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1. Biodeterioration of fruit juices and fruit juice 
concentrates 
2. Microbial spoilage of wine, beer and other 
fermented beverages 
3. Microbial deterioration of plant pectin and the 
development of soft rot in fruit and vegetables 
4. Microbial spoilage of milk 
5. Microbial spoilage of raw sugar and sugar 
confectionery
Microbial Deterioration of Plant 
Pectin and the Development of 
Soft Rot in Fruit and Vegetables
Pectic Substances 
A group of polysaccharides made up primarily of sugar acids. They 
are important constituents of plant cell walls and the middle lamella 
between adjacent cell walls. Normally they are present in an 
insoluble form, but in ripening fruits and in tissues affected by 
certain diseases they change into a soluble form, which is 
evidenced by softening of the tissues. 
They have multifunctional properties: 
โ€ข Control cell wall integrity and porosity, 
โ€ข Protect plants against phytopathogens, 
โ€ข Have gelling, emulsifying, stabilising, thickening and health-beneficial 
properties.
Types of Pectic Substances 
1. Protopectin A water-insoluble polymer that 
gives pectic acid on hydrolysis 
2. Pectic acid A high-molecular-weight polymer 
of galacturonic acid units, with no methoxyl 
groups, in which all the units are free 
3. Pectinic acid A polygalacturonic acid with 
some of its carboxyl groups methylated. It has a 
low methoxyl value and forms gels with sugars 
and water. 
4. Pectins Water-soluble pectinic acids 
containing about 6-7% methoxyl that form gels 
with sugars and acids
Pectins 
โ€ข All fruit and vegetables contain plant pectins. 
โ€ข Plant pectins are a mixture of polysaccharides from 
polymers of anhydrogalacturonic acid residues in 
which the carboxyl groups may be methylated. 
โ€ข In a typical plant pectin, galacturonic acid residues 
are linked by a-1-4 glycosidic bonds in a chain to 
which chains of rhamnose and galacturonic acid are 
bound and carboxyl groups are esterified to methanol 
in a random manner.
Pectin Backbone Structure
Schematic Structure of Pectin 
Pectin consists of four different types of polysaccharides 
Kdo, 3-Deoxy-d-manno-2-octulosonic acid; DHA, 3-deoxy-d-lyxo-2-heptulosaric acid 
Harholt J et al. Plant Physiol. 2010;153:384-395 
ยฉ2010 by American Society of Plant Biologists
The bioterioration of pectins is carried out by 
a mixture of biodeteriogen-produced 
pectolytic enzymes, of which there are three 
main classes. 
1. Polygalacturonidases 
2. Pectin transeliminases 
3. Pectin esterases
Polygalacturonidases 
โ€ข Polygalacturonide glycanohydrolases 
โ€ข Hydrolyze the a-1-4 glycosidic linkages between galacturonic 
acid residues 
โ€ข Have two subgroups that differ in substrate specificity and 
mechanism involved. 
1. Exo-polygalaturonidases 
Polygalacturonidase Polymethylgalacturonidase 
2. Endo-polygalacturonidases 
Endo-polygalacturonidase Endo-polymethylgalacturonidase
Pectin transeliminases 
โ€ข Pectic lyases 
โ€ข Cleave the a-1-4 glycosidic bonds between 
galacturonic acid residues by the transelimination of a 
proton from carbon atom 5 of the 
anhydromethylgalacturonate residue together with the 
oxygen of the adjacent atom of the a-1-4 glycosidic 
bond, to give a methyl galacturonide with a double 
bond between carbon atoms 4 and 5.
Pectin esterases 
โ€ข Hydrolyze methyl ester groups to give free 
carboxyl groups and methanol
โ€ข Most microorganisms produce AT LEAST ONE 
pectolytic enzymes. 
โ€ข Almost all fungi and many bacteria contain pectolytic 
enzymes that readily degrade pectin layers that bind 
the individual cells of fruit and vegetable tissue 
together into mixtures of oligosaccharides and 
galacturonic acids, a process that destroys the 
structural organization of plant tissue, which then 
becomes a soft amorphous mass. 
โ€ข This degradation of pectin layers in plant tissue is 
responsible for the spoilage process known as โ€œsoft 
rotโ€ in fruit and vegetables.
Soft Rot Microorganisms Associated with 
Rotting Fruits and Vegetables 
Apples Bacillus polymyxa 
Citrus fruits Penicillium spp. 
Grapes Rhizopus nigricans 
Plums Yeast 
Raspberries Rhizopus stolonifer 
Strawberries Bacillus cereus 
Tomatoes Byssochlamys fulva 
Carrots Erwinia carotovora 
Celery Sclerotinia sclerotiorum
Some strains of S. sclerotiorum produce 
photodynamic toxins, 8-methoxy psoralen and 
4,5,8-trimethyl psoralen, which are responsible 
for dermatitis among celery harvesters.
Microbial Deterioration of Milk
โ€ข Microbial spoilage of milk is a well-known 
phenomenon, milk being a perfect medium for 
microbial growth. 
โ€ข To prevent milk spoilage, heat treatment is a 
standard procedure in milk processing, i.e., 
pasteurization (i.e., LTHT, HTST, UHT) 
โ€ข Pasteurization is normally sufficient to destroy 
all bacteria in milk. However, even after this 
process, contamination by nonpathogenic 
bacteria may still occur, causing spoilage.
Milk microbial spoilage defects 
โ€ข Production of lactic acid (souring) 
โ€ข Gas production 
โ€ขDevelopment of a viscous ropy texture 
โ€ขCoagulation of milk proteins 
โ€ข Lipolysis of milk fats (rancidity) 
โ€ขDevelopment of off-flavors 
Predominant causal organisms: lactic-acid-producing 
bacteria, which ferment lactose to 
lactic acid.
Common lactic-acid-producing milk 
spoilage organisms 
โ€ข Homofermentative species (produce only lactic acid) 
Streptococcus lactis, S. cremoris, Lactobacillus casei, L. 
acidophilus, L. plantarum, L. helveticus, L. bulgaris 
โ€ข Heterofermentative species (produce lactic acid, acetic acid, 
ethanol, CO2) 
L. brevis, L. buchneri, L. fermenti, L. thermophiles, 
Leuconostoc citrovorum, L. mesenteroides, Microbacterium 
lacticum, Micrococcus luteus, M. varians, M. freudenreichii
Steps in lactose metabolism by lactic-acid- 
producing milk spoilage organisms 
1. Hydrolysis of lactose to galactose and glucose by lactase 
2. Conversion of galactose to glucose via a galactose inverting system, 
catalyzed by glucose-4-epimerase. 
3. Utilization of glucose to produce more galactose, which serves as an 
intermediate for the conversion of galactose-1-phosphate to glucose-1- 
phosphate, catalyzed by hexose-1-phosphate uridyl transferase. 
4. Conversion of glucose-1-phosphate to glucose-6-phosphate by 
phophoglucomutase 
5. Metabolism of glucose-6-phosphate to pyruvate via the EMP 
pathway. 
