Lesson 5 Food spoilage

Outcome of microbial contamination

• Persistence – remains viable but numbers remain unchanged

• Growth (multiplication)

• Death – Cannot multiply

• Sporulation

• Toxigenesis

Food Spoilage

• Food is considered spoiled when an undesirable change in the colour, flavour,

odour or texture has occurred which renders food unacceptable for human

consumption.

• Spoilage is a natural phenomenon and it occurs at varying rates depending on

• storage temperature,

• kind of food involved,

• kind of microorganisms present,

• packaging materials used,

• antimicrobial substances present in food,

• method of preservation, etc.

 Types of food spoilage

• Microbial spoilage: deterioration due to activity of microorganisms

• Enzymatic spoilage: undesirable changes due to enzyme catalyzed

reactions

• Chemical spoilage: due to non-enzymatic chemical reactions between

food components (Maillard browning) or between food and its

environ (lipid oxidation)

• Physical spoilage: undesirable changes to the physical structure of

food (crystallization, separation of emulsions)

Actions involved in microbial spoilage of foods

• Hydrolysis and fermentation of proteins (Putrefaction)

• releases foul smelling amino acids, H2S, peptides, amines, NH3, Indole.

• Breakdown of pectin – Pectinolysis

• produces methanol, uronic acid – spoilage seen as loss of fruit structure, soft

rots.

• Hydrolysis and fermentation of carbohydrates (Souring)

• Produces organic acids, CO2, mixed alcohols, and the effect is souring and

acidification.

• Hydrolysis of lipids – fat degradation

• Releases glycerol and mixed fatty acids – results in rancidity and bitterness.

Meat Spoilage

• Abundant nutrients for microbial growth

• Only a few of the microorganisms are associated with meat spoilage

• The intrinsic and extrinsic factors dictate which organisms

predominate

• Selective microbial associations during spoilage manifest

characteristic spoilage features

 Aerobic spoilage of meat

• Predominant Psychrotrophic organism: Pseudomonas:

• P. fragi

• P. fluorescens

Most Important

• P. lundensis

• Off odours evident at 107 growth

• Exhaustion of glucose and lactate

• Amino acid metabolism (putrefaction)

• Slime at 108 bacterial growth

Aerobic spoilage of meat

• Enterobacteriaceae: Serratia and Enterobacter

• Lactic acid bacteria and Brochothrix thermosphacta are

important

• B. thermosphacter associated with spoilage of lamb

• Causes souring due to carbohydrate hydrolysis

Anaerobic spoilage of meat

• Vacuum packaging/ Modified Atmosphere Packaging (N2, CO2, and O2)

• Late onset of spoilage

• Lactic acid bacteria (Carnobacterium, Lactobacillus and Leuconostoc)

• B. thermosphacter

• Clostridium

Types of meat spoilage

• Surface slime characterized by a shiny, viscous, moist covering

on the surface of the meat – Caused by Streptococcus,

Leuconostoc, Brucella, Micrococcus and some Lactobacilli

• Green discolouration: e.g. in sausage: caused by some

Lactobacilli and Leuconostoc.

• Hydrolysis (rancidity): Caused by Pseudomonas and

Achromobacter and Yeast.

Types of meat spoilage

• Phosphorescence: caused by Photobacteria and Pseudomonas

• Pigmentation, e.g. red spot caused by Serratia, and blue colour

caused by Pseudomonas

• Stickiness, Whiskers, Black spots, White spots, green patches,

off odours caused by molds

• Putrefaction caused by anaerobic microorganisms.

