1. –15 to –29 C for 30 min 1. –15 to –29 C for 3-4 hrs till 72 hrs
2. No food damage 2. Damage of food by crystal formation.
3. Done by: 3. Done by the natural air circulation or
a. Direct immersion of food / package in through electrical fans.
Refrigerant. E.g.: fish in brine
b. Indirect contact (-17.8 to –45.6 c)
c. Air blast freezing (-17.8 to –34.4C)
Rigid air is blown.
Advantage of quick freezing:
1. Shorter period.
2. Prompt prevention of microbial growth.
3. Rapid slowing of enzyme action.
Dehydrofreezing: fruits and vegetables have about half there moisture removed before freezing.
Changes during freezing:
1. Expansion in volume of food.
2. Ice crystals formation may crush cells.
3. Frozen condition chemical and enzymatic reaction proceed slowly.
E.g.: meat, poultry, fish products, proteins may irreversibly dehydrated.
Meat -> red myoglobin ------------- brown metmyoglobin
Fats (meat, fish) ----- oxidized and hydrolysed
4. Metacryotic liquid:
Unfrozen, concentrated solutions of sugars, salts may ooze from packages of fruits or concentrates during storage as a viruses material.
5. Fluctuation in temperature results in ice crystal formation.
6. Deracination may occur.
7. Freeze burn :
when ice – crystals evaporate from the area at the surface this defect is observed. The spot appears dry, grainy and brownish, tissues become dry and tough.
E.g.: fruits, vegetables, meat, poultry and fish.
8. During freezing vegetative cells die soon but some may remain for a longer period of time.
Changes during thawing:
1. Drip / bleeding:
The pink or reddish liquid that comes from meat during thawing.
The liquid oozing out of fruits or vegetables on thawing.
3. The wilting and flabiness of physical damage during freezing.
4. Thawing refers to sudden heating and sudden cooling. The damage of food is due to the freezing and storage but do not become evident earlier. Some of the liquid during thawing may be reabsorbed by the food particles or may remain as such.
If the thawed fleshed foods are below 3.3 C can be used but otherwise food should be discarded.
Effects of freezing:
1. Lethal effects:
Rapid cooling of cells from optimal to 00 c may also result in death and referred to as cold shock, where there is change in lipid membrane damage the permeability of cell or to the release of repair enzyme inhibitors. E.g.: ribonuclease inhibitors
2. Sub – lethal effects:
During enumeration of frozen food there may be reduction but not tree death of organisms. Some may be injured or damaged are called as freeze – injured, frost injured or metabolically injured. Freezing of micro organisms in a food may result in cryoinjuiry.
Response of microorganisms to freezing:
Freezing depends on type of microorganisms usually found in foods involved in preservation. There are various factors involving freezing.
1. On the basis of sensitivity of microorganisms during freezing they can be classified as 3 different groups:
a. Susceptible or sensitive => e.g.: yeast, mould, gram-negative bacteria, and vegetative cells.
b. Moderately resistance => e.g. Staphylococcus, Enterococcus, gram-positive bacteria.
c. Resistant => e.g. Spore forming organisms.
2. Freezing also depends on the freezing rate. Critical range of temperature lead to death of microbes than during rapid freezing.
3. It also depends on the kind of food normally used for presentation. The food used for preservation by freezing usually gets spoiled due to
a. High moisture content.
b. Availability of O2
c. Salt and sugary environment.
4. Freezing also depends on the change in PH or altered acidity or alkalinity in food.
5. During freezing there is increase in moisture content and formation of intracellular crystals. This usually results in altered permeability in membrane and cell wall. Thus results in osmotic imbalance or osmotic shock favoring cell lyses. Intracellular lie crystals are harmful to cells than extra cellular ice crystals.
6. The initial killing rate during freezing is rapid, but it is followed by a gradual reduction of microorganisms are referred as storage death.
PRESERVATION BY USE OF DRYING
Drying is referred to as the removal of water or lowers the water activity or reduces the amount of available moisture.
E.g., Dried fish => salt, condensed milk => sweet.
a). Sun Drying Drying of food by exposure to suns rays.
c). Condensed Drying where moisture removal from liquid substances
d). Evaporated Similar to dehydrated.
Before drying moisture %
After drying moisture %
Methods of drying:
1. Solar drying:
Direct sun’s rays
E.g., Raisins, figs, pears, peaches, rice, fish
2. Drying by mechanical dryers:
Passage of heated air to food under controlled RH.
a) Use of KLIN / EVAPORATOR:
They are used in form house
Natural draft from heated air brings drying.
b) Forced draft drying:
Heated air moves across the food usually in tunnels or food moved in conveyor belts through heated air.
c) Spray dried:
Spraying of liquid into a current of dry, heated air.
d). Drum dried:
Passage over a heated drum, with or without vacuum.