6. Reduction of pyruvate to lactic acid by lactate dehydrogenase.
Scheme of Lactose Metabolism 
1. Lactose ๏ƒ  Galactose and Glucose 
2. Galactose ๏ƒ  Galactose-1-phosphate ๏ƒ  D-UDP-galactose ๏ƒ  D-UDP 
glucose 
3. D-UDP-glucose ๏ƒ  D-Galactose-1-phosphate ๏ƒ  D-UPP-galactose + 
D-Glucose-1-phosphate 
4. Glucose-1-phosphate ๏ƒ  Glucose-6-phosphate ๏ƒ  Pyruvic acid 
5. Pyruvic acid ๏ƒ  Lactic acid 
glucose-4-epimerase 
phosphoglucomutase EMP pathway 
lactate dehydrogenase 
hexose-1-phosphate uridyl transferase 
lactase
HydrolSyscihs eomf leac otof sleactose metabolism
Scheme of Lactose Metabolism 
1. Lactose ๏ƒ  Galactose and Glucose 
2. Galactose ๏ƒ  Galactose-1-phosphate ๏ƒ  D-UDP-galactose ๏ƒ  D-UDP 
glucose 
3. D-UDP-glucose ๏ƒ  D-Galactose-1-phosphate ๏ƒ  D-UPP-galactose + 
D-Glucose-1-phosphate 
4. Glucose-1-phosphate ๏ƒ  Glucose-6-phosphate ๏ƒ  Pyruvate 
5. Pyruvate ๏ƒ  Lactic acid 
glucose-4-epimerase 
phosphoglucomutase EMP pathway 
lactate dehydrogenase 
hexose-1-phosphate uridyl transferase 
lactase
phosphoglucomutase EMP pathway 
๏ƒ  Glucose-6-phosphate ๏ƒ  Pyruvate 
Lactic acid 
lactate 
dehydrogenase 
NADH 
NAD+
Microbial Deterioration of Raw 
Sugar and Sugar Confectionery
The principal effect of microbial 
deterioration of raw sugar is the LOSS 
OF SUCROSE, due to the inversion of 
sucrose to fructose and glucose by 
invertase-producing yeasts and fungi, 
as catalyzed by invertase.
NOTE: This results in a reduction in sucrose 
content.
The susceptibility of raw sugar to microbial 
attack depends on the composition of the 
molasses film on sugar crystals, particularly on 
the water activity of the sugar. 
Most sugars have aw = 0.60-0.75; thus, only 
osmophilic yeasts (aw = 0.60) and xerophilic 
fungi (aw = 0.65) are the main contaminants of 
raw sugars. This is helped by the slightly acidic 
pH (5-6) of sugars, which inhibits bacterial 
growth.
Microorganisms Associated with 
Biodeterioration of Raw Cane Sugars 
โ€ข Fungi 
Aspergillus niger Penicillium expansum 
Alternaria brassicae Aspergillus flavus 
Monilla nigra Cladosporium herbarum 
โ€ข Yeasts 
Hansenula anomala Saccharomyces cerevisiae 
Candida utilis Pichia fermentans 
โ€ข Bacteria 
Bacillus subtilis B. megaterium 
Clostridia nigrificans C. thermosaccharolyticum
Sources of microbial infection 
โ€ข Airborne contamination 
โ€ขWaterborne contamination 
โ€ข Infected sugar residues 
โ€ข Unsuitable storage conditions
โ€ข Microbial deterioration of 
carbohydrates 
โ€ข Microbial deterioration of 
proteins and protein foods 
โ€ข Microbial deterioration of 
edible oils and fats
PROTEINS are large biological molecules 
or macromolecules consisting of one or more 
long chains of amino acid residues. 
The term comes from the Greek word proteios, 
meaning โ€œprimaryโ€, โ€œin the leadโ€ or โ€œstanding in 
frontโ€. 
โ€ข Essential to all life 
โ€ข They are the major constituents of enzymes, 
antibodies, many hormones and body fluids such 
as blood, milk, and egg white.
Of the many kinds of food spoilage, the 
microbial spoilage of proteins and protein 
foods is the most complex and perhaps the 
least understood, owing to the enormous 
complexity of structural proteins in nature 
and the wide variety of spoilage 
microorganisms associated with protein 
spoilage.
Structural Proteins of Protein Foods 
1. Myoglobin 
2. Myofibrillar proteins 
3. Collagen 
4. Elastins 
5. Keratins
โ€ข An iron- and oxygen-binding protein found in the muscle 
tissue of vertebrates in general and in almost all mammals. 
โ€ข Is formed from one polypeptide chain and one heme 
molecule. 
โ€ข Heme is a pigment responsible for the color of red meat; 
thus, the color that meat takes is partly determined by the 
degree of oxidation of myoglobin. 
โ€ข Degraded by microbial proteases to amino acids and 
oligopeptides 
Myoglobin
Myofibrillar proteins 
โ€ข Consist mainly of actomyosin and form the major part of 
muscle proteins. 
โ€ข Degraded by trypsin-like microbial proteinases
Collagen 
โ€ข Found in connective tissues such as tendons and bone cartilage 
โ€ข Very resistant to degradation 
โ€ข Contain a high proportion of nonpolar amino acids (valine, leucine, 
and isoleucine, with proline and hydroxyproline groups) but no 
cysteine 
โ€ข Degraded by collagenases from Clostridium 
Clostridiopeptidase A Hydrolyzes glycylprolyl bonds and has 
pH optima of 7.7-8.0. Activated by Ca and inhibited by EDTA. 
Clostridiopeptidase B Cleaves peptide bonds adjacent to 
lysine and arginine and has pH optima in the range of 7.2-7.4. 
Specific for collagen and gelatin.
Elastins 
โ€ข Found in connective tissues such as tendons and ligaments 
โ€ข Contain 90% nonpolar amino acids arranged in a random 
structure 
โ€ข Highly resistant to hydrolysis, heat, and maceration 
โ€ข Degraded by elastinases, which have been isolated from 
Flavobacterium elastolyticum, Aeromonas salmonicida, Bacillus 
subtilis, Pseudomonas aeruginosa, and P. mallei 
โ€ข These elastinases have the following characteristics: 
Have pH optima in the range of 7.0-9.0 
Hydrolyze peptide bonds adjacent to glycine and proline 
Highly specific for elastin 
Inhibited by diisopropylfluorophosphate
Keratins 
โ€ข The major constituents of wool, hair, nails, hooves, and fish scales 
โ€ข Contain large amounts of glycine and proline with about 8% 
cysteine. 
โ€ข Degraded by keratinases, which have been isolated from 
Streptomyces fradiae and S. microflavus. They have the following 
characteristics: 
Have pH optima in the range of pH 8.5-9.0 
Degrade keratin 
Activated by calcium and magnesium ions 
Inhibited by EDTA
In general, most bacteria are unable to colonize 
pure proteins unless sufficient peptides, amino 
acids and vitamins are present to enable them to 
produce proteases necessary for protein digestion. 
Food such as meat, fish, and cheese contain 
abundant quantities of amino acids and other 
nutrients; they are therefore readily colonized by 
most microorganisms.
Principal Changes Associated with 
Protein Spoilage 
1. Putrefaction 
Characterized by the production of foul odors and offensive 
textures and flavors, which arise from spoilage metabolites 
that result from the catabolic metabolism of low-molecular-weight 
peptides and amino acids by spoilage organisms. 
