L4: Food Manufacturing and Preservation

Practice flashcards covering food microbiology, preservation factors, thermal death modeling, and non-thermal processing methods.

Factors that influence microbial growth in foods:

1) Intrinsic Factors + 2 main examples

Factors contained within the food itself

eg. pH, water activity ($a_w$)

salt, sugars, nutrient content, preservatives

Factors that influence microbial growth in foods:

2) Extrinsic Factors + 2 key examples

Environmental factors to which food is exposed that affect microbial growth

eg. temperature, modified atmosphere (oxygen, CO2 levels)

heat treatment, UV, light, humidity, high pressure, electric field

Intrinsic factors

1) pH

Most microorganisms grow best at___

Exceptions___

Most microorganisms grow best at neutral levels, and few grow at a $pH < 4$ (acidic)

• Moulds & Yeasts ≥ 1.5

• Lactobacillus ≥ 3.5

• food preservation e.g. pickling and fermentation extend shelf life of food by lowering pH

Intrinsic factors

2) Moisture content of food

What is Water Activity ($a_w$)

The ratio of water vapour pressure in the food to the vapour pressure of pure water at same temperature

It is a measure of availability of water for use in chemical reactions and microbial growth

Pure Water $a_{w}$

Most fresh foods $a_{w}$

Chocolate/biscuits/pastry $a_{w}$

• Pure water aw = 1

• Most fresh foods aw = ~0.99

• Chocolate/biscuits/pastry aw = 0.6-0.85

Psychrophiles

Cold-loving

Optimum growth temp: $0^{\circ}C$ to $15^{\circ}C$

Minimum range: $-5^{\circ}C$ to $+5^{\circ}C$.

Psychrotrophs:

What are they

Optimum temp range for growth

Cold-tolerant (troph t= tolerant)

Optimum growth temp:$20^{\circ}C$ to $30^{\circ}C$

Minimum range of $-5^{\circ}C$ to $+5^{\circ}C$.

Mesophiles

Optimum growth temp

Moderate temp

Optimum growth temp: $30^{\circ}C$ to $40^{\circ}C$

Minimum growth temp $20^{\circ}C$.

Obligate Aerobes:

What are they?

2 microorganism categories that are obligate aerobes?

Microorganisms, including most yeasts and moulds, which require the presence of oxygen to grow.

Extrinsic factors – Temperature:

3 main groups?

Do mould or bacteria tolerate a wider temp range?

2 species that can grow at fridge temp?

• Psychrophiles/psychrotrophs and mesophiles

• Moulds can tolerate a wider temperature range (10^oC – 35^oC) than most bacteria with a few able to grow at lower temperatures i.e. in fridge

• Psychrotrophs Listeria monocytogenes and C. botulinum can grow at 4^oC so fridges do not ensure food safety

Facultative Anaerobes vs Obligate Anaerobes

Facultative Anaerobes: Bacteria that can grow in both the presence and absence of oxygen.

Obligate Anaerobes: Bacteria that can only grow in the absence of oxygen, such as those found in canned products.

1) Thermal Processing & Storage + the 2 methods

Heat treatment given to a food product to reduce pathogens and spoilage organisms to an acceptably low level and extend shelf life

2 Methods:

1) Pasteurisation

2) Sterilisation

1) Thermal Processing & Storage

Hermetic Package

An air-tight seal used to prevent recontamination of food after processing eg. thermal processing

1) Thermal Processing & Storage

A. Thermal Death Modelling

D Value

The time required at a constant temperature to reduce the number of viable bacteria 10-fold (i.e. 1 log or 90%)

i.e. it gives the microbes heat resistance at a single temp

1) Thermal Processing & Storage

A. Thermal Death Modelling

z Value

The number of degrees in temperature change required to reduce the $D\ value$ by a factor of $10$ (one log).

i.e. it describes how lethality changes with temp- higher temps have greater lethality (smaller D values) than lower temps

1) Thermal Processing & Storage

A. Thermal Death Modelling

F Value

The time at a specific temperature required to kill a specific number of cells having a specific $z\ value$.

$F_{121} = 1\ min$ means:

A heating process has a heating profile equivalent to instantaneously heating a product to $121^{\circ}C$ and holding it for $1\ minute$ .

The F value is dependent on the z value of an organism

$F_0$ Value

The sterilization value of a process where the reference temperature is $121^{\circ}C$ and the $z\ value$ is $10^{\circ}C$.

Botulinum Cook

A heat process that reduces the population of C. botulinum spores by a factor of 12 logs

–Provides a large safety margin for canned foods

Commercial Sterilization + what spoilage rate is considered commercially sterile

The destruction of all pathogenic and spoilage organisms that can grow in food under normal storage conditions, including vegetative cells and spores.

