Food preservation methods 1-Physical methods 2-Chemical methods 3-Biopreservation

Food processingAll the operations by which raw foodstuffs (animal

and plant tissue) converted into forms that

• will not spoil as quickly as the fresh, whole foods (raw materials) from which they were made

• is convenient and practical to consume.

• includes basic preparation of food, alteration of a food product into another form and preservation and packaging techniques.

Why foods are processed?

• to reduce or eliminate harmful microbes from growing in foods so that they remain fresh, wholesome, nutritious, safe, and free from the effects of spoilage for a certain length of time

• manufacture specific desirable food products that exhibit a certain shelf life

Physical methods

Those methods that utilize physical treatments to inhibit, destroy, or remove undesirable M.O or endogenous enzymes without involving antimicrobial additives or products of microbial metabolism as preservative factors.

1- Heat Treatments.

A- High- heat treatments

B- Low - heat treatments

2-Drying

3-Radiation

4-Filtrations

High-Heat Treatments

• It is the most effective method for inactivating M.O and enzymes

• It is either applied to foods in their final container or prior to packaging.

• It is depend on time-temperature relationship

Effect of heating on M.O

• Sufficient heating cause irreversible

denaturation of cell proteins and

metabolic enzymes and cause death.

• If heat is not severe it will cause cell

injury

Types of high-heat treatment

1-BlanchingThe food substance, usuallya vegetable or fruit, is plunged into boiling water, removed after a brief, timedinterval and finally plunged into iced wateror placed under cold running water(shocked) to halt the cooking process.

– Primarily used for fruits & vegetables

– Deactivates natural food enzymes

– Kills some bacteria

• Blanching time is crucial and varies with the vegetable and size of the pieces to be frozen.

• Under blanching speeds up the activity of enzymes and is worse than no blanching.

• Over blanching causes loss of flavor, color, vitamins and minerals.

• Carrots • Small, whole - 5 min• Diced, sliced or lengthwise strips - 2 min

• Mushrooms • Whole (steamed) - 5 min• Buttons or quarters (steamed) - 3½ min• Slices (steamed) - 3 min

• Okra • Small pods - 3 min• Large pods - 4 min

Resistance of M.O to Heat

• Psychrophiles are the most heat sensitive; thermophiles are the most heat resistant

• Sporeformers are more heat resistant compared to vegetative

• Cocci more resistant than rods

• Spores produced by mold are more heat resistant than the bacterial spores

• D value = decimal reduction time or time ( sec, min, hr) it takes to kill 90% of a population ( 1 log cycle) at a certain temperature under given conditions ( pH, food type etc.)

• Z-value = the increase in temperature required to reduce the thermal death time 10-fold

• F-value = Time in minutes needed to destroy a specific number of microbial cells or microbial spores at a reference temperature (121.1 ºC)

• Determine the D value for a M.O in a

particular food at a specified temperature

by assessing the number of survivors over

a specified time. Construct a thermal

death time curve

Example :

• Start with a population of 1,000,000 bacteria

• 1,000,000 x 90/100 = 900,000 killed

• 1,000,000 - 900,000 = 100,000 survive

• ( 1/10 survive; 9/10 killed)

• 1 D =90 % kill

• 100,000 x 90/100 = 90,000 killed

• 100,000 - 90,000 = 10,000 survive

• (1/100 survive; 99/100 killed)

• 2 D = 99% kill

• 10,000 x 90/100 = 9,000 killed

• 10,000 - 9,000 = 1,000 survive

• ( 1/1000 survive; 999/1000 killed)

• 3 D = 99.9%

• Example : D72C = 1.2 sec for a bacterium

• When heating a food at 72C, 90% of the bacterial population will be killed every 1.2 sec.

• After 1.2 sec 90 % killed

2.4 sec 99 % killed

3.6 sec 99.9 killed

4.8 sec 99.99 killed

• The higher the D value the more resistant the MO.

• Pooled raw milk at the processing plant has bacterial population of 4x106/mL.

• It is to be processed at 79°C for 21 sec.

• The average D value at 65°C for the mixed population is 7 min.

• The Z value is 7°C.

• How many organisms will be left after pasteurization? What time would be required at 65°C to accomplish the same degree of lethality?

Factors influencing the heat resistance of MO

1.Water activity

Decreasing relative humidity, moisture or aw increases heat resistance ( when heated in water versus air)

Wet heat Heat treatment in the presence of

water where M.O killed by denaturation

Dry heat It is less lethal and kills M.O by

dehydration and oxidation

2. Fat content

Increasing the fat content of a food generally increases the heat resistance (may protect the cell against moisture loss).

3. Carbohydrate content

Increasing the CHO content of foods generally results in an increase in heat resistance ( resistance varies depending on nature of CHO)

4.Salts

The presence of certain salt may either increase (NaCl) or decrease (CaCl2) heat resistance ( some salts may decrease water activity thereby promoting heat resistance)

5. Proteins

Increasing the level of protein in a food results in increased heat resistance

6.Most MO are maximally heat resistant at their optimum pH.