2. Degradation of protein constituents 
Indicated by protein coagulation and liquefaction, rot 
development, and destruction of structural proteins such as 
collagen and elastin.
Stages in the Protein Spoilage Process 
1. Initial contamination and colonization of the protein food by microorganisms. 
2. Rapid utilization and metabolism of low-molecular-weight compounds such 
as amino acids, dipeptides, lactic acid, and sugars present in meat or fish 
juices, which yield spoilage metabolites, e.g., cadaverine, putrescine, organic 
acids, CO2, H2S and NH3. At this stage, there is explosive microbial growth. 
3. Increased production of microbial proteases by proteolytic spoilage 
microorganisms. The proteolytic breakdown of high-molecular-weight proteins 
to oligopeptides provides a continued supply of nutrients. The oligopeptides 
are then hydrolyzed to free amino acids, which are then metabolized to 
additional metabolites. This accumulation of metabolites eventually poisons 
the microorganisms themselves, slowing down the putrefactive processes.
Amines Produced during Protein Spoilage 
Analysis of putrefied protein foods shows that a mixture of amines are 
produced by the anaerobic decarboxylation of amino acids. Such 
amines include the following: 
1. Cadaverine from L-lysine by Bacillus cadaveris, E. coli, and 
Clostridium histolycum 
2. Putrescine from L-ornithine by Clostridium septicum and C. welchii 
3. Aminoburyic acid from glutamic acid by S. faecalis 
4. Isobutylamine from L-valine by Proteus vulgaris and Pseudomonas 
cocovenans 
5. Tyramine from tyrosine by S. faecalis 
6. Tryptamine from tryptophan by S. faecalis and C. welchii 
NOTE: All reactions are catalyzed by decarboxylases.
Organic Acids Produced during Protein 
Spoilage 
1. Pyruvate from alanine by Bacillus subtilis, as catalyzed by 
alanine dehydrogenase (NAD-dependent) 
2. ๏ข-Methyl-ฮฑ-ketovalerate from isoleucine, as catalyzed 
isoleucine oxidase 
3. Indole from tryptophan to by E. coli and Proteus vulgaris 
4. ฮฑ-Ketoglutarate from glutamate by E. coli, S. cerevisiae, C. 
sporogenes, as catalyzed by glutamate dehydrogenase 
5. Fumarate from aspartate by E. coli, as catalyzed by 
aspartase 
6. Pyruvate from serine, as catalyzed by serine dehydratase
Organic Acids Produced during Protein 
Spoilage 
7. ฮฑ-Ketobutyrate from threonine, as catalyzed by threonine 
dehydratase 
8. Pyruvate from cysteine, as catalyzed by cysteine desulfhydrase 
9. Acetate from pyruvate by L. delbrueckii, Proteus vulgaris, and P. 
fluorescens 
9. Acetate from alanine and glycine by C. sporogenes 
10. Isobutyric acid from valine and alanine 
11. Isovaleric acid from leucine and alanine 
12. Methylbutyric acid from isoleucine and alanine 
13. Aminovaleric acid from proline 
14. Aminohydroxyvaleric acid from hydroxyproline
During the putrefactive process, anaerobic clostridia such 
as Clostridium butyricum, C. pasteurianum, C. acetobutylicum 
and C. sporogenes also produce large amounts of H2 via the 
reduction of hydrogen ions or protons as catalyzed by a specific 
hydrogenase requiring reduced ferredoxin as its cofactor. 
Ferredoxin 
2 H+ H2 
Fe3+ Fe3+ 
Other bacteria produce hydrogen and carbon dioxide from 
glucose, which is then degraded to pyruvate via the EMP 
pathway, which is then converted to acetate, formate and CO2.
โ€ข In the late stage of putrefaction, spoilage microflora 
also produce proteinases that degrade various 
protein constituents by hydrolyzing peptide bonds 
to give low-molecular-weight oligopeptides and 
free amino acids. 
โ€ข Spoilage microorganisms also utilize muscle 
glycogen to produce trimethylamine, ammonia and 
dimethylamine, whose chemical determination 
forms the basis of several methods for determining 
fish and fish product spoilage.
The pH optima of collagenase, elastinases, and 
keratinases lie in the alkaline range, i.e., 7.2-9.0; 
thus, the degradation of collagens, elastins, and 
keratins in protein foods is favored during the 
advanced stages of putrefaction when the various 
putrefactive amines such as cadaverine and 
putrescine produced increase the pH of the food 
from 5.5 to above 8.0 when proteolysis has 
become extensive.
During extensive proteolysis, both bacterial 
and fungal proteolytic enzymes hydrolyze 
casein proteins, elastins, gelatins and 
collagens.
โ€ข Microbial deterioration of 
carbohydrates 
โ€ข Microbial deterioration of 
proteins and protein foods 
โ€ข Microbial deterioration of 
edible oils and fats
FATSare a wide group of compounds composed of long-chain 
organic acids, called fatty acids. A typical fat molecule 
consists of glycerol combined with three fatty acids, i.e., it is 
a triol (i.e, it has three chemically active -OH groups). Fats 
are formed when each of these three -OH groups reacts 
with a fatty acid, resulting in triglycerides. 
โ€ข Hydrophobic, and generally soluble in organic solvents 
but insoluble in water. 
โ€ข Shorter-chain fats are usually liquid at room temperature, 
whereas longer-chain fats are solid. 
NOTE: Fats differ from carbohydrates and proteins in that they are not polymers 
of repeating molecular units
โ€œOilโ€, โ€œfatโ€, and โ€œlipidโ€ are often used 
interchangeably. Of these, lipid is the 
general term. Oil is the term usually 
used to refer to fats that are liquid at 
room temperature, while fat to those 
that are solid at room temperature.
In fat-containing foods, the biodeterioration of 
edible oils and fats by bacteria and fungi is the 
principal cause of spoilage indicated by the 
following: 
Rancidity 
Acidity 
Soapiness 
Off-flavors 
Discolorations
Butter and Margarine 
Butter is an emulsion of water in butterfat of the following 
composition: 80-83% butterfat 
16% water 
1% nonfat milk solids 
0-3% sodium chloride 
Margarine is also a water-in-fat emulsion: 
80% fat (a mixture palm, coco and marine oils) 
20% water 
Both are subject to microbial spoilage characterized by rancidity, 
acidity, off-flavors and discolorations.
Causes of Butter and Margarine Rancidity 
1. Autooxidative deterioration 
โ€ข Oxygen absorption and oxidation of unsaturated fatty acids (e.g., 
linoleic, linolenic and arachidonic) to hydroperoxides, which are 
oxidized to ketones and aldehydes. 
โ€ข Generally occurs during prolonged storage at ambient temperature 
โ€ข Catalyzed by cupric and ferric ions, UV and high storage 
temperatures (>5ยฐC) 
2. Lipolysis of natural and synthetic triglycerides in fats 
โ€ข Effected by milk and microbial lipases 
โ€ข Prevented by pasteurization of milk and heat treatment of butter 
3. Lipoxidation 
โ€ข Hydroperoxide production by specific microbial lipoxidases.
