Food Microbiology

Preservation, and Spoilage of Vegetables and Fruit

Contamination of Vegetables And Fruit Preservation of Vegetables  Microorganisms on the surfaces of freshly picked fruits and vegetables comprise not just the...

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This article writter by MN Editors on November 23, 2022

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Preservation, and Spoilage of Vegetables and Fruit
Preservation, and Spoilage of Vegetables and Fruit
  • It is believed that one-fourth of all harvested produce spoils before being consumed.
  • Fresh fruits and vegetables typically perish during storage, shipping, and while awaiting processing.
  • Unlike many other foods mentioned in this book, fruits and vegetables remain “alive” for an extended period of time after harvesting and prior to preparation.
  • The consequent respiration of these goods and the natural ripening process make it difficult to discuss the microbiological deterioration of fruits and vegetables independently.
  • Many of the reported microbiological deterioration issues are in fact “market illnesses” of these products and are described in plant pathology texts.
  • Produce may be consumed fresh, dried, frozen, fermented, pasteurised, or in a jar.

Contamination of Vegetables And Fruit

  • As soon as fruits and vegetables are harvested and placed into boxes, lugs, baskets, or trucks, they are susceptible to infection with rotting organisms from each other and from the containers if they have not been sufficiently cleaned.
  • Mechanical damage may enhance susceptibility to decay and microorganism growth may occur during transit to market or the processing plant.
General Microbiological Profile of Harvested Fruits and Vegetables
General Microbiological Profile of Harvested Fruits and Vegetables
  • Precooling the product and transporting it refrigerated will inhibit its growth. The washing of the fruit or vegetable may comprise a preparatory soaking, water agitation, or, preferably, a spray treatment.
  • Soaking and agitated washing have a tendency to spread rotting organisms from damaged to complete goods.
  • Recirculated or reused water is likely to add organisms, and the washing procedure may leave surfaces sufficiently moist to allow organism development during storage. The quantity of germs on food will be reduced by washing with detergent or germicidal treatments.
  • Microorganisms are eliminated by sorting and pruning damaged fruits and vegetables, but additional handling may cause mechanical damage and increase susceptibility to decay.
  • When these products are sold in the retail market unprocessed, they are typically not exposed to a great deal of additional contamination, with the exception of storage in contaminated bins or other containers, possible contact with decaying products, handling by salespeople and customers, and possibly spraying with water or packing with chipped ice.
  • This spraying gives vegetables a fresh appearance and prevents decomposition, but it also introduces organisms from water or ice, such as psychrotrophs, and provides a moist surface to promote their growth during longer storage.
  • In the processing facility, fruits and vegetables are exposed to additional contamination, and some procedures may lower the likelihood of microbial development or the number and variety of organisms.
  • Peeling by steam, hot water, lye, or blanching reduces the quantity of germs on the food, as does adequate washing at the plant (heating to inactivate enzymes, etc.,) Product sweating during handling increases quantities.
  • Trimming, mechanical abrasion or peeling, cutting, pitting or coring, and various types of disintegration may introduce pollutants from the involved equipment.
  • In reality, every piece of equipment that comes into touch with food can be a substantial source of bacteria if it has not been appropriately cleaned and sanitised. To assist such treatments, modern metal equipment with smooth surfaces and no cracks, dead ends, etc. is manufactured.
  • Trays, bins, tanks, pipes, flumes, tables, conveyor belts and aprons, fillers, blanchers, presses, screens, and filters are examples of potential sources of food contamination with microorganisms.
  • As they are difficult to clean and sanitise, wooden surfaces and textile surfaces, such as those on conveyor belts, are possible sources of contamination. The accumulation of germs in neglected areas of any food-handling system can taint the food.
  • Blanching in hot water, despite reducing the total number of organisms on the food, may result in the accumulation of spores of thermophilic bacteria, leading to the deterioration of canned items. e.g., peas have flat, sour spores.
  • Populations of microorganisms that accumulate on equipment as a result of microbial development in the exudates and residues of fruits and vegetables can have a significant impact on the level of contamination and the proliferation of pollutants.
  • Not only is it possible that enormous numbers of organisms might be added from this source, but it is also likely that these organisms will be in their logarithmic growth phase and therefore capable of sustaining rapid growth.
  • This effect is most noticeable after blanching vegetables. This heat treatment significantly lowers the bacterial content, destroys numerous surviving cells, and as a result lengthens their lag duration.
  • On the other hand, if enough time is allowed before freezing, drying, or canning, actively developing contaminants from the equipment might reach significant quantities; such development is typically the cause of extremely high bacterial counts.
  • The presence of fruit rot increases the amount of bacteria in fruit juices. When soft-rotted fruits are added to orange juice, for example, the amount of coliforms and the quantity of coliforms increase significantly.
  • Prior to extraction, heating grapes minimises the amount of microorganisms in the extracted juice, whereas pressing introduces contamination. The types of microorganisms on equipment will depend on the product being processed, as the product will serve as the organisms’ culture medium.
  • Thus, pea residues would support bacteria that thrive in a pea medium, whereas tomato residues would promote the growth of organisms that thrive in tomato juice. As the equipment is utilised during the course of the day, the organisms can continue to multiply.
  • At the end of the run, when equipment is cleaned and sterilised, the total number of germs on the equipment is drastically decreased, and if the procedure is effective, only the resistant forms survive.
  • Therefore, spores of bacteria are likely to survive, and if circumstances for growth exist while the equipment is idle, the quantity of these sporeformers may increase, particularly in badly cleaned sections. The accumulation of these thermophilic sporeformers, which are so problematic for vegetable canners, increases the difficulty of giving the goods a sufficient heat treatment.
  • On improperly cleaned and sterilised equipment, the quantity of such organisms may be large at the beginning of the day and decrease as the day proceeds, but the opposite is typically true.
  • A layoff during the run facilitates a resumption of population growth. Clearly, the amount of germs that enter foods from equipment depends on the growth possibilities provided to these organisms, which are the result of inadequate cleaning and sanitization paired with favourable circumstances of moisture and temperature over an extended period of time.
  • Added components such as sugar and starch may introduce spoiling organisms, particularly thermophilic bacterium spores.

