PROGRAM: FOOD SCIENCE AND TECHNOLOGY Course Title: …

PROGRAM: FOOD SCIENCE AND TECHNOLOGY

Course Code: FST 429

Course Title: FOOD MICROBIOLOGY IICourse Units: 2 Session: 2020/2021 First Semester

Course lecturer: DR. A.T. DAHUNSI

MICROBIAL FOOD SPOILAGE

Microbial Food Spoilage?

Any visible or invisible change which can makes food unacceptable for human consumption.

Food spoilage can be as result of:

Insect damage

physical injury due to; freezing, drying, burning, pressure, radiation

Activity of enzymes in plant and animal tissues

Chemical changes

Microbial activity

Causes of food rejection:

Changes in texture, smell, taste, or appearance

FST429 Session: 2020/2021 First Semester

Microbial Food Spoilage (Cont’d.) Foods serve as a good growth medium for many different

microorganisms.

It is estimated that about 25% of total food produced is spoilt due to

microbial activities

Microbial deterioration of food is evidenced by alteration in the

appearance (colour changes, pockets of gas / swelling), texture (soft

and mushy), colour, odour and flavour or slime formation.

Sources of microbial spoilage come from ubiquitous micro-organisms

which can originate from the natural habitat, e.g. soil, water, air, spoiled

raw materials, biofilms on the surface of equipment, personal hygiene

of food workers.


❖Roles of microorganisms present in foods

Role of microorganisms present in foods

Microorganisms

Commensals Probiotics PathogensFood

culturesSpoilers


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Microorganisms that causes food spoilage1. Lactic acid bacteria (LAB)

Gram-positive bacteria

Examples of LAB;

Lactobacillus, Pediococcus, Leuconostoc,

Oenococcus etc.

LAB are useful in production of fermented foods

But under low oxygen, low temperature, and acidic conditions, these bacteria

become the predominant spoilage organisms in foods.

Undesirable changes caused by LAB in wine, meats, milk,

and juices include;

greening of meat and gas formation in cheeses

pickles, and canned or packaged meat and vegetables.

Off-flavours, cheesy, malty, acidic, and buttery like changes


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Microorganisms that causes food spoilage (Cont’d.)

2. Acetic acid bacteria

• Are well adapted to sugar and ethanol rich

environments.

• AAB genera; Acetobacter, Gluconobacter, Acidomonas,

Gluconacetobacter, Asaia, Kozakia, Swaminathania,

Saccharibacter, Neoasaia, Granulibacter,

Tanticharoenia and Ameyamaea.

• Causes wine spoilage during production FST429 Session: 2020/2021 First Semester

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▪ 3. Filamentous fungi

Filamentous fungi causes mouldy appearance in foods

▪ Produces toxins in cereals

Fungal spoilage are characterized by highly visible, pigmented

growth, slime, off odours/off flavours.


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4. Yeasts

Causes spoilage of high acid foods, fruit

and fruit-based products, sugar syrups,

alcoholic or carbonated beverages, salad

dressings and other acid sauces, dairy

products etc.



5. Gram negative bacteria

Gram-negative rod shaped bacteria

Examples of Gram negative bacteria;

Enterobacter, Acinetobacter, Aeromonas,

Pseudomonas, Alcaligenes, Alteromonas,

Flavobacterium, Moraxella,

Archromobacter. Vibrio spp.

Grow at chill temperatures

Causes spoilage in chilled red meat, sea

fish, poultry, fish, milk and dairy

products.

Erwinia carotovora is one of

the most important bacteria

causing soft rot of vegetables

in the field or stored at

ambient temperatures FST429 Session: 2020/2021 First Semester


6. Spore forming bacteria

Spore forming bacteria produces heat resistance spores

Examples are;Alicyclobacillusspp., Bacillus spp. and Clostridium spp.

Associated with the spoilage of canned foods.


MICROBIOLOGICAL SPOILAGE OF MILK AND DAIRY PRODUCTS

Milk serves as an excellent medium for microbial growth.

Source of spoilage organisms;

✓ On the farm

✓ From the environment

✓ Milking equipment

✓ Processing plants

✓ Employees

✓ Air.

Milk and Dairy products; raw milk, pasteurized milk, dried milk,

butter milk, sour cream, cheese, yogurt etc.

Lactic acid bacteria are usually the predominant microbes in raw milk

Microbiological spoilage of milk and dairy products


Associated spoilage microorganisms in milk and dairy products; aerobic psychrotrophs,

coliforms, LAB, Gram-negative bacteria, yeasts, molds, heterofermentative lactobacilli,

and spore-forming bacteria.

Spoilage of dairy foods include the following characteristics; off-odours and flavours,

visible changes in colour or texture, gassing defects in cheeses etc.

Prevention and control of microbiological spoilage in

milk and dairy products

1. Quick consumption of raw milk

2. Heating of milk to 62.8 ºC (145 ºF) for 30 min or to 71.7 ºC (161 ºF) for 15 sec

3. Hygienic practices.

4. Ultra-pasteurized milk products, heated at or above 138 ºC for at least 2 sec.

5. Ultra-high-temperature (UHT) treatment destroys most spores in milk

6. Sterile water should be used for washing cottage cheese curd.


MICROBIOLOGICAL SPOILAGE OF MEAT AND POULTRY PRODUCTS❖ Meat is a perishable food

Meats are subject to spoilage by a wide range of microorganisms,including Gram-positive and Gram-negative bacteria, yeasts, and molds.

Animal species are used for red meat production include; cattle, swine, sheep, goat, deer, buffalo, camel, and horse.

