Intrinsic And Extrinsic Parameters Of Food That Affect Microbial Growth

Factors Affecting The Microbial Growth in Food:-

In most cases, micro-organism utilizes our food supply as a source of nutrient for their growth.This in the course can result in deterioration(decay)of food.The organism not only deteriorates the food but may also pose risks of disease to the human being on consumption of such contaminated food.However,the growth of microorganisms in food may be affected by several factors like physical,chemical and biological.These factors can broadly divide into two categories i.e.

1. Intrinsic parameters or intrinsic factors
2. Extrinsic parameters or extrinsic factors

Intrinsic Parameters:-

Intrinsic parameters are natural or inherent properties of food.These parameters greatly affect the number and types of microorganism that will colonize the food and food product.Intrinsic parameters affect only microorganisms, not to the food itself.Intrinsic parameters of food include:-

• pH value
• Moisture contains(water activity)
• Oxidation|reduction potential(OR potential)
• Nutrients contain
• Anti-microbial constituents
• Biological structure

pH:-

Every organism has a minimal,maximal and optimal pH for growth.Some organism can growth better at low pH or acidic pH,some can grow in alkaline pH and while other grow at some what neutral pH.pH influence both the growth rate and types of organism that will predominant the food.In general yeast and mold are more acidic tolerant than bacteria.Food with low pH value is more susceptible to spoilage by fungi.Fruits generally undergo yeast and mold spoilage because of low pH value.Most of meat and seafood products have pH of about 5.6 or above.This makes these food products susceptible to bacteria as well as yeast and mold spoilage.Most vegetables have higher pH and are subjected more toward bacterial spoilage than fungal spoilage.Some fruits may have a low PH because of inherent acidity while other (for example fermented products) may have low pH value because of developing acidity from the accumulation of acid(like lactic acid) during fermentation.

Food with different PH value and their spoilage causing micro-organism:-

Some foods are better able to resist change in pH than other those are said to be the buffer.In general, meats are highly buffer than vegetables.Various proteins in meat provide buffering capacity.Buffering properties for meats acid or alkaline fermentation to go longer low buffering power makes some foods less susceptible spoilage causing micro-organism(Mos).

For some organism,the initial PH may be suitable for growth but competitive flora on growth itself makes PH unfavorable,PH affects the transport of nutrients into the cell and microbial cells are greatly affected by the pH of food because they have no mechanism for adjusting internal pH.

Moisture content or water activity(a_w):-

Micro-organism(Mos) have an absolute demand for water, however,the exact amount of water needed for growth of microorganisms varies.This parameter helps us to understand the movement of water from the environment to the cytoplasm or from cytoplasm to the environment.The water requirement of microorganisms is expressed in physical form,called water activity(a_w).Water activity is the ratio of the vapor pressure of water present in food substrate(solution) to the vapor pressure of pure water at the same temperature.

a_{w=P/Po ,P=vapour pressure of water present in food}

_{Po=Vapour pressure of pure water at some temperature}

_{water activity is also expressed in term of EHR}

_{i.e. aw=P/Po=(1/100)ERH}

_{ERH=100. aw}

_Note:-

• a_w for pure water=1
• a_w for saturated NaCl solution=0.75
• a_w for sugar solution=0.84

Micro-organism(Mos) can utilize only unbound(free) water for their growth.Hydrophilic colloids,ions,sugar,gel etc bind some of the water to form shells of hydration and make it unavailable for microorganisms.In general,bacteria require the higher value of water activity for growth then fungi and gram-positive bacteria have higher requirement then gram negative.

→Minimal water activity requirement for different microorganisms:-

Micro-organism Minimum a_w for growth

1. Most bacteria 0.91
2. Most yeast 0.88
3. Most mold 0.80
4. Halophilic bacteria 0.75
5. Xerophilic fungi 0.65
6. Osmophilic yeast 0.60

of different foods are:-

1. 0.98 or above-Fresh meat,fish,fruits,vegetables,milk(85%),alcoholic beverages etc.
2. 0.93 to 0.98-Bread,tomato,evaporate milk,processed cheese etc.
3. 0.85 to 0.93-Cheeder cheese,sausage,sweetened condensed milk etc.
4. 0.6 to 0.85-Dried fruits,cereals,jam,gellies,nuts etc.
5. Below 0.6-Chocolate and confectionary product,honey,biscuits,crackers,potato chips etc.