Preservation of Vegetables 

Microorganisms on the surfaces of freshly picked fruits and vegetables comprise not just the regular surface flora, but also microorganisms from the soil and water, and possibly plant diseases. Additionally, a variety of moulds and occasionally a few yeasts may be present. Some germs may thrive between harvesting and processing or consumption of vegetables if the surfaces are damp or if the outer surface has been damaged. Temperature and humidity can be effectively managed to inhibit its growth.

Asepsis Technique

  • Even though there will be some contamination of vegetables between harvesting and preparation or consumption, gross contamination can be prevented.
  • Boxes, lugs, baskets, and other containers should be substantially free of microbial development between uses, and some will require washing and sanitation.
  • The lugs and other containers used to transport peas to the processing factory are examples. On their moist interior, these containers may support a substantial amount of bacterial development and serve as a source of a high number of organisms on the peas.
  • Vegetables in the process of rotting that come into contact with fresh produce can spread contamination and may result in losses.
  • Cleaning and sanitising the processing plant’s equipment adequately can reduce contamination.
  • The accumulation of heat-resistant spores of spoilage bacteria, such as those of flat sour bacteria, putrefactive anaerobes, or Clostridium thermosaccharolyticum*, is of particular concern.
  • Upon arriving to the plant, the bacterial count on fresh vegetables to be processed might range from 10^2 to 10 ^ 7 per gramme, depending on the species and condition.

Removal of Microorganisms 

Thoroughly washing vegetables removes the majority of surface pollutants but leaves the majority of the natural microbial surface flora intact. If the wash water is not of good bacteriological quality, it may introduce organisms, which may then cause growth on the damp surface. Sometimes, chlorinated water is used for washing, and detergents are added to aid in the removal of dirt and bacteria. Washing with a nonionic detergent solution can eliminate a portion of the mould growth on strawberries, for example.

