Food preservation through fermentation - Department of Chemical ...

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Dec 14, 2012 (4 years and 6 months ago)

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Other Food Preservation
Techniques

Food preservation through fermentation


Fermentation is breakdown of carbohydrates under limited
supply of oxygen or under anaerobic conditions.
(Yeasts: Sugar


Alcohol + CO
2
)


Some aerobic, specific conversions may also be referred as
fermentation; ( Acetobacter: Ethylalcohol + O
2



acetic acid).



In nature, natural fermentations occur continuously.



In technically advanced societies, fermented foods are produced
to add special tastes to human diet, in less developed areas
fermentation is still one of the major preservation methods.



In contrast to most preservation methods, fermentation
encourages growth and multiplication of selected microorganisms
in foods.

Fermentation Foods

alcohol, organic acids, pickles, olives,

yoghurt, bread, tarhana, vanilla,

sausages, salami, pastrami, cheese

Benefits of fermentation

1.
Preservation,


2.
Providing variety to the diet,


3.
Production of important compounds like organic acids and
alcohols,


4.
Fermented foods are often more nutritious than their
unfermented counterparts because of mainly three reasons:


a)

while growing in the medium the microorganisms also
synthesize several complex vitamins and other growth
factors like riboflavin and vitamin B
12
.


b)

nutrients locked into plant structures and cells by
indigestible materials can be liberated
c)

enzymatic splitting of plant materials like cellulose,
hemicellulose and related polymers normally not digestable
by human beings.

Changes caused in foods by fermentation


1.
Fermentative changes: Changes that involve carbohydrates

2.
Proteolytic or putrefactive changes:
Changes that involve proteins,

3.
Lipolytic changes: Changes that involve fats.




In fermentation, fermentative changes are encouraged,
whereas other changes are suppressed. Production of acid
helps in controlling proteolytic and lipolytic microorganisms.




The end products of fermentation depend upon:

(i)
The nature of the food,

(ii)
The types of microorganisms present,

(iii)
The environmental factors affecting microbial activity





Examples of some important fermentations

1.
Fermentation of sugar by yeasts (basis for wine and beer
production and leavening of bread):



C
6
H
12
O
6

+ yeast


2C
2
H
5
OH + 2CO
2


2.
Fermentation of alcohol by bacteria (basis for vinegar,
acetic acid, production):



C
2
H
5
OH + O
2

+ bacteria



CH
3
COOH + H
2
O


3.
Fermentation of milk sugar, lactose, by bacteria produces
lactic acid which causes precipitation of the curd in the milk
(basis for cheese production).

Controlling fermentation


The factors that affect fermentation are:


1.
Acidity
.

Fermentative microorganisms are more resistant than
proteolytic and lipolytic.

2.
Level of alcohol
. Yeasts cannot tolerate high (12
-
15%) alcohol
levels. (utilized in the production of fermented alcoholic drinks,
regular alcohol content of wine 9
-
13%, fortified up to 20%)

3.
Starter cultures
.

Competition for survival, large concentrations
dominate the others. In old times inoculation with parts of
previous batch at high concentration. Novadays pure, specific
starter cultures (concentrates, dehydrated, frozen) available
commercially.

4.
Temperature.

Growth rate, optimum growth temperature of
specific microorganisms. Optimum temperature regimes are
established for various fermentations.

5.
Level of oxygen
. Aerobic, anaerobic microorganisms. Extent
of fermentative changes depend on 0
2

concentration. In
yeasts growth favored by aerobic, fermentation favored by
anaerobic conditions.


6.
Level of salt.



(i)

10
-
18% selectively inhibits activity of proteolytic and
lipolytic and other spoilage organisms and favors growth of
fermentative organisms.


(ii)

draws juice out of fruits and vegetables through osmosis.

Preservation of foods as sugar concentrates


Based on reducing the water activity of foods to levels low
enough to stop microbial activity.



Increase in solids concentration reduces a
W
, increased acidity
helps in preservation.



65% or more solids plus acidity enough for preservation in the
absence of O
2
. Above 70%w solids no need for acidity.



Fruit jellies, jams, marmelades, sweetened condensed milk are
products. Syrups of different strengths are used as
preserving media and filling liquids for preserving fruits in
cans and jars. Contact with air should be avoided to stop
mould growth on the surface.



Gelling is an important property of fruits and their extracts
in the jam and confectionary industries. Four substances are
needed for forming a fruit gel; pectin, acid, sugar and water.
In gelling, the pectin conglomerates and forms a network of
fibers. This network is able to support liquids.



The concentration of pectin determines the continuity of the
network and the denseness of the formed fibers. The
concentration of sugar and level of acidity determine the
rigidity of the network. The higher the concentration of sugar,
the less water will be supported by the structure. The higher
the acidity the tougher are the fibers.



Optimum conditions for gel formation:
pH

= 3.2
-
3.5,
solids content

(fruit + sugar)


65%w,
pectin
concentration



1%



Separation of liquids from gel structures is called
synerisis
.
This effect is undesirable and should be avoided
(homogenization).

Preservation of foods with

chemical additives


Critical issue between food scientists, food manufacturers,
government agencies and consumers.