A spoilage rate of between 1 in 10⁵ to 10⁶ cans is usually accepted as “commercially sterile”

What is Pasteurisation and how does it work

Application of heat, usually below 100^oC, to food product in order to destroy microbes that can cause disease or spoilage

• designed to kill 99-99.5% of all vegetative bacterial cells, but not endospores (usually)

• inactivates spoilage enzymes and kills spoilage organisms

Vat (Batch) Pasteurisation

Original pasteurisation method, heating products to $62^{\circ}C - 64^{\circ}C$ for ~$30\ min$.

Shelf like eg. milk = several days when refrigerated

HTST (High Temperature/Short Time) / flash pasteurisation: How does it work, applications, milk shelf life

Heats food to $71.5^{\circ}C - 74^{\circ}C$ for $15 - 30\ seconds$

Most common method - metal plates and hot water to heat - then rapid cooling

Milk, fruit juices, beer etc

Shelf like eg. milk = 2-3 weeks when refrigerated

Thermoduric Microorganisms mean…..

Examples

Microorganisms capable of surviving pasteurisation

• Endospore forming bacteria e.g. Bacillus & Clostridium

• Gram positive vegetative bacteria e.g. Enterococcus, Microbacterium, Arthrobacter

Sterilisation

Heating food to temperatures higher than $100^{\circ}C$ for a specific duration to inactivate bacterial spores of public health significance

Canning

The commercial sterilisation of food in hermetically sealed containers which are heated and cooled repidly

• also known as appertisation

What is the most important factor that affects biodegradation of food

Storage temperature

Extrinsic factors

Oxygen availability

• The majority of yeasts and moulds are obligate aerobes

▪ e.g. aerobic bacteria would thrive in ground meat due to the introduction of oxygen during the grinding and mixing process

▪ facultatives or anaerobes might grow in canned food products in the absence of oxygen

Relative thermal resistance (D-value) for Thermophiles, Psychotrophs and Mesophiles

▪ Psychrotrophs < Mesophiles < Thermophiles (most thermally resistant)

Relative thermal resistance (D-value) for gram (-/+) bacteria and endospores vs vegetative cells

▪ Gram negative < Gram positive

(ie. Gram positive more thermally resistant than Gram negative)

▪ Vegetative cells <<<< endospores

Do bacteria or yeasts/moulds require a higher a_w for growth +
examples

In general bacteria require a higher a_w for growth than yeasts and moulds

• C. perfringens (typical food pathogen) minimum a_w for growth = 0.94

• S. aureus can grow at a_w as low as 0.84

• Most spoilage moulds grow at a_w 0.8 or above

Factors effecting microbial growth

Temperature

• Psychrophiles/psychrotrophs and mesophiles are the most important groups for food microbiology

• Moulds can tolerate a wider temperature range (10oC – 35oC) than most bacteria with a few able to grow at lower temperatures i.e. in fridge

• Psychrotrophs Listeria monocytogenes and C. botulinum can grow at 4oC so fridges do not ensure food safety

Examples of where aerobic bacteria and facultative/anaerobes would thrive in food

aerobic bacteria: ground meat - introduction of oxygen during the grinding and mixing process

▪ facultatives or anaerobes: canned food products - absence of oxygen

Majority of spoilt cans exhibit swelling due to the microbial break down of proteins (proteolysis), or carbohydrates usually caused by which spp

Clostridium spp.

Flat sour spoilage of canned food usually caused by

Bacillus spp

UHT processing (Ultra high temp)

A sterilisation process (however some sources call it a form of pasteurisation)

Very high temp (130 - 140˚C) for very short periods of time (2-5 seconds)

1) Thermal processing and storage

The 2 broad forms of thermal processing and storage

1) Pasteurisation

2) Sterilisation - canning, UHT

2) Food preservation- low temp

2 forms

Chilling

Freezing

3) Non-thermal processing of food (6)

• High Hydrostatic Pressure

• Gamma Radiation

• UV Radiation

• Ozonation

• Ultrasound

• Other methods in development

4) Modified atmosphere (2)

• Vacuum packing

• Modified atmosphere packaging (MAP)

2) Food Preservation – Low Temperature

A. Chilled foods

• Foods stored at 0 – 5˚C

• Prevents the growth of mesophilic organisms

• most food-borne pathogens are mesophilic

• reduces the rate of spoilage

2) Food Preservation – Low Temperature

A. Chilled foods

Spoilage that occurs is due to

• food spoilage due to chemical reactions caused by endogenous enzymes (those present in the food) and microbial enzymes