Shelf life

• Definition - Length of time a food remains wholesome (without

deterioration)

• It is related to total quality of food, production design, ingredient

specifications, manufacturing process, transportation and storage

• Depends on both intrinsic and extrinsic parameters of food

• Food materials vary in shelf life

• Highly perishable foods – e.g. meat, fish, milk, most fresh fruits and vegetables

• Semi-perishable foods – e.g. potatoes, apple, yam, some nuts

• Stable (durable) foods – e.g. sugar, flour, grain-legumes, dry products

Shelf-life indicators

• Direct determination and monitoring

• Batches of samples are taken at specified stages in food production

• Samples are stored under controlled conditions until quality becomes unacceptable

• Parameters tested include smell, texture, flavour, colour and viscosity mostly but also

microbiological indicators, chemical and functional indicators or nutritional

indicators.

• These are called End of Shelf Life Indicators

• Accelerated estimations

• Products are stored under raised temperatures (above normal storage temperatures)

to increase ageing process

• The elevated temperatures may enhance growth of microorganisms that differ from

those that will grow in food stored at un-accelerated temperatures

• May also cause different off-flavour notes as anticipated

Shelf-life determination

• Direct method

• Stored under selected conditions for longer than expected shelf life

• Product is monitored at regular time intervals (3-5 days using 8-10 data

points)

• Expected storage conditions and worst case scenarios used in shelf-life

determination.

• Benefits: No calculations necessary; observe the effects of the precise

conditions of storage

• Application: It is used for products with short shelf life

Shelf-life determination

Indirect methods (Accelerated shelf life)

• By increasing the storage temperature, the trial period is shortened and rate

of deterioration is increased so that it can be determined within a relatively

shorter period.

• Standard storage conditions used and elevated storage temperature by 10ºC.

• Benefits include:

• Avoid running a full length storage trial;

• See the impact of any changes much sooner

• Accelerated shelf-life study is applied to products with long shelf lives

The Rule of Q10

• Q10 is a unitless quantity

• Q10 is the factor by which the rate increases when the temperature is

raised by ten degrees.

• Temperatures MUST be in Celcius or Kelvin

• Assumption: For typical chemical reactions, Q10 values are 2.0

Calculation example

T1 T2 T3

20ºC 30ºC 40ºC

R1 R2 R3

15 24 38

• R values are the rater of chemical reaction or rate of deterioration of the

product at a specific temperature.

• Q10 = (24/15) (10/(30-20)) = 1.61 = 1.6

• Q10 = (38/24) (10/(40-30)) = 1.581 = 1.58

• Here the actual Q10 value is 1.6 and not 2

• If product reached end of shelf life after 8 weeks, Shelf life = 8 x (1.6 x 1.6)

=20.48 weeks = 5.12 months

Chemical indicators

• Glucose

• In red meats, glucose is utilized first, then amino acids. Therefore, monitoring

of glucose depletion can indicate onset of spoilage.

• Gluconic and 2-oxogluconic acid

• Pseudomonas metabolism of glucose results in accumulation of gluconic and

2-oxogluconic acids in beef.

• Lactic acid, acetic acid and ethanol

• The period between microbial concentrations and sensory detection of

spoilage is ill defined.

• Therefore, production of lactic acid, acetic acid and ethanol from glucose in

vacuum packaged meat are good indicators of meat spoilage, especially beef

and pork.

Chemical indicators

• Biologically active amines

• Tyramine is produced by some lactic bacteria but has no discernable sensory

property but has vasoactive properties

• It is detectable in vacuum packaged beef(Volatile compounds-mg/g meat)

with high microbial load (>106cfu/g)

• Volatile compounds

• These do not require extraction from food for detection

• It also allows for simultaneous determination of microbiological and chemical

activities in food

Microbiological indicators

• Storage trials

• Samples are taken at times intervals and analyzed for total

microbial load and specific spoilage organisms

• The viable counts are compared with chemical and sensory

evaluation of the product and correlations between the variables

determined to identify indicators of early food spoilage

• Challenge tests

• Food is inoculated with target organisms while replicating

conditions for production and storage in real life production

systems.

• Food products are then analyzed for survival or growth of target

organisms

• Predictive modelling

• Prediction of growth of microorganisms over a range of conditions

broader than feasible in the lab