A food additive is defined as a substance or a mixture of
substances, other than a basic foodstuff, which is present
in food as a result of any aspect of production, processing,
storage or packaging excluding contaminants.



Chemical additives can contribute substantially in the
preservation

and in
quality improvement

of foods. Legitimate
vs. illegitamate uses must be well understood and defined.





legitimate uses


1.
The maintenance of the nutritional quality of a
food,

2.
The enhancement of keeping quality or stability
with resulting reduction in food losses,

3.
Making foods attractive to the consumer in a
manner which does not lead to deception,

4.
Providing essential aids in food processing






illegitimate uses


1.
To hide the use of faulty processing and handling
techniques,

2.
To deceive the consumer,

3.
When the result is a substantial reduction of the
nutritive value of the food,

4.
When the desired effect can be obtained by good
manufacturing practices which are economically feasible



Safety of a food additive is very important. Thorough
examination of its physiological, pharmacological and
biochemical effects is required before allowing use.


Uses of chemical additives in foods

1.
Preservatives
( microbial spoilage, chemical
deterioration, control of insects and rodents )


2.
Nutritional supplements
(vitamins, amino acids,
minerals, calories)


3.
Color modifiers
(natural colors, certified food dyes,
derived colors)


4.
Flavoring agents

(synthetic, natural, flavor
enhancers or extenders)




5.
Chemicals which improve functional properties.


a)

Control of colloidal properties (gel, emulsion,


foam, suspensions),


b)

Firming agents,


c)

Maturing agents


6.
Chemicals used in processing.


a)

For sanitation, public health or aesthetic


purposes


b)

To facilitate removal of unwanted coverings


(skins, feathers, hair etc.)


c)

Antifoaming agents,


d)

Chelating agents


e)

Yeast nutrients


7.
Chemicals to control moisture

(waxes, anticaking
agents)

8.
Chemicals to control pH

(acids, bases, salts)

9.
Chemicals used to control physiological functions

(ripening agents)

10.
Others

(gases for pressure dispensing)




Important chemical preservatives


1.

Natural preservatives
. Table salt, sugars, vinegars,
spices


2.

Inorganic chemicals


a) SO
2

: preservation of fruit concentrates,
controlling enzymatic browning, protecting from
attacks by microorganisms and insects and avoiding
discoloration in dried fruits.


b) H
2
O
2

: sterilization of milk, surface
sterilization of many commodities by spraying,
disinfecting packaging materials in aseptic
processing.


c) Cl
2
:

disinfectant for water


d) CO
2
:

preserving effects at high pressures for
carbonated drinks, controlling maturation, improving
storage quality of fresh fruits (slowing down respiration).


3.
Organic chemicals


a) Benzoic acid (benzoates):

Protecting acid foods from
yeasts and molds. Mostly fruits and their products.


b) Fatty acids:

Effective mold inhibitors. Used in bread,
pickles.


c) Sorbic acid:

Used in controlling mould growth in
packaged cheese, margarine. Effective against many
moulds found on meats.


Emerging Food Preservation
Techniques

1.
Biotechnology and reduced spoilage rate


2.
Membrane filtration techniques


3.
High intensity light


4.
Ultrasound


5.
Modified atmosphere packaging


6.
Pulsed electric fields


7.
High hydrostatic pressure

Biotechnology and reduced spoilage rate




Genetic engineering applied to slow down natural
senescence process
.



Plant resistance to diseases and pests increased
.

Membrane filtration techniques


Separating bacterial cells from process streams
.


Concentration
.


High intensity light


Pulsed (1
-
20 flashes per second) broad
-
spectrum
white light, non
-
ionizing
.


Rapid inactivation of microorganisms on food
surfaces,

equipment and food packaging materials
(decontamination)
.


Destroying insects, pests, parasites, viruses
.

Ultrasound



P
ower ultrasound; higher intensity and
frequency
.


S
urface cleaning by dislodging dirt and
bacteria from surfaces, especially useful for
non
-
smooth surfaces.


I
nactivation of microorganisms

(cell damage)
and enzymes

(structural change)


Modified atmosphere packaging


Extend shelf life of food products by slowing down
their rates of spoilage
.


Absence of ox
y
gen retards aerobic MO.


Carbon dioxide has a strong bacteriostatic effect on
aerobic MO and inhibitory effects on some enzymes.


Nitrogen used to replace oxygen in the headspace or
as a filler gas in packages
.


Experimental success with CO, SO2, N2O, NO, He,
H2, Ar,Cl2, ethylene oxide, propylene oxide on
specific applications
.


Pulsed electric fields




O
peration at low and moderate temperatures
below 50

deg.
C


A
pplying short electric pulses (1
-
20μs) with a high
field strength(15
-
80kV per cm) to samples placed
between two electrodes
.


M
icrobial cell undergoes
ir
reversible membrane
permeabilization. MP is correlated to microbial
inactivation caused by breakdown of cell
membrane locally.

Sudden local
ohmic heating

of the cell membranes
also contributes to the breakdown

High hydrostatic pressure



E
nzymes and microorganisms inactivated
by temperature (60
-
70

deg.
C) / pressure
(400
-
200MPa) combination.


D
estruction results from structural
changes caused in enzymes and physical
damage on microbial cells
.