• rate of chemical reactions is reduced with lowered temperatures

2) Food Preservation – Low Temperature

A. Chilled foods

As temperature approaches 0˚C the rate of growth of psychrotrophs/psychrophiles decreases - example

Pseudomonas on fish – generation time 6.7h at 5˚C but 26.6h at 0˚C

2) Food Preservation – Low Temperature

B. Freezing

Long term preservation of foods while retaining nutrient content and resemblance to the fresh material (better than appertisation)

• Food generally stored at <-20˚C

2) Food Preservation – Low Temperature

B. Freezing

Food generally starts freezing between

• food generally starts freezing between -0.5 and -3˚C

• has the effect of reducing a_w

2) Food Preservation – Low Temperature

B. Freezing

If food held above -10˚C what organisms may grow

psychrotrophic xerotolerant organisms

Xerotolerant = microbes that can survive in environments with extremely limited water availability

• Some yeasts and moulds, e.g. Cladosporium herbarum (spots on surface of cured meat/ham) – causes allergy

2) Food Preservation – Low Temperature

B. Freezing

Is freezing considered a lethal process

No

although it is more likely to kill mesophilic organisms than chilling

2) Food Preservation – Low Temperature

B. Freezing

Example of survivor that can grow during thawing

Listeria

can grow even at refrigeration temps after thawing

3) Non-thermal Processing of Foods

Why are Non-thermal processing methods advantageous

Often do not alter the nutrient content, colour, texture of foods

Typically applied at R/T or cooler, and rapid – saves energy

Many different techniques

3) Non-thermal Processing of Foods

A. High Pressure Processing (HPP)

Foods are placed in sealed pouches, put in water bath in sealed vessel, subjected to

high pressure

• Pressures 100-1000 Mpa, temps -20 to 60^oC, secs to mins

3) Non-thermal Processing of Foods

A. High Pressure Processing (HPP) was developed by? When?

Bert Hite in 1890s

3) Non-thermal Processing of Foods

A.High Pressure Processing (HPP) induces conformational changes in …. , denatures….inactivates …., decreases ….

induces conformational changes in cell membranes, denatures proteins

Inactivates enzymes

decreases DNA synthesis

3) Non-thermal Processing of Foods

A.High Pressure Processing (HPP) advantages and applications

• minimal loss of taste, smell, colour, nutrients (if raised temp not included)

• extends shelf life of fruits and vegetables, meat and fish products, juices

• e.g. HPP protects deli meats like ham from post processing contamination with Listeria or norovirus, kills Vibrio on oysters

3) Non-thermal Processing of Foods

B. Ionising radiation

Gamma or X-ray radiation

kill microorganisms by causing breaks in DNA helix but do not effect food matrix

3) Non-thermal Processing of Foods

B. Ionising radiation advantages

• They kill microorganisms by causing breaks in DNA helix but do not affect the food matrix

• They do not cause radioactivity of the food or packaging

• They are relatively easily accessible and cheap to produce, good penetration ability

3) Non-thermal Processing of Foods

B. Ionising radiation

Use in food production

• Inhibition of sprouting

• Decreasing the ripening of fruits and vegetables

• Killing and sterilisation of insects

• Reduction of microbial populations in foods, e.g. meats, seafood, fruits, veg, eggs, spices

– but consumer acceptance is still low

3) Non-thermal Processing of Foods

B. Ionising radiation

Microbial susceptibility to ionising radiation - bacteria g +ve/-ve

• Gram negs are very susceptible

• Gram pos Lactobacillus slightly more resistant (thicker cell wall)

• Endospore-forming Gram pos much more resistant (thick protective envelope; proteins protecting spore DNA; DNA repair occurs as spore germinates)

3) Non-thermal Processing of Foods

B. Ionising radiation

Microbial susceptibility to ionising radiation - moulds/yeasts,viruses

• Moulds similar to non-sporing bacteria

• Some yeasts quite resistant (efficient DNA repair systems)

• Viruses much more resistant partly due to small genome size

3) Non-thermal Processing of Foods

C. UV Radiation

What wavelength range?

How does it kill microorganisms?

• UV light at wavelength between 240 – 280 nm [UV-C] is most antimicrobial (especially 260 nm)

• UV acts on nucleic acid bases

o Pyrimidine bases most susceptible (especially thymine T)

o Cross linkage of Ts in DNA results in thymine dimers, DNA unable to replicate or be transcribed → cell death

3) Non-thermal Processing of Foods

C. UV Radiation

Some microorganisms are more resistant to UV radiation than others (but variable, strain-dependent): on average :

• Gram neg resistance < Gram pos, yeasts, mould resistance < bacterial spores <<< viruses

3) Non-thermal Processing of Foods

C. UV Radiation - uses

• Commonly used as disinfectant for surfaces, water and air, (BUT poor penetration)

• has now gained use in the food industry

o Studies demonstrate UV decontamination of fresh fruits and vegetables, meats and fish surfaces

o FDA-approved for control of pathogens in fruit juice processing and seafood industries

o Used for milk treatment but reduces vitamin C content

3) Non-thermal Processing of Foods

D. Ozonation

How does it work?

