food microbiologyb, factors affecting growth and survival of microorganisms

Created by Natasha-Zombie 

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exponential growth

in perishable products sudden spoilage reflects _____________of microorganisms

polymer degradation

microbial spoilage includes _____________ occurring in the fruit and vegetables is the breakdown of pectin leading to soft rot

gas production

microbial spoilage includes: visible growth , e.g. mould on bread, pigment production, off odour/the flavour production , and _____________

polymer production

microbial spoilage: _____________ formation of "ropey" bread, or beer from the formation of a complex polysaccharide slime in beer produced by certain bacteria

vinegar-forming Acetobacter, Acetobacter species

ropey beer is caused by 2 ifferent kinds of bacteria. The aerobic, _____________ are known to secrete slime Ropiness from the _____________ is only common in beer exposed to oxygen, like cask ale and beer left in beer taps. the 2nd bacteria Pediococcus grows best under anaerobic conditions and is commonly carried on yeast

passive carrier

food is a _____________ of microbes- some do not multiply in foods, e.g. Campylobacter , whilst others can grow under certain conditions

nutritional quality

beneficial effects of fermentation, include: preservation, they can affect desirable transformations in a food such as the improvement of _____________ and safety as well as improving the acceptability

lactic acid bacteria and yeasts

the overwhelming majority of fermented food is produced by the activity of_____________

low pH and reduced water activity

food fermentation: lactic acid bacteria and yeasts share a common ecological niche, being able to grow under conditions of _____________

anaerobic conditions

food fermentation: only lactic acid bacteria and facultative yeast will prosper under _____________. As a consequence, they frequently occur together in fermented foods

the spoilage organism

food fermentation: LAB and yeast in some cases, members of both groups act in concert to produce a product while in others one group plays the role of _____________


effects of food fermentation: Meat and Fish: definite improvement of _____________ , some improvement of safety, no improvement of nutrient value, increases acceptability e.g., salami

improvement of nutritive value

effects of food fermentation: milks: definite improvement of stability, improved safety, _____________ , improved acceptability, e.g. yoghurt/cheese

improvement of safety

effects of food fermentation: vegetables, improvement of stability, _____________ , no improvement of nutritive value, some improvement of acceptability e.g., sauerkraut

no improvement of safety or nutritive value

effects of food fermentation: , fruits: some improvement of stability, _____________, definite improvement of acceptability, e.g. wine

no improvement of stability

effects of food fermentation legumes: _____________, some improvements in safety and nutritive value pounds. Usually some improvement of acceptability, e.g. , soy sauce

does not improve safety or stability

effects of food fermentation cereals:_____________, may improve nutritive value, and some improvement of acceptability , e.g., beer and Lambic beer

chelates metal ions

fermentation effects : improvement of nutritional quality , or maintains nutritional quality, Klebsiella produce vitamins B12 which can remove anti-nutritional factors e.g. phyatic acid, _____________ removing them and making them unable to be absorbed

hydrolyse sugars in milk and cheese

lactose intolerance , due to a lack of beta galactosidase enzyme . Therefore cannot _____________ leading to increased bacterial fermentation in the guts and upset bowel and gas production

properties of the food itself

intrinsic factors:_____________ include nutrients, PH/buffering capacity, redox potential, water activity and antimicrobial constituents

factors of storage and preparation environment

extrinsic factors _____________ include relative humidity, temperature and gaseous atmosphere


intrinsic factors, nutrients: microbes derive the chemical elements constitute microbial biomass. Those molecules essential for growth , that the organism cannot synthesise and a _____________ that can be used as an energy source

competitive disadvantage

IF nutrients: the inability of an organism to utilise a major component of food material will limit its growth and put it at a _____________ to those that can

carbohydrates available

IF nutrients: the addition of fruits containing sucrose and other sugars to yoghurt increases the range of _____________ and allows the development of a more diverse spoilage microflora of yeasts

post-mortem acidification

IF nutrients:Fish spoils faster than meatfor a number of reasons such as very low levels of carbohydrate in the flesh, which has 2 important consequences for spoilage. Firstly, it limits the degree of _____________ of tissues so that the ultimate pH of the muscle is 6.2-6.5 compared with around 5.5 in mammalian muscle.