Microorganisms Associated with Butter and 
Margarine Rancidity 
Butter and margarine rancidity is generally associated with 
lipolytic molds and yeasts. 
Aspergillus tamari A. chevalieri 
Cladosporium suaveolens Cladosporium butyri 
Candida lipolytica Ospora lactis 
Paecilomyces aureocinnamoneum Pseodomonas fluorescens 
Margarinomyces bubaki Penicillium glaucum 
Epicoccum purpurescens Micrococci
Rancidity and acidity are caused by the production 
of free fatty acids, particularly butyric, caproic, caprylic 
and capric acids, and their corresponding methyl 
ketones. These volatile free fatty acids and methyl 
ketones directly arise from the metabolism of liberated 
free fatty acids by ๏ข-oxidation to the corresponding ๏ข-keto 
acid, which is decarboxylated to methyl ketones or is 
cleaved to give acetyl-coA and a lower fatty acid that is 
two carbons shorter. 
Secondary alcohols are also formed by the 
reduction of various methyl ketones.
On the other hand, characteristic soapy flavors are 
produced by the liberated lauric and myristic acids that 
are present as triglycerides in butterfat and coconut 
oils.
Factors Affecting Microbial 
Growth in Food 
1. Temperature 
2. Water activity 
3. Humidity 
4. pH 
5. Oxygen availability 
6. Osmotic pressure
Temperature 
Storage temperature is considered the most important factor 
that affects food spoilage, as it determines the type of 
microfolora that will cause spoilage; however, the relative 
humidity and availability of oxygen must also be controlled. 
Microorganisms have been reported to grow over a wide 
temperature range, the lowest being โˆ’34ยฐC and the highest 
being 90ยฐC. All microorganisms, however, have an optimum 
temperature as well as a range in which they will grow. This 
preference for temperature forms the basis of dividing 
microorganisms into the following groups: 
Psychrohiles Psychrotrophs Mesophiles Thermophiles
Types of Organisms by Growth 
Temperature 
Psychrophiles Grow best between -2 and 7ยฐC 
Psychrotrophs Optimum growth from 20 to 30ยฐC, 
but can grow at ca. 7ยฐC 
Mesophiles Optimum growth at 30โ€“40ยฐC, but 
can grow between 20 and 45ยฐC 
Thermophiles Optimum growth between 55 and 
65ยฐC, but can grow at temperatures 
as low as 45ยฐC.
Temperature 
Just as molds can grow over a wide range of pHs 
and moisture contents, they can also tolerate a 
wider temperature range than bacteria. Many 
molds can grow in the refrigerator. Yeasts are not 
usually found growing in the thermophilic 
temperature range, but prefer psychrotrophic and 
mesophilic temperatures.
Water Activity 
Microorganisms cannot grow in a water-free environment, 
as enzyme activity is absent and most chemical reactions 
are greatly slowed down. Fresh vegetables, fruit, meat, fish 
and some other foods naturally have a high moisture 
content, averaging about 80%. Drying is one of the oldest 
methods of food preservation as it reduces moisture 
availability, thereby limiting the number and types of 
microorganisms that can grow and reducing the rate at 
which they can do so. A measure of this parameter is 
called water activity, denoted by aw.
Water Activity 
Water activity is a measure of water available to 
microorganisms. Pure water has a water activity of 1.0 
while most fresh foods have a water activity of about 0.99. 
In general bacteria require a higher aw than yeasts 
and molds. Most spoilage bacteria cannot grow at aw < 
0.91, with Clostridium botulinum having a minimum growth 
level of 0.94. Staphylococcus aureus, has, however, been 
found to grow at aw as low as 0.84. The lowest reported aw 
for bacterial growth is 0.75. Most spoilage molds cannot 
grow at aw < 0.80. The lowest reported aw for any mold 
growth is 0.65, and that for yeasts 0.61.
Minimum Water Activities for growth of 
Different Microorganisms 
Normal bacteria 0.91 
Normal yeasts 0.88 
Normal molds 0.80 
Xerophilic molds 0.65 
Osmophilic yeasts 0.60
Humidity 
The humidity of the environment is important as it 
affects the aw of the food as well as the moisture 
content of the food surface. Food can pick up 
moisture from the atmosphere. Under conditions of 
high relative humidity storage (e.g., in a 
refrigerator), surface spoilage can take place, 
unless food is adequately protected by packaging.
pH 
Most microorganisms grow best at neutral pH and only a 
few are able to grow at a pH lower than 4.0. Bacteria are 
more fastidious about their pH requirements than yeasts 
and molds. The fact that pH can limit microbial growth is a 
basic principle of food preservation and has been exploited 
for thousands of years. Fermentation and pickling extend 
the shelf-life of food products by lowering the pH. The fact 
that no known spore-forming pathogenic bacteria can grow 
at pH < 4.6 is the basis for the food sterilization principle for 
low-acid and acid foods.
Oxygen Availability 
Controlling the availability of free oxygen is one means of 
controlling microbial activity in food. Although oxygen is essential for 
carrying out metabolic activities that support all forms of life, some 
microorganisms use free atmospheric oxygen, while others metabolize 
oxygen (reduced form) bound to other compounds such as 
carbohydrates. 
Microorganisms can be broadly classified into two groups: 
aerobic and anaerobic. Aerobes grow in the presence of atmospheric 
oxygen, while anaerobes, in the absence of atmospheric oxygen. In 
between these two extremes are facultative anaerobes, which adapt 
and grow with or without atmospheric oxygen, and microaerophilic 
organisms, which grow in the presence of reduced amounts of 
atmospheric oxygen.
Oxygen Availability 
At the surface and within protein foods, oxygen availability 
and oxygen tension govern the numbers and type of food-colonizing 
spoilage microorganisms. The exposed surface of 
fresh meat and fish have a high oxygen tension and therefore 
support a large number of aerobic microorganism, such as 
Pseudomonas spp., Achromobacter spp., bacilli, micrococci, 
yeasts and fungi.
Osmotic Pressure 
Osmotic pressure is inversely related to water activity. 
As the osmotic pressure of any system increases, water 
activity decreases. Thus, high osmotic pressures are normally 
incompatible with living organisms due to the osmotic effects 
that tend to dehydrate living cells.
