Food Microbiology & Safety
Helen Billman-Jacobe
Sources of microorganisms in food_Plants
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Ray and Bhunia Chapter 3
Intended learning outcomes
Differentiate between normal flora in plant and animal derived foods, contaminants, spoilage organisms, pathogens and functional organisms
Relate the intrinsic and extrinsic factors of food substances to how microorganism can grow and survive
There are many types of plants products and the intrinsic factors are specific for each type . As a general guide they are higher in carbohydrates and lower in lipids than meat.
Eh, Aw and pH vary for each type
PLANTS- natural microflora
Plants grown for food will have natural microbial flora
Some plant associated microbes are plant pathogens –some fungi, bacteria or viruses Some are spoilage organisms
Some are contaminants
pH values of some fruits and vegetables ——————————————————————————————- Fruits pH Vegetables pH ——————————————————————————————-
Grapefruit
Raspberries
Tomatoes ——————————————————————————————-
2.9 – 3.3 3.3 – 4.4 4.5 – 5.2 3.2 – 4.7 3.0
Cauliflower
Parsnip 5.3
3.4 – 4.5 2.0 – 2.4 6.2 – 6.7 3.3 – 4.3 3.4 – 4.7 2.8 – 4.6 2.9 – 3.5 3.4 – 4.9
5.4 – 5.8 5.2 – 6.5 5.2 – 6.3 4.9 – 6.3 6.0 – 6.7 5.6 – 6.0 6.0 – 6.4
Runner beans 4.6
Spinach Sweet potato Turnips
5.1 – 6.8 5.3 – 5.6 5.2 – 5.6
Natural protection
Plants have mechanisms to prevent microbial invasion of their tissues.
• phenolic metabolites with antimicrobial activity
• phytoalexins, metabolites produced by plants in response to microbial attack i.e allicin (onions), phaseollin (beans)
Adams & Moss 2008
Physical factors affecting spoilage: Aw
Another physical factor influencing the pattern of spoilage is the availability of water.
Physical factors affecting spoilage: Aw
Another physical factor influencing the pattern of spoilage is the availability of water.
Concentration reduces water activity to aw=0.8- 0.83
Keep frozen -9°C
Concentrated citrus juice
Minimally processed, no heat treatment Spoilage by Candida and Saccahromyces (yeasts)
Spoilage by Lactobacillus and Leuconostoc produces buttery flavour (diacetyl)
Water activity of some foods of plant origin ———————————————————————————-
Foods Water activity ———————————————————————————-
Fresh and raw fruit and vegetable Fresh Bread
Flour, rice, bean, peas
Jam, marmalades
0.98 < 0.95 - 0.98 0.80 - 0.87 0.75 - 0.80 0.60 - 0.65 0.20 - 0.60
stewed fruits
Pastes, spices ---------------------------------------------------------------------------------
Cereals are rich in carbohydrates They are susceptible to fungi Field fungi
• Form on maturing plant
• Need fairly high water activities for optimal growth
• Can cause spoilage after harvest at higher aw
Fusarium blight in wheat heads
http://files.campus.edublogs.org/blogs.cornell.edu/dist/5/293/files/2012/06/head-blight-28zi6d3.jpg
Fusarium mycotoxins
Some fungi produce toxins which are harmful mycotoxins
Berthiller et al 2005
Storage fungi are adapted to grow at the lower water activities that are usually used to store grain important genera Penicillium and Aspergillus
Infection of grains by Claviceps purpura causes ergotism
The fungus produces hallucinogenic alkaloids which can cause altered behaviour, abortion or death if eaten
Yellow-green powdery growth of Aspergillus flavus on a corn rootworm-damaged ear. ( )
Aflatoxins
Fungus derived carcinogens include aflatoxins and fumonisins
Aflatoxins
Discovered in 1960s-turkey X disease caused by fungus infected peanut meal Aflatoxins intercalate with the cells nucleic acids and causes mutations
Occurs primarily in the liver where the toxins are converted to unstable derivatives
If cattle ingest aflatoxin-contaminated feed then the aflatoxins can occur in the milk Aflatoxins observed in beer, cocoa, raisins and soymeal
Fumonisins
Produced by Fusarium moniliforme
Linked wih oesophageal cancer in humans
Occurs in corn and corn-based feeds that have not been stored dry
: Minimum water activity requirements of some spoilage fungi ------------------------------------------------------------------------------------------- Species Minimum aw ------------------------------------------------------------------------------------------- Field fungi
Fusarium culmorum 0.89 Fusarium graminearum 0.89 Alternaria alternata 0.88 Cladosporium herbarum 0.85
Storage fungi
Penicillium aurantiogriseum 0.82 Penicillium brevicompactum 0.80 Aspergillus flavus 0.78 Aspergillus candidus 0.75 Eurotium amstelodami 0.71 Willemia sebi 0.69
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Penicillium italicum and Penicillium digitatum commonly found on citrus fruits,
blue mould and green mould respectively of oranges, lemons and other citrus fruits.