nitrogenous materials

IF nutrient fish spoilage 2: secondly, the absence of carbohydrates means that bacteria present on the fish will immediately results to using the soluble pool of readily assimilated _____________producing off odours and flavours for sooner

increases glycogen levels in liver and muscle

IF nutrient fish spoilage 3: off odours and flavour is less pronounced in fish produced by intensive aquiculture since they are normally fed to saturation, which _____________

trimethylamine oxidise TMAO

Fish spoilage: the composition of nonprotein nitrogen fraction differs significantly from that in meat._____________ occurs in appreciable quantities in marine fish is part of the Osmo regulatory system

terminal electron acceptor

fish spoilage: TMAO it is used as a _____________ by non-fermentative bacteria such as Shewanella , putrifaciens and this allows them to grow under microaerophilic and anaerobic conditions. The product of this reduction is trimethylamine which is an important components in the characteristic odour of fish

PH = log 1/ [H+]

PH and buffering capacity _____________ since pH is a logarithmic scale differences in PH of 1/2/3 units corresponds to 10- 100- and 1000 fold differences in the hydrogen ion concentration

growth and metabolism

PH: the acidity or alkalinity of an environment has a profound effect on the activity and stability of macromolecules such as enzymes. So it is not surprising that the _____________ of micro organisms are all influenced by pH

bell shaped curve

plotting a microbial growth rate against PH produces an approximately symmetrical _____________Spanning 2-5 pH units of the maximum rate exhibited over a range of 1-2 units


in general bacteria grow fastest in the pH range of_____________. With the exception being those bacteria to produce quantities of acid as a result of their energy yielding metabolism such as lactobacilli and acetic acid bacteria with optimum, usually between pH 5 and 6


yeasts grow fastest in the pH range _____________


filamentous fungi grow fastest in the pH range_____________


most foods are only slightly acidic, St materials with an alkaline pH generally have a rather pleasant taste. egg white pH increases to around _____________ as carbon dioxide is lost from the aid of delaying is a commonplace exemption to this


another exception is fermented shark produced in Iceland, known as hakar which has a pH of _____________

soft rot producing bacteria

the acidity of products can have important implications for its microbial ecology and the rate and character of the spoilage for example, plant products classed as vegetables generally have moderately acid pH and _____________such as pseudomonas play a significant role in the spoilage

yeast and moulds

fruits have a lower pH that prevents bacterial growth and therefore spoilage is predominantly caused by _____________


those fish that have a naturally low pH such as Hallibut PH _____________ have better keeping qualities and other fish

deliberately employed

the ability of low pH to restrict microbial growth has been _____________ since the earliest times in the preservation of foods with ascetic and lactic acids

weak organic acids , H+ A-

with the exception of those soft drinks that contain phosphoric acid. Most foods owe their acidity in the presence of _____________ these do not disociate completely into protons and conjured base in solution, but establish an equilibrium

[H+] * [A-] / [HA]

equilibrium constant of weak organic acids Ka is given by _____________. taking logarithm to the base 10 = pH = pKa + log [A-] / [HA]

Henderson Hasselbach equation

the _____________ describes the relationship between the pH of a solution, the strength of the acid present and the degree of the Association. When the pH is equal to the acid's pKa than half of the acid present will be undissociated

partial disassociation of weak acids

the_____________ such as acetic acid plays an important part in their ability to inhibit microbial growth.