Water is indispensible for mos.It brings nutrients into the cell and helps in expelling.Waste materials out of the cell.Different metabolic reaction required water molecules to run within the cell.

Oxidation/Reduction potential(O/R potential):-

Oxidation/reduction(OR) potential of a substrate(food)may be defined as ease of with which substrate lose or gain the electron.OR potential is usually written as E_h and expressed in term of millivolt(mv).The oxidized substrate has +ve E_h(+mv)and reduced substrate have -ve E_h(-mv)value.Aerobic organism including Micrococcus,Pseudomonas,Bacillus,Acinetobacter etc.required +ve E_h value and anaerobes including Clostridium,Bactericides etc.required -veE_h value.The redux potential of food is influenced by its chemical composition as well as specific processing and treatment.Also,OR potential is influenced by storage condition.

Fresh food of plant and animal origin undergo anaerobic spoilage because an anaerobic condition is maintained by reducting sugar and cysteine(SH group) respectively.Some food has the capacity to resist change in OR potential called "Poising capacity".Reducing agent such as cysteine and reducing sugar provides poising capacity.OR potential of some foods are:-

• Raw meat:- +ve 200mv
• Raw minced meat:-+ve 225mv
• Cheese:- -ve 200mv
• Plant/fruit juice:- -ve 300mv to +ve 400mv

Nutrients contain:-

The kinds and proportional of nutrient in food are all important in determining which micro-organism(microorganisms) is most likely to grow.In general, the simple compound is utilized first by the measuring microorganisms.The carbohydrate(simple sugar) are most commonly utilized as an energy source.Protein rich food like meat,egg,fish etc.are always spoiled by protolytic organism because they can utilized protein as a source of energy if sugar is not available.In fact protein rich food promotes more growth of bacteria then yeast and mold.similarlly,in the general mold can grow in the higher concentration of sugar,yeast in fairly high concentration but most bacteria grow best in the low concentration of sugar.In the same way,some microorganisms are unable to manufacture or synthesis some of their requirement(like amino acid,vitamins etc) and most depends on the food for their supply.In general gram-positive bacteria are least synthetic,gram-negative bacteria and mold are able to synthesize most of their requirement.

Anti-microbial constituents:-

Some foods posses naturally occurring substances which influence the activity of invading microorganisms,for example:-

1. Plant

-Clove:-Essential oil,Eugenol

-Garlic:-Allicin

-Common:-Cinnamic,aldehyde

-Sage:-Eugenol,Thymol

-organic:-Thymol and isothermal

-Mustard oil:-Allyl isothiocyanate

-Fruits,vegetables,tea:-Hydrocinnamic acid and derivatives

Animal

-cows milk:-Lactoferrin,conglutinin,lactoperoxidase system.

-Egg:-Lysozyme,0vatransferrin(inhibit Salmonella enteritidis)

Biological structure:-

The natural covering of some foods provides excellent protection against the energy of micro-organism and spoilage of food by such microorganisms. Natural covering of food like,

-Testa of seed

-Shell of egg/nuts

-peel of fruits/vegetable

-Hide of animal may limit the entry of microorganisms

And hence subsequently prevent from spoilage of food.In general, the interriorinner part of healthy tissues of living plants and animal are either sterile or may contain low microbial load,unless the penetrating microorganisms get opportunity to invade interiorof food.

Extrinsic Parameters:-

Extrinsic parameters are environmental factors,in which food and food products are kept.Extrinsic parameters substrate independent and affects both micro-organism (mos) as well as food.Unlike intrinsic parameters,extrinsic parameters can be maintain and regulated well.The extrinsic parameters includes:-

1. Temperature of storage
2. Relative humidity of the environment
3. Presence and concentration of gases
4. Presence and activities of micro-organism

Temperature of storage:-

Temperature of storage is highly important parameters that affects the spoilage of highly perishable food.Mos are reported to grow between -34⁰c to 100⁰c and each organism exhibit a minimum,optimum and maximum temperature for growth and these are known as cardinal temperature.

yeast and mold can grow at temperature range of 20 to30⁰c.Most bacteria can grow well at ordinary temperature(37⁰c),however some(thermophiles) grow at high temperature and other(psychrophilic) grow at low temperature.