What is it effective against?

• forms reactive oxygen species eg. hydroxyl radicals

• oxidises many cellular components incl. lipids, protein, DNA

• O₃ effective against bacteria, fungi, protozoa, viruses

• some research shows activity against bacterial spores

3) Non-thermal Processing of Foods

D. Ozonation - uses

• O₃ effective against bacteria, fungi, protozoa, viruses

• Now widespread in the food industry - effectiveness and being a “natural” disinfectant

• Used in all sorts of meats, eggs, seafood, fruit, veg, juices etc.

• Ozone can be produced on site and it spontaneously decomposes into oxygen

3) Non-thermal Processing of Foods

E. Ultrasound

• High-frequency sound waves cause cell disruption and lead to the inactivation of bacteria, moulds, yeasts and viruses

• This occurs through cavitation, where bubbles form in a liquid, expand, and collapse violently (implode)

• The energy of this destroys chemical bonds

3) Non-thermal Processing of Foods

E. Ultrasound

Bacterial susceptibility

Gram negative << Gram positive << Endospores

3) Non-thermal Processing of Foods

E. Ultrasound

Applications

• Many applications in food processing, from a pre- treatment to fruit/veg drying, meat tenderisation, reduction in spoilage of fruit juices etc.

3) Non-thermal Processing of Foods

F. Other methods in development

i) Pulsed electric fields

• High voltage discharges to kill microorganisms

• Works best on liquid foods

• Kills vegetative cells - pasteurisation process

• Uses less energy than thermal pasteurisation treatments

3) Non-thermal Processing of Foods

F. Other methods in development

ii) Pulsed light

• Broad spectrum of radiation in pulses 1-20 per sec

• Bad penetration ability but can surface disinfect foods

• Potential as non-chemical alternatives to chorine washes e.g. of fruits and veg

• Kills vegetative cells – pasteurisation process

3) Non-thermal Processing of Foods

F. Other methods in development

iii) Oscilating magnetic fields

• Oscillation of very high magnetic fields perpendicular to each other

• Potentially pasteurise products with low electrical conductivity

• Very inconsistent results to date

4. Modified Atmosphere:

What is it used for

2 Methods

• Used in combo with other methods (e.g. thermal processing and cold storage)

• Primarily used to inhibit the growth of fast growing aerobic spoilage organisms

• Two methods used are:

A. Vacuum packing

B. Modified atmosphere packaging (MAP) – gas flushing

Modified Atmosphere

A. Vacuum packing

• Product placed in plastic bag and air is removed

• Residual O2 absorbed through chemical reactions and residual respiration of normal flora present on food

• Low oxygen tension restricts the growth of aerobes.

Modified Atmosphere

A. Vacuum packing

Applications

• Used extensively for smallgoods (ham, bacon, salami)

• Cuisine sous-vide – food is vacuum packed and then cooked in warm water

• Used for chilled ready to eat meals

• Gives longer shelf-life for chilled products

Modified Atmosphere

B. Modified Atmosphere Packaging (MAP)

• Packed food flushed with a gas mixture of carbon dioxide, oxygen, and nitrogen

• Gas mixtures vary from product to product

• CO2 added to inhibit the growth ofaerobes

• O2 added to red meats to ensure bright red colour

• Balance of gas usually made up of N2– inert gas

• C. botulinum and other pathogens could grow so cold storage and other methods needed too; lower a_w , reduce pH and/or add preservatives

Modified Atmosphere

B. Modified Atmosphere Packaging (MAP)

Applications

• Used for packaging a wide variety of foods

• Fresh meat, cured and processed meats

• Eggs, Poultry, Fish

• Pasta

• Bakery products

• Sandwiches e.g. 45 days shelf life!

Explain the difference between intrinsic and extrinsic factors that influence microbial growth in foods. List four of the intrinsic factors.

▪ Define the terms “D value” and “z value” in thermal death modelling of bacteria

▪ Explain the causes of spoilage of canned food, and describe two types of canned food spoilage

▪ Who discovered high pressure processing (HPP) and when did this occur?

What is the current process for HPP of foods?

▪ What is modified atmosphere packaging (MAP) of food? What gasses are used and why? Name two foods that are currently packaged by MAP.