Sources of spoilage organisms;

• Carcasses are contaminated by the skin, feaces, intestinal contentduring slaughtering

• Water

• Personnel and equipment

• Cross-contamination

• Poor hygienic conditions and handling

• Working surfaces

• Avian gut microbiota (in poultry)

• Air and environment

Some of the characteristics of spoilage in meat includes; discolouration and unpleasant odour, formation of gas bubbles, pack swelling, mucus formation, discoloration.


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Commonest microbial genera isolated from fresh meat are; Acinetobacter,

Pseudomonas, Brochothrix, Flavobacterium, Psychrobacter, Moraxella,

Staphylococcus, Micrococcus, lactic acid bacteria, and Enterobacteriaceae.

Poultry meat is spoiled quickly, even under cold storage conditions.

Specific microorganisms inducing spoilage of cooled poultry meat are Pseudomonas,

Enterobacteriaceae, lactic acid bacteria, and Brochothrix thermosphacta.

Some molds cause spoilage by forming black, white, or blue-green spots

on the surface of meat and meat products.

• Cladosporium cladosporioides, Cladosporium herbarum, Penicillium hirsutum, and

Aureobasidium pullulans causes black spots on dry-cured ham.

• Chrysosporium pannorum causes white spot spoilage of frozen meat

• Penicillium expansum is responsible for the appearance of blue-greenish spots on

meat.


Factors that influence the microbiological shelf life of meat andpoultry

• Pre-slaughter husbandry practices

• Age of the animal at the time of slaughter

• Sanitary handling during slaughter, evisceration, and processing

• Temperature controls during slaughter, processing, and distribution

• Preservation controls

• Type of packaging

• Consumer handling and storage

Prevention and control of microbiological spoilage in meat and poultry

1. Good sanitation practices during slaughtering and processing

2. Removal of spoilage microorganisms

3. Maintenance of low temperature during processing, transportation, and storage


MICROBIOLOGICAL SPOILAGE OF FRUITS AND VEGETABLES

Healthy fruits have many microbes on their surfaces but can usually inhibit

their growth until after harvest.

Sources of spoilage organisms in fruits and vegetables;

Soil

Water

Air

Other environmental sources

• The common spoilage defects are caused by molds belonging to genera Penicillium,

Phytophthora, Alternaria, Botrytis, and Aspergillus. Bacterial genera,

Pseudomonas, Erwinia, Bacillus, and Clostridium.

• Spoilage of fruits and vegetables include the following; rot, changes in the

appearance, such as black rot, gray rot, pink rot, soft rot, stem-end rot

• Examples of fresh-cut vegetables are; shredded lettuce, diced cabbage, trimmed

spinach, peeled carrots, cauliflower and broccoli florets, sliced tomatoes, sliced

potatoes, snapped green beans, cleaned and diced onions, and mixed salads. FST429 Session: 2020/2021 First Semester

Ripening weakens cell walls and decreases the amounts of antifungal chemicals in fruits, and physicaldamage during harvesting causes breaks in outer protective layers of fruits that spoilage organisms canexploit.

Fruits juices generally have relatively high levels of sugar and a low pH and this favours growth of yeasts,molds and some acid-tolerant bacteria.

Erwinia carotovora is the most common spoilage bacterium and has been detected in all kind of vegetable,because it can grow at refrigeration temperatures.

Bacteria are more often responsible than fungi for postharvest spoilage of refrigerated fresh-cut vegetablesand melons, with the majority being psychrotrophic and Gram-negative. The most common and importantspoilage microorganisms of refrigerated fresh-cut vegetables are the fluorescent

Prevention and control of microbiological spoilage of fruits and vegetables

1. Removing dead and decaying plant matter and other organic material from the crop plant

2. Fungicide applications before harvesting.

3. Insect pest management

4. Minimizing bruises

5. Rapid cooling after harvesting


MICROBIOLOGICAL SPOILAGE OF CANNED FOODS Canning: Food rendering commercially sterile by the application of heat.

The sterility of canned food is maintained by the hermetic seal.

Food products can be classified according to their acidity into

• Low-acid

• Acid

• High-acid

• Acidified foods.

Gases produced by microorganisms in canned foods causes leakages, swelling and

spoilage

Spoilage may be detected by appearance or odour, shift in pH, by the appearance of

large numbers of microorganisms in a product smear, or by isolating viable

microorganisms in subculture.

Spoilage microorganisms in canned foods; Bacillus sp., Clostridium sp.,

Byssochlamys sp., Neosartorya sp., Talaromyces sp. Eupenicillium sp. etc.


PREVENTION OF FOOD PRODUCTS FROM SPOILAGE MICROORGANISMS

Food products are perishable by nature

The development of food preservation processes; extending the shelf-life of foods

Preservative agents such as; acetic, lactic, benzoic and sorbic acid, inhibit microbial growth

Use of chemical preservatives

• Retards or inhibit the activity of microorganisms such as fermentation, putrefaction and decompositionof the food.

• Commonly used preservatives include, common salt, sugar, dextrose, spices, vinegar, ascorbic acid,benzoic acid and its salt, SO2 and the salts of sulphuric acid, nitrates, sorbic acid, propionic acid, lacticacid.

Use of food additives

• Food additives are; sugar, salt, acids, spices.

• 68-70% concentration of sugar is used for preparation of jam, jelly, marmalades

• 15-20% concentration of salt is used for the preparation such as pickles

• Many processed foods and beverages need the addition of acids to impart their characteristicflavour and taste in the final product


PROGRAM: FOOD SCIENCE AND TECHNOLOGY Course Title: …

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