food microbiologyb, factors affecting growth and survival of microorganisms

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