Food spoilage associated with 
protein degradation 
Type of food Spoilage 
Milk Cogulation of caseins, off-flavors, racidity, putrefaction, 
cadaverine 
Meats Surface slimes, liquefaction, degradation of collagen, elastin, 
keratin, putrefaction, cadaverine, putrescine 
Fish Fishy odor, TMA, DMA, surface slimes, H2S, cadaverine, 
putrescine, indole 
Hams, bacon, 
chicken, turkey 
Greening, putrefaction, liquefaction, bone taint, rancidity 
Eggs White, rot, black rot, mixed rot, fungalinfections 
Cheese moldy
Food spoilage associated with fats 
degradation 
Food Spoilage 
Milk Souring 
cream Rancidity, free fatty acid 
butter Free fatty acid 
Margarine Rancidity, methyl ketones 
Oats Bitterness 
Wheat Soapiness 
Rapeseed oil Lipoxidation
Microbial lipases 
โ€ข Cleave triglycerides at either 
โ€ข 1,3 position 
โ€ข 2-position
๏ข-oxidation 
โ€ข Yield keto-acids, methyl ketones, secondary alcohols, shorter fatty 
acids such as butyric, propionic acid, acetic acid
Acyl CoA dehydrogenase 
Enoyl CoA hydratase 
๏ข-hydroxylCoA dehydrogenase 
Thiolase 
Pathway for ๏ข-oxidation 
Of a Fatty Acid

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Biodeterioration2

  • 1. 1. Biodeterioration of fruit juices and fruit juice concentrates 2. Microbial spoilage of wine, beer and other fermented beverages 3. Microbial deterioration of plant pectin and the development of soft rot in fruit and vegetables 4. Microbial spoilage of milk 5. Microbial spoilage of raw sugar and sugar confectionery
  • 2. Microbial Deterioration of Plant Pectin and the Development of Soft Rot in Fruit and Vegetables
  • 3. Pectic Substances A group of polysaccharides made up primarily of sugar acids. They are important constituents of plant cell walls and the middle lamella between adjacent cell walls. Normally they are present in an insoluble form, but in ripening fruits and in tissues affected by certain diseases they change into a soluble form, which is evidenced by softening of the tissues. They have multifunctional properties: โ€ข Control cell wall integrity and porosity, โ€ข Protect plants against phytopathogens, โ€ข Have gelling, emulsifying, stabilising, thickening and health-beneficial properties.
  • 4. Types of Pectic Substances 1. Protopectin A water-insoluble polymer that gives pectic acid on hydrolysis 2. Pectic acid A high-molecular-weight polymer of galacturonic acid units, with no methoxyl groups, in which all the units are free 3. Pectinic acid A polygalacturonic acid with some of its carboxyl groups methylated. It has a low methoxyl value and forms gels with sugars and water. 4. Pectins Water-soluble pectinic acids containing about 6-7% methoxyl that form gels with sugars and acids
  • 5. Pectins โ€ข All fruit and vegetables contain plant pectins. โ€ข Plant pectins are a mixture of polysaccharides from polymers of anhydrogalacturonic acid residues in which the carboxyl groups may be methylated. โ€ข In a typical plant pectin, galacturonic acid residues are linked by a-1-4 glycosidic bonds in a chain to which chains of rhamnose and galacturonic acid are bound and carboxyl groups are esterified to methanol in a random manner.
  • 7. Schematic Structure of Pectin Pectin consists of four different types of polysaccharides Kdo, 3-Deoxy-d-manno-2-octulosonic acid; DHA, 3-deoxy-d-lyxo-2-heptulosaric acid Harholt J et al. Plant Physiol. 2010;153:384-395 ยฉ2010 by American Society of Plant Biologists
  • 8. The bioterioration of pectins is carried out by a mixture of biodeteriogen-produced pectolytic enzymes, of which there are three main classes. 1. Polygalacturonidases 2. Pectin transeliminases 3. Pectin esterases
  • 9. Polygalacturonidases โ€ข Polygalacturonide glycanohydrolases โ€ข Hydrolyze the a-1-4 glycosidic linkages between galacturonic acid residues โ€ข Have two subgroups that differ in substrate specificity and mechanism involved. 1. Exo-polygalaturonidases Polygalacturonidase Polymethylgalacturonidase 2. Endo-polygalacturonidases Endo-polygalacturonidase Endo-polymethylgalacturonidase
  • 10. Pectin transeliminases โ€ข Pectic lyases โ€ข Cleave the a-1-4 glycosidic bonds between galacturonic acid residues by the transelimination of a proton from carbon atom 5 of the anhydromethylgalacturonate residue together with the oxygen of the adjacent atom of the a-1-4 glycosidic bond, to give a methyl galacturonide with a double bond between carbon atoms 4 and 5.
  • 11. Pectin esterases โ€ข Hydrolyze methyl ester groups to give free carboxyl groups and methanol
  • 12. โ€ข Most microorganisms produce AT LEAST ONE pectolytic enzymes. โ€ข Almost all fungi and many bacteria contain pectolytic enzymes that readily degrade pectin layers that bind the individual cells of fruit and vegetable tissue together into mixtures of oligosaccharides and galacturonic acids, a process that destroys the structural organization of plant tissue, which then becomes a soft amorphous mass. โ€ข This degradation of pectin layers in plant tissue is responsible for the spoilage process known as โ€œsoft rotโ€ in fruit and vegetables.
  • 13. Soft Rot Microorganisms Associated with Rotting Fruits and Vegetables Apples Bacillus polymyxa Citrus fruits Penicillium spp. Grapes Rhizopus nigricans Plums Yeast Raspberries Rhizopus stolonifer Strawberries Bacillus cereus Tomatoes Byssochlamys fulva Carrots Erwinia carotovora Celery Sclerotinia sclerotiorum
  • 14. Some strains of S. sclerotiorum produce photodynamic toxins, 8-methoxy psoralen and 4,5,8-trimethyl psoralen, which are responsible for dermatitis among celery harvesters.
  • 16. โ€ข Microbial spoilage of milk is a well-known phenomenon, milk being a perfect medium for microbial growth. โ€ข To prevent milk spoilage, heat treatment is a standard procedure in milk processing, i.e., pasteurization (i.e., LTHT, HTST, UHT) โ€ข Pasteurization is normally sufficient to destroy all bacteria in milk. However, even after this process, contamination by nonpathogenic bacteria may still occur, causing spoilage.
  • 17. Milk microbial spoilage defects โ€ข Production of lactic acid (souring) โ€ข Gas production โ€ขDevelopment of a viscous ropy texture โ€ขCoagulation of milk proteins โ€ข Lipolysis of milk fats (rancidity) โ€ขDevelopment of off-flavors Predominant causal organisms: lactic-acid-producing bacteria, which ferment lactose to lactic acid.
  • 18. Common lactic-acid-producing milk spoilage organisms โ€ข Homofermentative species (produce only lactic acid) Streptococcus lactis, S. cremoris, Lactobacillus casei, L. acidophilus, L. plantarum, L. helveticus, L. bulgaris โ€ข Heterofermentative species (produce lactic acid, acetic acid, ethanol, CO2) L. brevis, L. buchneri, L. fermenti, L. thermophiles, Leuconostoc citrovorum, L. mesenteroides, Microbacterium lacticum, Micrococcus luteus, M. varians, M. freudenreichii
  • 19. Steps in lactose metabolism by lactic-acid- producing milk spoilage organisms 1. Hydrolysis of lactose to galactose and glucose by lactase 2. Conversion of galactose to glucose via a galactose inverting system, catalyzed by glucose-4-epimerase. 3. Utilization of glucose to produce more galactose, which serves as an intermediate for the conversion of galactose-1-phosphate to glucose-1- phosphate, catalyzed by hexose-1-phosphate uridyl transferase. 4. Conversion of glucose-1-phosphate to glucose-6-phosphate by phophoglucomutase 5. Metabolism of glucose-6-phosphate to pyruvate via the EMP pathway. 6. Reduction of pyruvate to lactic acid by lactate dehydrogenase.
