microbial behaviour

traditional food preservation

stored in cool caves - cheese, fruit and veg

frozen/ freeze driend at high latitudes and altitudes - meat, potato

dried in sun - tomato, fish, meat, biltong

smoked/dried over fires - kipper

salted and dried - bacaalhau

fermented - tempeh, cheese, wine, beer, yogurt, kefir

veterinary public health approach

characteristics of the original product and aspects of the processes that may represent hazards to health (public and animal)

• Biological agents and their toxins

• Environmental effects

• Human interventions

Establish standards and procedures that protect consumer health and secure food trade

how do different methods contribute to meat preservation

principles of food microbiology

• different factors that affect microbial growth

food/meat characteristics

• raw meat and post-mortem changes

• raw milk and further processing

food processing methods

Fate of microorganisms in the food products

Consideration should be given to:

• contamination events during primary production;

• contamination events during meat processing;

• whether processing includes a microbicidal step and if any survivors exist;

• whether post-microbiocidal step contamination can occur;

• identification of antimicrobial factors acting in the final product, and their interaction;

• whether conditions during storage and retail stages may enable growth of, or toxin production by, pathogens;

• usual pre-consumption practices by consumers relevant to behaviour of pathogens; and

• infective dose for given pathogen(s).

different kind of microorganisms affecting food

spoilage microorganisms

• pseudomonas, bronchotric, proteus, clostridia and bacilli

• penicillium, aspergillus

food poisoning microorganisms - foodborne pathogens

• bacteria - salmonella, campylobacter spp, listeria, E.coli

• fungi

• viruses

• toxins

probiotics and fermentation agents

• useful - can use to advantage to make new products e.g cheese, wine etc

phases of bacterial growth

Iag phase - (short) period of adjustment to environment

• can be few mins to yrs

logarithmic growth phase - growth and accelerated to a phase of rapid, constant exponential growth

stationary phase - depletion of nutrients and accumulation of toxic metabolic products growth is slowed to a point where all cell division and cell death are in balance

death phase - population decreases due to death of cells

bacterial death

usually logarithmic in vitro(in broth) and in foods if not instantaneous

D-value (decimal reduction time) = time required to destroy 90% of population at given temp

Z-value = number of degrees required for thermal death time curve(D-value) to decrease by 1 log cycle(10 times) in other worlds

• increase in temperature required for 10 fold decrease in D value

D-value

Z-value

saves time and efficiency

factors that affect microbial growth in food

intrinsic - physiochemical properties of food

extrinsic - storage environment

implicit - properties and interactions of micro-organisms

processing factors - can modify all or some of above

intrinsic factors in food

nutrients

pH and buffering capacity

redox potetnial

water activity

presence of antimicrobial factors

nutrient content

are available nurtients suitable for growth?

which micro-organisms are favoured

• high sugar content(fruit addition in yogurt) increases risk of fungal growth

• different microorganisms have different enzymes allowing them to grow on different media

• knowledge of composition of food will help predict the micro-organisms to control

pH and buffering capacity

pH = negative logarithm of H+ capacity

affects transmembrane transport of nutrients, ATP synthesis

affects stability of enzymes and consequently the growth and metabolism of micro-organism growth

different micro-organisms like different pH

• bacteria - 6-8

• yeasts - 4.5-6

• filamentous fungi - 3.5-4

different food commodities have differnt pH

• muscle -5.6

• fish - 6.4-6.6

• milk - 6.4-6.6

• egg white - 9.2

redox potetnial Eh

tendency of medium to accept or donate electrons to oxidise or to reduce is termed redox potential

different kind of microorganisms are favoured in different potentials

• obligate aerobes need high redox potential

• obligate anaerobes grow in low or negative redox potential but depends in presence of oxygen and ability to destroy superoxides

different food commodities have differnt redox potential

• Meat E(mV) @ pH:5.6=-200

• Mince meat E(mV) @pH:5.9=+225

• Cooked sausages E(mV) -20 to -150

• Lemon E(mV) @pH:2.2 =+383

water activity

Water activity (aw) is defined as the ratio of the water vapour pressure of a food (p) to that of pure water (p0) at the same temperature: aw = p / p0”

or:

Amount of free (available) water for microorganisms and their metabolic needs

On a scale from 0 (absolute dry matter) to 1 (distilled water)

aw of a food may be affected by the relative humidity of the atmosphere

Methods of reducing aw:

• Drying

• Freezing

• Addition of solutes (NaCl, sugars) to food

• Altering the microstructure of a food (for example, butter is a water-in-fat emulsion prepared from cream, a fat-in-water emulsion)

3 types of microorganisms based on aw requirements

halophilic microorganisms - can grow in higher salt concentrations - NaCl

osmophilic - can grow in high concentration of ionised substances - sugars

xerophilic - can grow in dry food - staph. aureus, some fungi and yeasts

presence of antimicrobial factors

egg-white proteins - ovotransferrin

milk proteins - lactoferrin, lysozyme

egg shell and cuticle contain some antimicrobial factors too

nitrites - added to cured meat products to stabilise red colour and inhibit cl. botulinum

woodsmoke compounds - phenols, low-molecule acids

extrinsic factors that affect food

relative humidity

gaseous atmosphere

temperature

relative humidity

affects water activity -

• takes long time for all product to get affected

affects surface of food -

• white spots by sportrichium carnis which affects only the white surface of meat

• black spots by cladosporium herbarum often on imported chilled carcases

• usually accompanied by decomposition

• penicillium gives green-blueish mould

gaseous atmosphere

oxygen concentration affects redox potential

CO2 and N2 used as basis of modified atmosphere packaging

• affects pH - produces carbonic acid with water, reduces surface contamination

• acts as a weak organic acid penetrating membrances, affecting nutrient transport and enzyme availability

• extends shelf-life

Four types of microorganisms based on gaseous atmosphere requirements:

• Aerobic microorganisms – grow in the presence of O2 (Pseudomonas);

• Facultatively anaerobic – can grow in the presence and absence of O2 (most foodborne pathogens, particularly from Enterobacteriacea family);

• Strictly anaerobic – grow only in the absence of O2 (Clostridium);

• Micro-aerophilic – grow preferentially in atmospheres with reduced O2 tension (usually with other gases such as CO2, Campylobacter)

temperature

different micro-organisms like differnt temps

membrane integrity is affected in high temp, nutrient availability in low temp

• thermophiles - 40 to 90 degrees

• mesophiles - 5 to 47 degrees

• psychophiles - -5 to 20 degrees

mesophiles and psychotrophs the most important in food preservation

implicit factors

properties and interactions of microorganisms

specific growth rate - bacteria grow faster than moulds in fresh meat

mutualism - growth of one organism helps another

antagonism - lactic acid bacteria restricts bacterial growth of pathogens

physiological status, strain diversity and adaption

implicit factors - bacterial stress response

Bacteria “feel” a change in the environment

The stress response can alter bacterial behaviour = survival:

• High temp. (sub-lethal heat) → response→ change in cell’s membrane lipids

• Simple attachment can induce response

~ 30% Salmonella genome has ‘stress-response’ genes, making bacteria more:

• fast-growing; max growth rate can be increased even if the lag phase is lengthened;

• toxigenic/virulent, and therefore more likely to cause food-borne disease;

• resistant to the factor that initially caused injury; and

• able to adapt to other adverse factors subsequently imposed along the food chain.