Spoilage of Fruits and Vegaetables PDF

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Dr. Indu Sharma...

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Spoilage of Fruits and Vegetables

Fruits and vegetables are an integral part of daily diets of man all over the world. The spoilage of fruits and vegetables normally occurs after harvest. However, some plant pathogens cause spoilage even before harvesting. The post-harvest spoilage of fruits and vegetables occur mostly during packing, transport and storage, before being processed or consumed. Unlike other food products, the fruits and vegetables will be undergoing physiological changes for quite some time even after harvesting due to the process of ripening. During the ripening process, the autolytic enzymes are active, breaking down the carbohydrates. Apart from the enzymes, the microorganisms are also involved in causing the spoilage of the fruits and vegetables from preharvest to post-harvest stage. These are the fungi and bacteria. Spoilage by fungi and bacteria The dominant spoilage organisms in fruits are fungi, as their pH is low (normally 5), whereas the pH of vegetables is from 5.0 to 7.0, which makes them susceptible to both fungi and bacteria. The spoilage is affected through the formation of rot, which is due to the ability of fungi and bacteria to secrete pectolytic enzymes. The rot is characterized by the softening of the tissue due to the action of pectolytic enzymes. Most important moulds involved in the spoilage of fruits and vegetables are Penicillium and Rhizopus. Although bacterial spoilage of fruits and vegetables is less when compared to moulds, nevertheless, certain strains of bacteria still cause spoilage. The important organisms belong to Erwinia Spp. and Pseudomonas Spp. Among them, Erwinia carotovora is important. Fungi responsible for the spoilage of fruits and vegetables Fruits and vegetables Majority of fruits and some vegetable Several fruits and vegetables Several fruits and vegetables

Genus Penicillium

Spoilage Effects Blue rot

Rhizopus Sclerotinia

Soft rot Watery soft rot in vegetables,

Potato, tomato, citrus fruits and many Geotrichum

brown rot in fruits Sour rot

vegetables Cabbage, cauliflower,

Black rot

potato,

lemon,

Alternaria

orange, apple and pear

Spoilage of Soft Drinks, Fruit Juices, Fruit Preserves

Soft drinks, fruit juices and fruit preserves are the unique products, as they are acidic by nature, to which antimicrobial ingredients are added. The soft drinks are of two types. One, carbonated, and the other one is non-carbonated. The carbonated soft drinks are manufactured by the absorption of carbon dioxide in the potable water containing flavours and colours. Other ingredients like sweeteners, foaming, emulsifying and stabilizing agents may also be added. The non-carbonated drinks, apart from the above ingredients, may contain fruit juices and vitamin C. The fruit juices are liquids, extracted from mature fruits. The bottled commercial fruit juices are diluted and blended with additives to give the required taste. The fruit preserves or jams are viscous or semi-solid products containing single fruit or mixed fruits. Other ingredients like jellying agents, pectin, gums etc., are added. The concentrated drinks are both with and without fruits. They have to be diluted appropriately before consumption, to suit the taste. The above mentioned types of beverages contain either natural antimicrobial properties like the low pH, low water activity or added preservatives to enhance the shelf life. But the spoilage of these products still takes place. Yeasts and moulds are the principal organisms that can grow in soft drinks, juices and fruit preserves. Certain lactic and acetic acid organisms among bacteria also can grow. Spoilage by yeasts Yeasts dominate in the spoilage of fruit products which contain high acid content due to their ability to tolerate high acid environment. Yeasts are osmophiles but they can tolerate only high sugar environment but not salt. They also have the ability to grow anaerobically and have low nutritional requirements coupled with the ability to synthesize the nutrients required for growth and survival. Among the types of yeasts, the ascospore-forming and heat-resistant organisms like Saccharomyces cerevisiae and S. chevalieri are found responsible for the spoilage in canned fruit-products. The growth of yeast in a product results in the formation of CO2, development of turbidity,

clumping and flocculation (forming wooly cloudlike

aggregations). Spoilage by moulds Like yeasts, moulds also can tolerate high acid environment, sustain low water activity and grow with minimum nutrients. But majority of the moulds are strict aerobes which

restrict their growth in fruit containing products due to low redox potential prevailing in them. The moulds which have been found responsjble for the spoilage of fruit products are Penicillium riotatum, Penicillium roquefortii, Cladosporium spp or Byssochlamys spp. Moulds can also tolerate high temperatures, when compared to yeasts and as such they are found in pasteurized fruit products. The organisms include Byssochlamys fulva, Thermoascus aurantiacum etc. Spoilage by bacteria The Lactobacillus and Leuconostoc species have been reported in spoiled fruits and soft drinks. Gluconobacter species is also responsible for the spoilage of fruit products. The spoilage of fruit products due to lactic acid bacteria causes opalescence in soft' drinks, gas bubbles and bursting of containers.