  • 20. Scheme of Lactose Metabolism 1. Lactose ๏ƒ  Galactose and Glucose 2. Galactose ๏ƒ  Galactose-1-phosphate ๏ƒ  D-UDP-galactose ๏ƒ  D-UDP glucose 3. D-UDP-glucose ๏ƒ  D-Galactose-1-phosphate ๏ƒ  D-UPP-galactose + D-Glucose-1-phosphate 4. Glucose-1-phosphate ๏ƒ  Glucose-6-phosphate ๏ƒ  Pyruvic acid 5. Pyruvic acid ๏ƒ  Lactic acid glucose-4-epimerase phosphoglucomutase EMP pathway lactate dehydrogenase hexose-1-phosphate uridyl transferase lactase
  • 21. HydrolSyscihs eomf leac otof sleactose metabolism
  • 22. Scheme of Lactose Metabolism 1. Lactose ๏ƒ  Galactose and Glucose 2. Galactose ๏ƒ  Galactose-1-phosphate ๏ƒ  D-UDP-galactose ๏ƒ  D-UDP glucose 3. D-UDP-glucose ๏ƒ  D-Galactose-1-phosphate ๏ƒ  D-UPP-galactose + D-Glucose-1-phosphate 4. Glucose-1-phosphate ๏ƒ  Glucose-6-phosphate ๏ƒ  Pyruvate 5. Pyruvate ๏ƒ  Lactic acid glucose-4-epimerase phosphoglucomutase EMP pathway lactate dehydrogenase hexose-1-phosphate uridyl transferase lactase
  • 23. phosphoglucomutase EMP pathway ๏ƒ  Glucose-6-phosphate ๏ƒ  Pyruvate Lactic acid lactate dehydrogenase NADH NAD+
  • 24. Microbial Deterioration of Raw Sugar and Sugar Confectionery
  • 25. The principal effect of microbial deterioration of raw sugar is the LOSS OF SUCROSE, due to the inversion of sucrose to fructose and glucose by invertase-producing yeasts and fungi, as catalyzed by invertase.
  • 26. NOTE: This results in a reduction in sucrose content.
  • 27. The susceptibility of raw sugar to microbial attack depends on the composition of the molasses film on sugar crystals, particularly on the water activity of the sugar. Most sugars have aw = 0.60-0.75; thus, only osmophilic yeasts (aw = 0.60) and xerophilic fungi (aw = 0.65) are the main contaminants of raw sugars. This is helped by the slightly acidic pH (5-6) of sugars, which inhibits bacterial growth.
  • 28. Microorganisms Associated with Biodeterioration of Raw Cane Sugars โ€ข Fungi Aspergillus niger Penicillium expansum Alternaria brassicae Aspergillus flavus Monilla nigra Cladosporium herbarum โ€ข Yeasts Hansenula anomala Saccharomyces cerevisiae Candida utilis Pichia fermentans โ€ข Bacteria Bacillus subtilis B. megaterium Clostridia nigrificans C. thermosaccharolyticum
  • 29. Sources of microbial infection โ€ข Airborne contamination โ€ขWaterborne contamination โ€ข Infected sugar residues โ€ข Unsuitable storage conditions
  • 30. โ€ข Microbial deterioration of carbohydrates โ€ข Microbial deterioration of proteins and protein foods โ€ข Microbial deterioration of edible oils and fats
  • 31. PROTEINS are large biological molecules or macromolecules consisting of one or more long chains of amino acid residues. The term comes from the Greek word proteios, meaning โ€œprimaryโ€, โ€œin the leadโ€ or โ€œstanding in frontโ€. โ€ข Essential to all life โ€ข They are the major constituents of enzymes, antibodies, many hormones and body fluids such as blood, milk, and egg white.
  • 32. Of the many kinds of food spoilage, the microbial spoilage of proteins and protein foods is the most complex and perhaps the least understood, owing to the enormous complexity of structural proteins in nature and the wide variety of spoilage microorganisms associated with protein spoilage.
  • 33. Structural Proteins of Protein Foods 1. Myoglobin 2. Myofibrillar proteins 3. Collagen 4. Elastins 5. Keratins
  • 34. โ€ข An iron- and oxygen-binding protein found in the muscle tissue of vertebrates in general and in almost all mammals. โ€ข Is formed from one polypeptide chain and one heme molecule. โ€ข Heme is a pigment responsible for the color of red meat; thus, the color that meat takes is partly determined by the degree of oxidation of myoglobin. โ€ข Degraded by microbial proteases to amino acids and oligopeptides Myoglobin
  • 35. Myofibrillar proteins โ€ข Consist mainly of actomyosin and form the major part of muscle proteins. โ€ข Degraded by trypsin-like microbial proteinases
  • 36. Collagen โ€ข Found in connective tissues such as tendons and bone cartilage โ€ข Very resistant to degradation โ€ข Contain a high proportion of nonpolar amino acids (valine, leucine, and isoleucine, with proline and hydroxyproline groups) but no cysteine โ€ข Degraded by collagenases from Clostridium Clostridiopeptidase A Hydrolyzes glycylprolyl bonds and has pH optima of 7.7-8.0. Activated by Ca and inhibited by EDTA. Clostridiopeptidase B Cleaves peptide bonds adjacent to lysine and arginine and has pH optima in the range of 7.2-7.4. Specific for collagen and gelatin.
  • 37. Elastins โ€ข Found in connective tissues such as tendons and ligaments โ€ข Contain 90% nonpolar amino acids arranged in a random structure โ€ข Highly resistant to hydrolysis, heat, and maceration โ€ข Degraded by elastinases, which have been isolated from Flavobacterium elastolyticum, Aeromonas salmonicida, Bacillus subtilis, Pseudomonas aeruginosa, and P. mallei โ€ข These elastinases have the following characteristics: Have pH optima in the range of 7.0-9.0 Hydrolyze peptide bonds adjacent to glycine and proline Highly specific for elastin Inhibited by diisopropylfluorophosphate
  • 38. Keratins โ€ข The major constituents of wool, hair, nails, hooves, and fish scales โ€ข Contain large amounts of glycine and proline with about 8% cysteine. โ€ข Degraded by keratinases, which have been isolated from Streptomyces fradiae and S. microflavus. They have the following characteristics: Have pH optima in the range of pH 8.5-9.0 Degrade keratin Activated by calcium and magnesium ions Inhibited by EDTA
  • 39. In general, most bacteria are unable to colonize pure proteins unless sufficient peptides, amino acids and vitamins are present to enable them to produce proteases necessary for protein digestion. Food such as meat, fish, and cheese contain abundant quantities of amino acids and other nutrients; they are therefore readily colonized by most microorganisms.
  • 40. Principal Changes Associated with Protein Spoilage 1. Putrefaction Characterized by the production of foul odors and offensive textures and flavors, which arise from spoilage metabolites that result from the catabolic metabolism of low-molecular-weight peptides and amino acids by spoilage organisms. 2. Degradation of protein constituents Indicated by protein coagulation and liquefaction, rot development, and destruction of structural proteins such as collagen and elastin.
