Transcript File - Nutrition and Food Technology-just

Food preservation methods
1-Physical methods
2-Chemical methods
3-Biopreservation
Food processing
All 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-Blanching
The food substance, usually
a vegetable or fruit, is
plunged into boiling water,
removed after a brief, timed
interval and finally plunged into iced water
or 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 : D72C = 1.2 sec for a bacterium
• When heating a food at 72C, 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)
72C for16 sec. and then immediately
cooled to less than 10°C
B - Low temperature, long time (LTLT)
63C 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 pasteurization
a- 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- Sterilization
Destruction 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 121C will
achieve this effect.
Holding time
the food should be heated at specific
temperature for a specific time
2.52 min at 121C
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 at
Cold Point in Can
Protective Effects of
Food 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 can
shape of can
consistency 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.