1. Lactic acid bacteria The lactic acid bacteria are Gram-positive, catalase negative organisms which can grow under anaerobic conditions. These are rod-shaped (lactobacilli), or cocci (pediococci and leuconostocs) . The homofermentative species produce mainly lactic acid from hexose sugars; the heterofermenters produce one molecule of lactic acid, one molecule of carbon dioxide, and a two-carbon compound, which is usually acetic acid or ethanol or a combination of the two. Growth of lactic acid bacteria in juices and other fruit products cause the formation of haze, gas, acid, and a number of other changes. Certain heterofermentative lactobacilli lead to slime in cider. The lactobacilli and leuconostocs that are present in citrus juices generate acetylmethylcarbinol and diacetyl, compounds that give the juices an undesirable, buttermilk-like flavor. Some strains, being extremely tolerant to ethanol grow in wines. Lactobacillus fructivorans can grow in appetizer and dessert wines containing as much as 20% ethanol. Lactic acid bacteria have the ability to decarboxylate malic acid to lactic acid. This malolactic fermentation is often desirable in high-acid wines because the acidity is reduced and

desirable flavors are

produced. Oenococcus oenos is the most acid and alcohol-tolerant species and often is isolated from wines that are undergoing a malo-lactic fermentation.

2. Acetic acid bacteria These

are

Gram

negative,

aerobic

rods

having

two

genera,

viz. Acetobacter and Gluconobacter . Both of these species oxidize ethanol to acetic acid

under acidic condition, Acetobacter species can oxidize acetic acid to carbon dioxide thus, the genus is called as over oxidizer. Because the bacteria are obligate aerobes, juices, wines, and cider are most susceptible to spoilage while held in tanks prior to bottling. Some strains of Acetobacter pasteurianus and Gluconobacter oxydans produce microfibrils composed of cellulose, which leads to formation of flocs in different fruit juice beverages.

3. Spore formers Spores are heat resistant, so role of organisms producing spores is important in heat treated juices and beverages. Variuos spore formers such as Bacillus coagulans, B. subtilis, B. macerans, B. pumilis, B. sphaericus , and B. pantothenticus have been found to grow in different types of wines. Some of these organisms have also been involved in canned fruits. Spore-forming bacilli that actually prefer a low pH have been responsible for spoilage of apple juice and a blend of fruit juices. Types of microbial spoilage in Fruits and Vegetables The most commonly occurring types of microbial spoilage of fruits and vegetables are as follows: 1. Bacterial soft rot: caused by Erwinia crtatowa and related species, which are fermenters of pectins, Pseudomonas

marginalis, Clostridium and Bacillus spp.

It

results

water-soaked

appearance, a soft, mushy consistency, and often a bad odor. 2. Gray mold rot: caused by species of Botrytis, eg: B.cinerea, which is favored by high humidity and warm temperature. 3. Rhizopus soft rot: caused by species Rhizopus, eg R.stolonifer. A rot results that often is soften and mushy. The cottony growth of the mold with small, black dots of sporangia often covers masses of the foods. 4. Anthracnose: usually caused by Colletotrichum lindemuthianum, C. coccodes and other species. The defect is a spotting of leaves and fruit or seedpods. 5. Alternaria rot, caused by Alternaria tenuis and other species. Areas become greenish-brown early in the growth of the mold and later turn to brown or black spots. 6. Blue mold rot: caused by species of Penicilfium digitatum and other species. The bluish-green color that gives the rot its name results from the masses of spores of the mold.

7. Downy mildew: caused by species of Phytophthora, Bremia, and other genera. The molds grow in white, woolly masses. 8. Watery soft rot: caused chiefly by Sclerotinia sclerotiorum, is found mostly in vegetables. 9. Stem-end rots, caused by species of molds of several genera, e.g., Diplodia, Alternaria, Phomopsis, Fusarium, and others, involve the stem ends of fruits. 10. Black mold rot, caused by Aspergillus niger. The rot gets its name from the dark-brown to black masses of spores of the mold, termed "smut". 11. Black rot, often caused by species of Alternaria but sometimes of Cera?tostomella, Physalospora, and other genera. 12. Pink mold rot, caused by pink-spored Trichothecium roseum. 13. Fusarium rots, a variety of types of rots caused by species of Fusarium. 14. Green mold rot, caused usually by species of Cladosporium but some?times by other greenspored molds, e.g., Trichoderma. 15. Brown rot, caused chiefly by Sclerotinia (Monilinia fructicola) species. 16. Sliminess or souring, caused by saprophytic bacteria in piled, wet, heating vegetables.

EAT HEALTHY TO A LONG LIFE Dr. Indu Sharma

Assistant Professor, Microbiology department Assam University...