  • 41. Stages in the Protein Spoilage Process 1. Initial contamination and colonization of the protein food by microorganisms. 2. Rapid utilization and metabolism of low-molecular-weight compounds such as amino acids, dipeptides, lactic acid, and sugars present in meat or fish juices, which yield spoilage metabolites, e.g., cadaverine, putrescine, organic acids, CO2, H2S and NH3. At this stage, there is explosive microbial growth. 3. Increased production of microbial proteases by proteolytic spoilage microorganisms. The proteolytic breakdown of high-molecular-weight proteins to oligopeptides provides a continued supply of nutrients. The oligopeptides are then hydrolyzed to free amino acids, which are then metabolized to additional metabolites. This accumulation of metabolites eventually poisons the microorganisms themselves, slowing down the putrefactive processes.
  • 42. Amines Produced during Protein Spoilage Analysis of putrefied protein foods shows that a mixture of amines are produced by the anaerobic decarboxylation of amino acids. Such amines include the following: 1. Cadaverine from L-lysine by Bacillus cadaveris, E. coli, and Clostridium histolycum 2. Putrescine from L-ornithine by Clostridium septicum and C. welchii 3. Aminoburyic acid from glutamic acid by S. faecalis 4. Isobutylamine from L-valine by Proteus vulgaris and Pseudomonas cocovenans 5. Tyramine from tyrosine by S. faecalis 6. Tryptamine from tryptophan by S. faecalis and C. welchii NOTE: All reactions are catalyzed by decarboxylases.
  • 43. Organic Acids Produced during Protein Spoilage 1. Pyruvate from alanine by Bacillus subtilis, as catalyzed by alanine dehydrogenase (NAD-dependent) 2. ๏ข-Methyl-ฮฑ-ketovalerate from isoleucine, as catalyzed isoleucine oxidase 3. Indole from tryptophan to by E. coli and Proteus vulgaris 4. ฮฑ-Ketoglutarate from glutamate by E. coli, S. cerevisiae, C. sporogenes, as catalyzed by glutamate dehydrogenase 5. Fumarate from aspartate by E. coli, as catalyzed by aspartase 6. Pyruvate from serine, as catalyzed by serine dehydratase
  • 44. Organic Acids Produced during Protein Spoilage 7. ฮฑ-Ketobutyrate from threonine, as catalyzed by threonine dehydratase 8. Pyruvate from cysteine, as catalyzed by cysteine desulfhydrase 9. Acetate from pyruvate by L. delbrueckii, Proteus vulgaris, and P. fluorescens 9. Acetate from alanine and glycine by C. sporogenes 10. Isobutyric acid from valine and alanine 11. Isovaleric acid from leucine and alanine 12. Methylbutyric acid from isoleucine and alanine 13. Aminovaleric acid from proline 14. Aminohydroxyvaleric acid from hydroxyproline
  • 45. During the putrefactive process, anaerobic clostridia such as Clostridium butyricum, C. pasteurianum, C. acetobutylicum and C. sporogenes also produce large amounts of H2 via the reduction of hydrogen ions or protons as catalyzed by a specific hydrogenase requiring reduced ferredoxin as its cofactor. Ferredoxin 2 H+ H2 Fe3+ Fe3+ Other bacteria produce hydrogen and carbon dioxide from glucose, which is then degraded to pyruvate via the EMP pathway, which is then converted to acetate, formate and CO2.
  • 46. โ€ข In the late stage of putrefaction, spoilage microflora also produce proteinases that degrade various protein constituents by hydrolyzing peptide bonds to give low-molecular-weight oligopeptides and free amino acids. โ€ข Spoilage microorganisms also utilize muscle glycogen to produce trimethylamine, ammonia and dimethylamine, whose chemical determination forms the basis of several methods for determining fish and fish product spoilage.
  • 47. The pH optima of collagenase, elastinases, and keratinases lie in the alkaline range, i.e., 7.2-9.0; thus, the degradation of collagens, elastins, and keratins in protein foods is favored during the advanced stages of putrefaction when the various putrefactive amines such as cadaverine and putrescine produced increase the pH of the food from 5.5 to above 8.0 when proteolysis has become extensive.
  • 48. During extensive proteolysis, both bacterial and fungal proteolytic enzymes hydrolyze casein proteins, elastins, gelatins and collagens.
  • 49. โ€ข Microbial deterioration of carbohydrates โ€ข Microbial deterioration of proteins and protein foods โ€ข Microbial deterioration of edible oils and fats
  • 50. FATSare a wide group of compounds composed of long-chain organic acids, called fatty acids. A typical fat molecule consists of glycerol combined with three fatty acids, i.e., it is a triol (i.e, it has three chemically active -OH groups). Fats are formed when each of these three -OH groups reacts with a fatty acid, resulting in triglycerides. โ€ข Hydrophobic, and generally soluble in organic solvents but insoluble in water. โ€ข Shorter-chain fats are usually liquid at room temperature, whereas longer-chain fats are solid. NOTE: Fats differ from carbohydrates and proteins in that they are not polymers of repeating molecular units
  • 51. โ€œOilโ€, โ€œfatโ€, and โ€œlipidโ€ are often used interchangeably. Of these, lipid is the general term. Oil is the term usually used to refer to fats that are liquid at room temperature, while fat to those that are solid at room temperature.
  • 52. In fat-containing foods, the biodeterioration of edible oils and fats by bacteria and fungi is the principal cause of spoilage indicated by the following: Rancidity Acidity Soapiness Off-flavors Discolorations
  • 53. Butter and Margarine Butter is an emulsion of water in butterfat of the following composition: 80-83% butterfat 16% water 1% nonfat milk solids 0-3% sodium chloride Margarine is also a water-in-fat emulsion: 80% fat (a mixture palm, coco and marine oils) 20% water Both are subject to microbial spoilage characterized by rancidity, acidity, off-flavors and discolorations.
  • 54. Causes of Butter and Margarine Rancidity 1. Autooxidative deterioration โ€ข Oxygen absorption and oxidation of unsaturated fatty acids (e.g., linoleic, linolenic and arachidonic) to hydroperoxides, which are oxidized to ketones and aldehydes. โ€ข Generally occurs during prolonged storage at ambient temperature โ€ข Catalyzed by cupric and ferric ions, UV and high storage temperatures (>5ยฐC) 2. Lipolysis of natural and synthetic triglycerides in fats โ€ข Effected by milk and microbial lipases โ€ข Prevented by pasteurization of milk and heat treatment of butter 3. Lipoxidation โ€ข Hydroperoxide production by specific microbial lipoxidases.
  • 55. Microorganisms Associated with Butter and Margarine Rancidity Butter and margarine rancidity is generally associated with lipolytic molds and yeasts. Aspergillus tamari A. chevalieri Cladosporium suaveolens Cladosporium butyri Candida lipolytica Ospora lactis Paecilomyces aureocinnamoneum Pseodomonas fluorescens Margarinomyces bubaki Penicillium glaucum Epicoccum purpurescens Micrococci
  • 56. Rancidity and acidity are caused by the production of free fatty acids, particularly butyric, caproic, caprylic and capric acids, and their corresponding methyl ketones. These volatile free fatty acids and methyl ketones directly arise from the metabolism of liberated free fatty acids by ๏ข-oxidation to the corresponding ๏ข-keto acid, which is decarboxylated to methyl ketones or is cleaved to give acetyl-coA and a lower fatty acid that is two carbons shorter. Secondary alcohols are also formed by the reduction of various methyl ketones.