extracellular proton concentration

microbial inhibition by weak acids is not solely due to the creation of a high _____________ there is also directly related to the concentration of undisociated acid

preserves the product

the acidity of pickled products low pH by acetic acid weak acid _____________protecting it from bacterial spoilage

right to favour the dissociated molecule

the undissociated form of the weak acid is lipophilic and therefore can diffuse freely into the bacterial cell through the cell membrane; in doing so, they pass from external environment of low pH where equilibrium favours the undissociated molecule to the high pH of the cytoplasm shifting the equilibrium _____________ so the acid ionisers producing protons, which will tend to acidify the cytoplasm and break down the pH component of the proton motive force

neutralising or expelling the protons leaking in

the cell will try to maintain its internal pH by _____________ but this will slow growth as it diverts energy from growth related functions

growth is no longer possible and the cell dies

if the external pH is sufficiently low and the extracellular concentration of massive high burden on the cell becomes too great and cytoplasmic pH drops to a level where _____________

accumulation of lactic acid

the breakdown of glycogen continues leading to an _____________ and a decrease in muscle PH. Providing sufficient glycogen is present, this process will continue until the glycolytic enzymes are inactivated by the low pH


in a typical mammalian muscle the pH will drop from an initial value of around 7 to _____________ with an accumulation of 1% lactic acid

exercised before slaughter

if there is a limited supply all of glycogen in the muscle acidification will continue only until the glycogen runs out, and the muscle will have a higher ultimate pH. This can happen if the muscle has been _____________but can also result from stress or exposure to cold

isoelectric point (no net charge)

because the pH of meat is relatively high the meat proteins are above their _____________ and will retain much of the moisture present

impeding oxygen transfer

the muscle fibres in meat are tightly packed together giving meat dry, firm texture and _____________

myoglobin, Oxymyoglobin

the tight packing of muscle fibres in combination with the higher residual activity cytochrome enzymes will mean that the meat has a dark colour of_____________ rather than a bright red paste that colour. The higher pH will also mean that microbial growth is faster, so spoilage will occur sooner

redox potential

_____________ ability to exchange electrons for chemical reactions (low: aerobic respiration; high: anaerobic respiration)


_____________ reaction occurs as a result of transfer of electrons between atoms or molecules. In living cells and ordered sequence of both electron and hydrogen transfer reactions is an essential feature of the electron transport chain and energy generation by oxidative phosphorylation

oxidising environment

the lower the redox potential is the longer the shelf life of the product will be. A high positive redox potential leads to a strong _____________ there is a tendency to accept electrons from the electrode.

a large positive redox potential

_____________ indicates that the oxidise species of the couple is a strong oxidising agent and the reduced form only weakly reducing

a large negative redox potential

_____________ indicates that the reduced species is a strong reducing agent and the oxidised form only weakly oxidising

donate electrons to the electrode

if the redox couples present favourably, reduce state, then there will be a tendency to _____________ creating a negative potential and signifying a reducing environment

oil rig

mnemonic: oxidation is loss reduction is gain

increases by 58 mV

from every unique decrease in pH, the redox potential _____________

oxidised state

if sufficient air is present in a food a high positive redox potential occurs, and most after Redox couples are in a _____________.

increasing the surface area of meat by chopping

redox potential is directly linked to storage atmosphere and processing e.g. _____________ leads to a high positive redox potential because more air can reach the surfaces

decreases redox potential

microbial growth _____________ the combination of oxygen depletion and the production of reducing compounds such as hydrogen ions

high positive redox potential

oxidative microorganisms such as strict aerobic bacteria have a high requirement of oxygen and a _____________ will predominates at food surfaces exposed to the air or where air is readily available. Example pseudomonas, which grows at and redox potential of +100 - +500 mV

low or negative redox potentials

obligate anaerobes tend only to grow at _____________ reducing environment. an aerobic metabolism gives the organism, a low yield of utilisable energy than aerobic respiration. So a reducing environment that minimises the loss of valuable reducing power from the microbial cells


the high _____________ content in meat products tends to establish reducing conditions.


an oxidising agent is a chemical compound that readily transfers oxygen atoms, or a substance that gains electrons in a redox chemical reaction
In both cases, the oxidizing agent becomes _____________in the process.