An increase or decrease from this value normally results in an increase in heat sensitivity

7. Initial number of MO in a food. Increasing the levels normally result in greater survivors ( some bacteria may release protective substances in the food or liquid, menstruum , they are heated in).

8. Heat resistance tends to increase with an increase in growth temperature ( especially for spore forming MO)

9.The heat resistance of a MO decrease when heated in the presence of an inhibitory compound ( acid, bacteriocin, NO2, etc)

• Holding time:

the food should be heated at specific temperature for a specific time

• Cold point:

the centre of can filed with solid food

2-Pasteurization • The process of heating food to ensures

destruction of all non spore forming pathogens (bacteria, viruses, protozoa, molds, and yeasts) and a large number of spoilage M.O (99 to 99.9%) and heat sensitive enzymes

• Products that can be pasteurized : eggs, sports drinks, canned food, water , juice, honey, apple cider, milk

• Time/temperature kills all pathogens or reduces them to levels which are safe; incapable of growing in milk under proper storage conditions.

A - High temperature, short time (HTST)

72C for16 sec. and then immediately cooled to less than 10°C

B - Low temperature, long time (LTLT)

63C for 30 min. and then immediately cooled to less than 10°C

• Small number of spore forming MO survive and cause spoilage

• Two groups can survive milk pasteurizationa- Thermoduric • Can survive exposure to relatively high temperature

but do not necessary grow at these temp.b- Thermophilies• They requires high temperature for their growth

and metabolic activity

3- SterilizationDestruction of all microorganisms vegetative

and spores• Commercial sterility No viable MO can be detected by

conventional cultural methods or that the number of survivors is too low to be significant under condition of storage

12-D concept minimum heat process that should reduce the

probability of survival of the most heat resistant Cl. botulinum spores to 10-12.

In other words, minimum heat that would allow for the survival of one Cl. botulinum spore in 1012 cans (1 billion cans).

Processing for 2.52 min at 121C will achieve this effect.

Holding time

the food should be heated at specific temperature for a specific time

2.52 min at 121C

Cold point

the centre of can filed with solid food

Time-Temperature Combinations

From thermal death curves, the following time/temperature treatments yield the same microbe killing effect:

0.78 min @ 127oC 10 min @ 116oC

1.45 min @ 124oC 36 min @ 110oC

2.78 min @ 121oC 150 min @ 104oC

5.27 min @ 118oC 330 min @ 100oC

Thermocouple Placement atCold Point in Can

Protective Effects ofFood Constituents

• Sugar protects bacterial spores in canned fruit

• Starch & protein protect spores

• Fats & Oils protect bacterial spores

• Every food particle inside a can must reach the critical temperature for the required time

• Factors affecting heat penetration include:size of canshape of canconsistency of the food item (thick or thin)nature of the food (particulate vs liquid)

From a microbiological point of view canned foods

are divided into groups depending on the final pH

of their product in order to prevent food

poisoning results from Clostridium botulinum

1) low acid: pH > 4.6

meats, some vegetables (corn and lima beans)

2) medium acid to acid: pH 3.7 - 4.6

tomatoes, pears

3) high acid: pH < 3.7

sauerkraut, pickles , grapefruit

Infrequently canned food undergoes microbial spoilage due to:

1- Underprocessing:

• Inadequate time/temperature applied; incorrect calculation used for determining the heat process.

• In acid canned foods this is largely due to spores that survive then germinate.

• In hot filled foods spoilage may result from yeast and mold and aciduric bacteria.

2- Post process leakage ( PPL):

• Most common form of spoilage.

• Can is contaminated ( leakage at the ‘canners end’) following retorting perhaps during water cooling.

• Most of the MO causing this problem are viable ( since the can has already been heated).

• From a ‘lot’, only a few cans show this condition.

3. Pre-process spoilage (incipient spoilage)

• Product is canned but held too long before retorting especially at abusive temperatures ( perhaps due to a power failure).

• Microscopic inspection of the retorted can contents will show evidence of a mixed microflora of dead MO.

• All cans in the lot will be effected and soft swells are common.

Flat-sour bacteria

• Endo-spore forming bacteria that produce acid but little or no gas in canned food and usually there are more resistant than Cl. botulium spores and they need 4 to 5 min at 121 ºC.

• Temperature abuse > 40 ºC but not < 30 ºC

• Bacillus sterothermophilus

Aseptic Packaging

• Food is sterilized outside the can

• Placed into a sterile container and sealed under aseptic conditions

• Paper and plastic packaging materials most commonly used

• Most suitable for liquid-based food products

Hot Pack/Hot Fill

Filling unsterilized containers with sterilized food that is still hot enough to render the package commercially sterile.