1. Use of Heat 

  • To deactivate the enzymes in vegetables that will be dehydrated, frozen, or canned, they are scalded or blanched before drying, freezing, or canning.
  • Simultaneously, the quantity of microorganisms is lowered significantly, maybe by 1,000 to 10,000 times.

2. Use of Low Temperatures 

  • As previously stated, a few types of relatively stable vegetables, such as root vegetables, potatoes, cabbage, and celery, can be maintained for a limited duration through conventional or basement storage.

a. Chilling

  • The majority of vegetables that are to be preserved without extra processing are rapidly cooled and stored at chilling temperatures.
  • Utilizing cold water, ice, mechanical refrigeration, or vacuum cooling (moistening with evacuation), as with lettuce, chills the product.
  • In many instances, precooling, or cooling prior to regular cold storage, is accomplished immediately after harvesting with the application of a cold water spray, a method known as hydrocooling.
  • Each type of vegetable has its own recommended storage temperature and relative humidity. The “refreshment” of leafy vegetables (lettuce, spinach) with a water spray can cool the produce if cold water is used and will aid in the preservation of the produce.
  • Controlling the atmospheric composition during the preservation of vegetables has not been as prevalent as with fruits.
  • Some employees have advocated the addition of carbon dioxide or ozone to the air.
  • Ultraviolet rays have not proven effective since they do not reach all of the vegetable’s surfaces during packaging and handling.
  • Sweet potatoes are an example of a vegetable that requires particular cold storage conditions. Normal potatoes get sweet at temperatures between 2.2 and 4.4 degrees Celsius and must be stored at higher temperatures if they are to be used to make potato chips.
  • Before storage, sweet potatoes and onions undergo unique curing processes.

b. Freezing 

  • The selection and preparation of vegetables, as well as their blanching and freezing, as well as the changes that occur during these processes. Microorganisms of the natural flora and pollutants from the soil and water are found on the surface of plants.
  • If the surfaces are damp, some of these organisms will grow before the vegetable reaches the freezing facility.
  • There, washing reduces the quantity of some organisms and increases the number of others, while scalding or blanching (86 to 98 C) reduces the number of organisms by as much as 90 to 99 percent.
  • During the chilling and handling prior to freezing, however, there is a chance for recontamination by equipment and the proliferation of organisms, so that under inadequate conditions one million or more organisms per gramme of vegetable may be present at the time of freezing.
  • The freezing procedure reduces the quantity of organisms by a percentage that varies depending on the types and numbers of organisms that were initially present, but on average, around half of them are killed.
  • Tabulate the changes in the quantity of organisms on snap beans as they undergo various freezing plant activities. During storage in a frozen state, the number of organisms decreases steadily, but there are at least a few survivors of most types of organisms after the typical storage duration.
  • The temperature and duration of thawing will determine the type of bacteria most likely to develop. When the thawing temperature is relatively low, Micrococcus species predominate on thawing vegetables such as sweet corn and peas. However, Achromobacter* and Enterobacter spp. additionally prevalent.
  • Lactobacilli are prevalent on peas under these conditions. The growth of one Micrococcus species may be followed by the growth of another species. At higher temperatures, Flavobacterium species may also proliferate.
  • As the small packages of quick-frozen veggies are often handled in the house, where the frozen food is poured quickly into boiling water, there is no possibility for microbial growth.
  • Most veggies wilt and become floppy while freezing, and frozen vegetables may suffer colour changes during storage.
  • There is a potential that food-poisoning bacteria will thrive and create toxin when thawed veggies are stored at room temperature for an extended period of time. Jones and Lochhead (1939) discovered, for instance, enterotoxin-producing staphylococci in frozen corn.
  • Sterilized maize infected with staphylococci of the food-poisoning type, frozen, thawed, and stored at 20 degrees Celsius for one day allowed for the growth of these cocci and the synthesis of enough enterotoxin to elicit food poisoning symptoms.
  • This enterotoxin would not be completely eliminated by the vegetable’s typical cooking process.
  • Clostridium botulinum has been discovered in frozen veggies, therefore its presence can be believed to be common.
  • Fortunately, the conditions for growth and toxin generation are rare; a power outage lasting several days in freezers after floods or hurricanes is an example of such settings.
  • Cooking frozen veggies does not eliminate all Clostridium botulinum spores, and cooked vegetables should not be left at room temperature for an extended period of time.
  • Counts of bacteria per gramme in frozen veggies might range from a few to 102. Coliforms and enterococci are frequently recoverable. Prevalence of E. coli is uncommon and may cause one to doubt hygienic practises.
Numbers of Organisms Per Gram on Snap Beans after Passing through Unit
Operations in a Freezing Plant
Numbers of Organisms Per Gram on Snap Beans after Passing through Unit Operations in a Freezing Plant