Strong reducing agents easily lose (or donate) electrons. A reducing agent (is the element or compound in a redox reaction that reduces another species. In doing so, it becomes _______________, and is therefore the electron donor in the redox. In organic chemistry, reduction more specifically refers to the addition of hydrogen to a molecule


factors contributing to redox potential of foods:_______________, between oxidise averages states of the redox couples present


factors contributing to redox potential of foods: the total _______________, of redox couples

surface area to volume ratio

factors contributing to redox potential foods: access to oxygen to the food surface (_______________,) , PH and microbial activity

raw meat

_______________, has a negative redox potential of -200 mV at pH 5.7

high positive redox potential of +225 mV

as the pH of meat increases from 5.7 to 5.9 by the processing i.e. grinding of minced meat leads to a _______________,

low pH of 6.5

cooked sausages and canned meats , have a low negative redox potential , which can be attributed to their _______________, and due to depletion of oxygen via the canning procedure and lack of surface area availability of sausages to oxygen

oxygen depletion

_______________, appears to be principal mechanism of reducing redox potential; as the oxygen content of the medium decreases, so the redox potential declines from a value of around 400 millivolts at a saturation by around 60 mV for each tenfold reduction in the partial pressure of oxygen

methylene blue and resazurin

the decrease in redox potential as a result of microbial activity is the basis of some long est rapid tests applied to food, particularly dairy products. redox dyes such as _______________, sometimes used to indicate changes in redox potential which are correlated with microbial levels


water activity functions of water in a living cell: reactant,_______________,mechanical and structural

partial pressure

water activity aw is most conveniently defined as the ratio of _______________,of water in the atmosphere in equilibrium with the substrate (P) compared with the_______________, of the atmosphere in equilibrium with pure water at the same temperature (P0) Aw= P/P0

Raoult's Law

_______________, for a solution of an ideal non-electrolyte the relative lowering of vapour pressure is equal to the mole fraction of the solute

Aw x 100 =

______________ = equilibrium relative humidity . ERH %

total number of moles

thus for an aqueous solution the water activity is approximately given by the ratio of the number of moles of water to the ______________ aw= Nw/ (Nw+Ns)

decreases the water activity

pure water has a water activity of 1.00 the addition of solutes to pure water ______________ e.g. sucrose solution has an aw of 0.9806

water activity

______________ amount of water available to organisms; reduced by interaction with solute molecules(osmotic effect) and absorption to surfaces (matric effect), which can clearly be demonstrated by the rise of water in a capillary tube

colligative property

it should be noted that water activity is a _______________ that is to say it depends on the number of molecules or ions present in solution rather than their size.thus a compound like sodium chloride which dissociates into 2 ions in solution is more effective at reducing the water activity than a compound like sucrose on a mole-to-mole basis

osmotic pressure

a parameter related to water activity is _______________ which can be foretold as the forced the unit area required to stop the net flow of water molecules from a region of high to one of low-water activity

lower water activity

cytoplasm is an aqueous solution and so must have a _______________ than pure water; thus a microorganism in an environment of pure water will experience a net flow of water molecules into the cytoplasm. If it can control this it will increase in size and burst. Most microorganisms cope by having a rigid, strong wall capable of withstanding the osmotic pressure of the cytoplasm

even at lower, or its osmotic pressure even higher

as water activity is decreased or osmotic pressure is increased in the environment. It is essential that water activity of the cytoplasm is _______________. This is achieved by the production of increasing concentrations of solutes which must not interfere with cytoplasmic function. They are known as compatible solutes and include such compounds such as mannitol, and amino acids


with a reduction of water activity in their environment and number of groups of microorganisms capable of active growth_______________ and at a water activity of below 0.6. There is no active growth


most gram negative bacteria have a minimum water activity of_______________


most gram positive bacteria have a minimum water activity of _______________


most yeasts have a minimum water activity of _______________


most filamentous fungi have a minimum water activity of_______________


Halophilic bacteria can withstand high Salt concentrations and have a minimum water activity of _______________


xerotolerant/ xerophilic fungi are able to grow on dry foods and have a minimum water activity of_______________