  • 57. On the other hand, characteristic soapy flavors are produced by the liberated lauric and myristic acids that are present as triglycerides in butterfat and coconut oils.
  • 58. Factors Affecting Microbial Growth in Food 1. Temperature 2. Water activity 3. Humidity 4. pH 5. Oxygen availability 6. Osmotic pressure
  • 59. Temperature Storage temperature is considered the most important factor that affects food spoilage, as it determines the type of microfolora that will cause spoilage; however, the relative humidity and availability of oxygen must also be controlled. Microorganisms have been reported to grow over a wide temperature range, the lowest being โˆ’34ยฐC and the highest being 90ยฐC. All microorganisms, however, have an optimum temperature as well as a range in which they will grow. This preference for temperature forms the basis of dividing microorganisms into the following groups: Psychrohiles Psychrotrophs Mesophiles Thermophiles
  • 60. Types of Organisms by Growth Temperature Psychrophiles Grow best between -2 and 7ยฐC Psychrotrophs Optimum growth from 20 to 30ยฐC, but can grow at ca. 7ยฐC Mesophiles Optimum growth at 30โ€“40ยฐC, but can grow between 20 and 45ยฐC Thermophiles Optimum growth between 55 and 65ยฐC, but can grow at temperatures as low as 45ยฐC.
  • 61. Temperature Just as molds can grow over a wide range of pHs and moisture contents, they can also tolerate a wider temperature range than bacteria. Many molds can grow in the refrigerator. Yeasts are not usually found growing in the thermophilic temperature range, but prefer psychrotrophic and mesophilic temperatures.
  • 62. Water Activity Microorganisms cannot grow in a water-free environment, as enzyme activity is absent and most chemical reactions are greatly slowed down. Fresh vegetables, fruit, meat, fish and some other foods naturally have a high moisture content, averaging about 80%. Drying is one of the oldest methods of food preservation as it reduces moisture availability, thereby limiting the number and types of microorganisms that can grow and reducing the rate at which they can do so. A measure of this parameter is called water activity, denoted by aw.
  • 63. Water Activity Water activity is a measure of water available to microorganisms. Pure water has a water activity of 1.0 while most fresh foods have a water activity of about 0.99. In general bacteria require a higher aw than yeasts and molds. Most spoilage bacteria cannot grow at aw < 0.91, with Clostridium botulinum having a minimum growth level of 0.94. Staphylococcus aureus, has, however, been found to grow at aw as low as 0.84. The lowest reported aw for bacterial growth is 0.75. Most spoilage molds cannot grow at aw < 0.80. The lowest reported aw for any mold growth is 0.65, and that for yeasts 0.61.
  • 64. Minimum Water Activities for growth of Different Microorganisms Normal bacteria 0.91 Normal yeasts 0.88 Normal molds 0.80 Xerophilic molds 0.65 Osmophilic yeasts 0.60
  • 65. Humidity The humidity of the environment is important as it affects the aw of the food as well as the moisture content of the food surface. Food can pick up moisture from the atmosphere. Under conditions of high relative humidity storage (e.g., in a refrigerator), surface spoilage can take place, unless food is adequately protected by packaging.
  • 66. pH Most microorganisms grow best at neutral pH and only a few are able to grow at a pH lower than 4.0. Bacteria are more fastidious about their pH requirements than yeasts and molds. The fact that pH can limit microbial growth is a basic principle of food preservation and has been exploited for thousands of years. Fermentation and pickling extend the shelf-life of food products by lowering the pH. The fact that no known spore-forming pathogenic bacteria can grow at pH < 4.6 is the basis for the food sterilization principle for low-acid and acid foods.
  • 67. Oxygen Availability Controlling the availability of free oxygen is one means of controlling microbial activity in food. Although oxygen is essential for carrying out metabolic activities that support all forms of life, some microorganisms use free atmospheric oxygen, while others metabolize oxygen (reduced form) bound to other compounds such as carbohydrates. Microorganisms can be broadly classified into two groups: aerobic and anaerobic. Aerobes grow in the presence of atmospheric oxygen, while anaerobes, in the absence of atmospheric oxygen. In between these two extremes are facultative anaerobes, which adapt and grow with or without atmospheric oxygen, and microaerophilic organisms, which grow in the presence of reduced amounts of atmospheric oxygen.
  • 68. Oxygen Availability At the surface and within protein foods, oxygen availability and oxygen tension govern the numbers and type of food-colonizing spoilage microorganisms. The exposed surface of fresh meat and fish have a high oxygen tension and therefore support a large number of aerobic microorganism, such as Pseudomonas spp., Achromobacter spp., bacilli, micrococci, yeasts and fungi.
  • 69. Osmotic Pressure Osmotic pressure is inversely related to water activity. As the osmotic pressure of any system increases, water activity decreases. Thus, high osmotic pressures are normally incompatible with living organisms due to the osmotic effects that tend to dehydrate living cells.
  • 70.
  • 71. Food spoilage associated with protein degradation Type of food Spoilage Milk Cogulation of caseins, off-flavors, racidity, putrefaction, cadaverine Meats Surface slimes, liquefaction, degradation of collagen, elastin, keratin, putrefaction, cadaverine, putrescine Fish Fishy odor, TMA, DMA, surface slimes, H2S, cadaverine, putrescine, indole Hams, bacon, chicken, turkey Greening, putrefaction, liquefaction, bone taint, rancidity Eggs White, rot, black rot, mixed rot, fungalinfections Cheese moldy
  • 72. Food spoilage associated with fats degradation Food Spoilage Milk Souring cream Rancidity, free fatty acid butter Free fatty acid Margarine Rancidity, methyl ketones Oats Bitterness Wheat Soapiness Rapeseed oil Lipoxidation
  • 73. Microbial lipases โ€ข Cleave triglycerides at either โ€ข 1,3 position โ€ข 2-position
  • 74. ๏ข-oxidation โ€ข Yield keto-acids, methyl ketones, secondary alcohols, shorter fatty acids such as butyric, propionic acid, acetic acid
  • 75. Acyl CoA dehydrogenase Enoyl CoA hydratase ๏ข-hydroxylCoA dehydrogenase Thiolase Pathway for ๏ข-oxidation Of a Fatty Acid

Editor's Notes

  1. Schematic structure of pectin. Pectin consists of four different types of polysaccharides, and their structures are shown. Kdo, 3-Deoxy-d-manno-2-octulosonic acid; DHA, 3-deoxy-d-lyxo-2-heptulosaric acid. HG and RGI are much more abundant than the other components (see text). 3-C-carboxy-5-deoxy-l-xylose aceric acid
  2. Some strains of S. Sclerotiorum produc photodynamic toxins, 8-methoxy psoralen and 4,5,8-trimethyl psoralen, which are responsible for dermatitis among celery harvesters.
  3. Pyruvate to lactic acid --REDOX
  4. Xerophilic fungi are yeasts and moulds that are capable of growth at or below a water activity (aw) of 0.85. E.g. Aspergillus,ย Penicilliumย andย Eurotium. Osmophilic organismsย are microorganisms adapted to environments with high osmotic pressures, such as high sugar concentrations.