Quality of food products also be taken into account in selecting storage temperature.for example banana is kept better if stored at (13-17)⁰c rather than at (5-7)⁰c.Similarly alarge number of vegetables are favoured by the temperature of 10 to 15⁰c.

Atlow temperature, there is change in membrane structure that affects the uptake and supply of nutrients to enzymes system within the cells.As the temperature increases above the optimum,the growth rate declines much more sharply as a result of irreversible denaturation of protein and thermal breakdown of cells plasma membrane.

Relative humidity:-

Humidity is the concentration of water vapour in the atmosphere.Relative humidityis the ratio expressed as the percentage of moisture in air to the moisture present in food under the saturation condition at temperature and pressure.Relative humidity and water activity are inter-related i.e.when food with low water activity are stored in the environment of high humidity,water will transfer from gas phase (air) to the food and thus increaseswater activity pf food,leading to spoilage by viable mos.Mos also produced water as an in product of respiration(metabolism).Thus,they increases the water activity(a_w)) of their on immediate environment.So,that eventually mos requiring high water activity are able to grow and spoiled food,which was initially consider to be microbiological safe.

The storage of fresh fruits and vegetables required very careful control of relative humidity.If it is too low than many vegetables will loose water and become flaccid or shrinkage.If it is too high condensation may occurs.Hence,R_H should be choosen such that there will be neither condensation nor desication i.e.food and environment should be in equillibrium state.

There is also relationship between temperature and humidity .i.e.higher the temperature lower will be the R_H and vice versa.The highest humidity near saturation is required for bacterial growth,less by yeast and even less by mold.Food that undergo surface spoilage from mold,yeast and certain bacteria should be stored under condition of low relative humidity(R_H).Improperlly wrapped food (such as meat item) suffer from surface spoilage in the refrigeratordue to highr R_H .

Presence and concentration of gases:-

Presence of different gases and its varying concentration may significantlly affects the colonizing mos on the food i.e.surface spoilage is prevented by altering gaseous composition.Oxygen is one of the most important gas which affects both food products as well as Mos.Oxygen gas when come in contect with food ,influence redox potential of food and finally the microbial growth.

The storage of food in atmosphere containing increased amount of co₂ up to 10% is reffered as "controlled atmospheric storage"(CAS).The inhibitor effects of co₂ on microbial growth is applied in modified atmospheric packing of food and food drinks.Mold and oxidative gram negative bacteria are more sensetive and gram positive bacteria like Lactobacillus tends to more resistant to co₂.The inhibitory effects of co₂ increases with decreases with decrease in temperature.This is because co₂ is highly soluble at lower temperature.Some mos are killed by prolong exposure to co₂.co₂ when come in contect with water,form weak acid i.e. H₂CO₃(carbonic acid).H₂CO₃ penetrate the plasma membraneand acidity the cells interior.

Ozone added to food as a preservative action on certain food.Ozone has give GRAS(generally reconized as safe) status in US,effective range is 1-5 ppm.However,it has some demerits like strong oxidizing agents,causes the rancidity of high lipid containing food.

Presence and activity of micro-organism:-

Inhibition or destruction of one population of mos by presence of other population of mos present in same habitat is the microbial interference.Some mos produced substances/metabolites (like secondary metabolites),that are either lethal or inhibitory to others.For example

1. Tetracyclin:-used in feed,medicine (to control microbial infection)

2.Nisin (isolated from Lactococcus lactic)

-Permitted in more than 50countries

-Effective against gram positive bacteria

-ADI (acceptible daily intake ) containing -0.13 mg/kg

-Mainly used in cheese,tomato,mushroom etc

3.Natamycin (isolated from Streptococcus natalensis

-Effective against yeast and mold

-ADI:-less than 0.3mg/kg

-used in cheese,sausages etc

NOte:-

-Halophilic bacteria:-salt loving bacteria

-Xerophilic fungi:-Dry loving fungi

-Osmophilic yeast:-Sugar loving yeast

References:-

1.JM, Jay. modern Food microbiology. third edition. New Delhi: CBS publication, 1987.

2.WC, Frazier and Westhoff DC. Food Microbiology. Third Edition. New Delhi.: Tata McGraw-Hill Publishing Company Limited, 1986.