_______________ organisms are able to grow in the presence of high concentrations of unionised organic compounds such as sugars


fresh vegetables and meat, milk and fish have a water activity of between _______________


cured meets such as ham and salami, which can contain 6% NaCl have a water activity of approximately _______________


dried cheeses, flour cakes, beans and cereals have a water activity of approximately _______________

0. 76

pickles and jams have water activity of approximately _______________


dried fruits, have a water activity of _______________

integrity of their cell wall

Halophilic bacteria are obligately Halophilic , because the _______________ depends on a high concentration of NaCl in their environment. They may cause spoilage of dried salted fish


the limiting value of water activities with growth of any microorganism is about 0.6 and below this value the spoilage of foods is not microbiological and maybe due to chemical reactions such as _______________

survive at very low water activity levels

it is important to note that even at water activity of 0.6. Even if active growth is impossible microorganisms may still _______________

direct inhibition of enzyme activity

effects of CO2: alteration of cell membrane,_______________,effect on intracellular pH and direct changes to physio-chemical properties of proteins


antimicrobial constituents: the 1st microbial barrier is the _______________ a physical barrier to infection such as the skin, Shell, husk or rind of a product. Relatively resistant to degradation and provides an inhospitable environment microorganisms by having a low water activity, a shortage of readily available nutrients, and often antimicrobial compounds

essential/volatile oils

as a 2nd line ofdefencea product may contain antimicrobial components to local concentration of which often increases as a result of physical damage. Implants injury can rupture storage cells containing_______________ or may bring together an enzyme substrate which were separated in the intact tissue.


mustard, horseradish, watercress and cabbage,produce antimicrobial _______________(mustard oils)


Allium species such as garlic, onions and leak produce ______________ such as allicin


the antimicrobials produced by plants such as garlic and onions, cabbage and mastered are known collectively as ______________ and are producedby many plants in response microbial , invasion

pigments, alkaloids and resins

many natural constituents of plant tissues such as ______________ have antimicrobial properties

benzoic and sorbic acid

______________ found in cranberries and mountain ash buries respectively are used in their pure forms as food preservatives

aromatic essential oils

herbs contain antimicrobial activity in the ______________t they containsuch as eugenol from allspice, thymol from thyme. As a consequence, herbs and spices may contribute to the microbial stability of foods in which they are used


animal products have a range of non-specific antimicrobial constituents for example______________ of the hens eggs possesses a whole battery of a inhibitory compounds. Many of the same compounds are found in milk, but in low concentrations so less effective


______________ catalyses to hydrolysis and like acidic linkages in peptidoglycan of the bacterial cell wall. Destruction or weakening of this layer causes to sell to rupture under osmotic pressure. Pace that is most active against gram positive bacteria were the peptidoglycan is more readily accessable


other compounds limit microbial growth by restricting the availability of key nutrients. ______________ in egg white and lactoferrin in milk are proteins that scavenge iron from the medium. Iron is an essential nutrient for all bacteria, and many have evolved means of overcoming iron limitation by producing their own iron-binding compounds known as siderophores

avidin and ovoflavoprotein

a quite has powerful cofactor binding proteins such as ______________ which sequester biotin and riboflavin restricting the growth of those bacteria for which they are essential nutrients


milk also has the capacityto generate antimicrobials in the presence of hydrogen peroxide. The enzyme ______________ constitutes about 0.5% of whey proteins and catalyses the oxidation of thiocyanate by hydrogen peroxide. Hydrogen peroxide can be generated by the endogenous activity by aerobic metabolism of lactic acid bacteria. the reaction produces short lived oxidation products sucha shypothicyanate which can kill gram negative and inhibit G+ bacteria by damaging the bacterial cytoplasmic membrane

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