Penicillium expansum
• causes a soft rot of apples
• Produces a mycotoxin called patulin
• Mycotoxin detected in unfermented apple juice
is a mycotoxin that is produced by certain species of Penicillium and Aspergillus molds
grow on a variety of foods including fruit, grains, and cheese (patulin in activated in cheese).
Patulin has been found to occur in a number of foods including apple juice, apples and pears with brown rot Patulin is reported to be destroyed by fermentation
Thermal processing causes only moderate reductions in patulin levels
Gray mould Botrytis cinerea
Infection of grapes on the vine by this same
Grapes dry out and an increase in sugar concentration
referred to as the noble rot.
http://4.bp.blogspot.com/-fAf5x74Fshg/TXtqZiucBxI/AAAAAAAACpA/vdkNozIu9NQ/s200/noble%2Brot.jpg
Vegetables
Vegetable generally have a higher pH than many fruit
more susceptible to spoilage by bacteria than fungi
There are also important spoilage fungi of stored vegetables.
Spoilage bacteria are generally able to break down the pectin in the plant tissue pectinolytic species of the Gram-negative genera Erwinia, Pseudomonas and Xanthomonas
Not usually plant pathogens but opportunistic microorganisms gain access to plant tissue through wounds Relative humidity below 90-95% (limited because you don’t want the veges to wilt)
Low temperature, gas phase with reduced oxygen (ca. 2-3%) and enhanced CO2 (ca. 2-5%)
Some microorganisms causing spoilage of fresh vegetables ------------------------------------------------------------------------------------------ Microorganisms Vegetable Symptom ----------------------------------------------------------------------------------------- Bacteria
Potato Potato Potato Potato Brassicas
Botrytis cinerea
Botrytis allii
Mycocentrospora acerina
Trichotecium roseum
Fusarium coeruleum
Aspergillus alliaceus -----------------------------------------------------------------------------------------
Corynebacterium sepedonicum Pseudomonas solanacearum Erwinia carotovora Streptomyces scabies Xanthomonas campestris
ring of tubers soft rot
Many Onions Carrots Tomato Potato Onion, Garlic
gray mould neck rot liquorice rot pink rot
dry rot black rot
Sources of microorganisms in food
The internal tissues of healthy plants and animals are sterile. i.e. there are no bacteria present The outer surfaces have normal flora and contaminants
Diseased plants or animals may have microorganisms in the tissues however we generally do not use diseased plants or animals as food.
Contamination
direct contamination of food plants from faeces of birds and animals, the use of manure or sewage sludge as fertilizer, or the use of contaminated waters for irrigation is a serious public health concern
Minimally processed fresh vegetables such as celery, watercress, lettuce, endive, cabbage and bean sprouts have all been associated with Salmonella and pathogenic E. coli infections
Sources of microorganisms in food: Plants
Proper methods of growing food plants reduces the risk of contaminants.
- Using untreated sewage for watering or fertilising may introduce human pathogens
- Damage reduction during harvest reduces tissue damage and minimises spoilage
- Washing with good quality water removes external contaminants
- Storage at low temperature reduces the opportunity for microbes to multiply
A tanker dumps untreated faecal sludge in a wheat field. Photo:
Sources of microorganisms in food: Plants
Most foods from plant sources are essentially sterile in the inside
Most fruit and vegetables carry microorganisms on the surface The sources of surface microbes are soil, fertilisers, water and air
HBJ 2020 24
Sources of microorganisms in food
There are many ways that microorganisms can get into food
Natural sources of contaminants for food of plant origin: – surfaces of fruits vegetables and grains
– damaged tissues
Natural sources of contaminants for food of animal origin: – The natural microflora of the host
Other sources of contamination
– Air, soil, sewage, water, feeds, humans, food ingredients, equipment, packages and insects
Normal flora on surfaces of plants
Some plants have associated microbial communities as part of their normal flora
- Leafy vegetables of the brassica family ( e.g. cabbage, broccoli) have a microbial community on the surface
- Expected microbes: moulds, yeasts, lactic bacteria and bacteria from the genera Pseudomonas, Micrococcus, Erwinia, Bacillus, Clostridium
https://www.frontiersin.org/articles/10.3389/fmicb.2018.03028
“Cook sprouts thoroughly to reduce the risk of illness. Sprouts are a particular concern because the warm, humid conditions needed to grow sprouts also are ideal for germs to multiply. Therefore, eating raw or lightly cooked sprouts may lead to food poisoning. It’s especially important to avoid raw sprouts if you are in a group more likely to get seriously sick from food poisoning: pregnant women, young children, older adults, and people with weakened immune systems” CDC
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