3. Freeze drying:
Sublimation of water from frozen food by means of a vacuum and heat.
E.g., Meat, Poultry, seafood’s and fruits.
4. Drying during smoking:
E.g., wool smoke desired flavors and preservative are uses.
Meat 43 – 71 C for few hrs to several days prevents mold growth.
It has HCHO, phenol, cresol, methyl and ethyl esters, ketones etc.
5. Other methods:
Foam – mat drying ->Lipid whipped to foam, dried with warm air, crushed to powder, as is pressure – gun puffing of partially dried foods to give a porous structure facilitate further drying.
Tower Drying ->Dehumified air at 30 C or less.
E.g. Tomato concentrate, milk and potatoes.
Relative humidity of air
velocity of air
time of drying
If all these not accounts may lead to case hardening where rapid, evaporation of moisture from the surface than diffusion from the interior leads to hard, horny, impenetratable surface film that hinders further drying.
Milk -> Pure from udder in low, may be contaminated by handlers, process, and equipments.
Meat/ Poultry -> Due to soil, intestinal activity, handlers, equipments.
Fish -> By intestinal activity, surface slime, and handlers.
Egg -> Handlers, equipments, hatched hen and soil.
a. Elimination of spoiled foods.
Rejection of cracked, dirty foods.
Sorting for size, maturity and soundness.
Especially fruits and vegetables. These procedures are followed to remove soil and adhering materials and removes microbes. Water must be pure as it may also acts as a source of contaminate if poor quality of water is used.
E.g., Egg -> Moisture helps the bacteria to penetrate the shell.
May be done by hand, machine, lye bath or abrasion. It reduce the number of microorganisms are on the surface.
4. Sub division:
Slicing, cutting should not increase number of organisms but will do so if equipment is not adequately cleansed and sanitized
Sulfuring of light colored fruits and certain vegetables.
Fruits -> 1000 – 3000 ppm of SO2 gas
Vegetables -> dipping after blanching or spraying of sulfite solution.
Helps to maintain an attractive light color, conserve vit C, vit A, repels insect, kills many microorganisms.
Storage in boxes or tins. It is for equalization of moisture or addition of moisture to a desired level.
E.g., Dehydration of meat at 60 C -> leads to growth of Staph. aureus ., so that 1000C applicable.
Packed the foods after drying for protection against moisture contamination with microbes, insects.
Fruits usually during package -> 30 to 70 min – time, 70 to 100% - RH, 65.6 to 850C – Temperature.
Microbiology of dried foods:
Dried fruits: Mold spores may be seen.
Dried vegetables: Few 100’s per gram to million of organisms due to the improper pretreatment. E.g.: Bacillus, Micrococcus, Clostridium, E.coli, Enterobacter, Pseudomonas, Streptococci and Lactobacillus, Leuconostoc.
1. Commercially prepared foods haves 20-40% moisture and are non-refrigerated shelf stability are IMF.
2. They have reduced water activity.
E.g.: Candies, Jams, jellies, honey, bakery items etc.
3. Aw may be 0.75 and 0.85 for IMF.
4. They can be adjusted by the addition of sugar, salt or glycerols.
4. WRITE SHORT NOTES ON PRESERVATION OF FOODS BY FOOD ADDITIVES. PRESERVATION OF FOODS BY FOOD ADDITIVES
INTRODUCTION A food additive is a substance or mixture of substances, other than the basic food stuff, is present in food as a result of any aspect of production, processing, storage or packaging.
The definition emphasizes one interpretation of a food additive, i.e.; it is an intentional additive. There food additives are specifically added to prevent the deterioration or decomposition of a food have been referred to as chemical preservatives.
This decomposition may be caused by micro organisms, by food enzymes, or by purely chemical reactions. The inhibition of the growth and activity of micro organisms is one of the main purposes of the use of chemical preservatives
Preservatives may inhibit micro organisms by interfering with their cell membranes, their enzymes activity or their genetic mechanisms.
Factors that influence the effectiveness of chemical preservatives in killing micro organisms or inhibiting their growth. Concentration of the chemical
Kind, number, age & previous history of the organism
The chemical & physical characteristics of the substrate in which the organism is found.
The ideal antimicrobial preservative:
A chemical preservative should have a wide range of antimicrobial
Should be nontoxic to human being or animals
Should be economical
Should not have an effect on the flavor, taste or aroma of the original food
Should not be inactivated by the food or any substance in the food
Should encourage the development of resistant strains