorate quickly

n    The levels of O2 are decreased and CO2 increased to 21%

n    Total concentration of O2 and CO2 are reduced to 4-5%

 

 

Limitations of CAS

n    Low levels of O2 and high levels of CO2 are toxic to many foods

n    Conditions may increase ethylene

n    May produce off odors and flavors

n    Lead to anaerobic respiration

 

MAS

n    Stored under air tight conditions

n    [O2] decreased down to 0%

n    [CO2] increase up to 20%

n    Limitations are the same as CAS