3. Drying 

  • As techniques for drying vegetables and vegetable products have evolved, public acceptance has increased, and a variety of dried food items are now widely available.
  • Dried vegetables and vegetable derivatives are used to flavour dried soups, together with dried spices and sauces.
  • Using the explosive puffing method, numerous vegetables can be dried. Typically, diced, partially dehydrated veggies are inserted in a revolving, closed chamber.
  • After applying heat and pressurising the chamber to a predetermined level, the pressure is released instantly.
  • This causes more water loss, but more importantly, a porous network of capillaries is created in the product.
  • Increased porosity facilitates additional drying and confers excellent rehydratability. The microorganisms that survive blanching may continue to grow until the time of drying and contribute to the total count of the dried product.
  • Yeasts and the majority of bacteria are killed by heat drying, however spores of bacteria and moulds typically survive, as do the more heat-resistant vegetative cells.
  • Microbial counts on dried vegetables, either after drying or as purchased on the retail market, are typically substantially higher than those on dried fruits, due to the likelihood of a higher number of microorganisms present before to drying and a greater proportion surviving drying.
  • The majority of veggies are less acidic than fruits, and as a result, their sensitivity to heat is diminished.
  • Samples of dried vegetables from retail stores contain hundreds of thousands or even millions of germs per gramme, despite the fact that these items can be manufactured to have a much smaller amount of organisms.
  • The microbial content of dried vegetables is diminished when they are sulfured to preserve their pale colour. There will be no growth of germs in the veggies if they are dried and stored properly.
  • During storage, there is a gradual drop in the number of live organisms, which is more fast in the first few months and then slows down.
  • The spores of bacteria and moulds, as well as some micrococci* and microbacteria, are resistant to desiccation and will survive better than other microorganisms; as the storage period lengthens, they will account for an increasing proportion of the survivors.

4. Use of Preservatives 

  • It is uncommon to apply preservatives to vegetables, however some veggies may receive particular surface treatment.
  • Rutabagas and turnips are sometimes paraffinized to increase their shelf life. On lettuce, beets, and spinach, zinc carbonate reportedly eradicates the majority of mould formation.
  • On potatoes, biphenyl vapours will control Fusarium. Experimentally, a controlled environment of carbon dioxide or ozone has been applied to refrigerated vegetables, but there has been limited practical application.

a. Added Preservative 

  • Sodium chloride is the sole commonly used chemical preservative. The amount of salt added to vegetables can range from 2.25 to 2.5% for sauerkraut to saturation for cauliflower.
  • Lower quantities of salt allows acid fermentation by bacteria; as the proportion of salt increases, the rate of acid production decreases until a concentration of salt is reached that prohibits bacterial growth and acid generation.
  • High-protein vegetables, such as green peas and lima beans, and vegetables that soften easily, such as onions and cauliflower, are maintained by adding sufficient salt to prevent fermentation: from 70 to 80° salometer (18.6 to 21.2 percent salt) to saturation (26.5 percent salt, or 100° salometer).
  • It should be noted that when brine or salt is added to vegetables, water is extracted, which decreases the salt content in the liquid.
  • Increased usage of sulfites as salad fresheners, i.e. to avoid enzymatic browning of lettuce, cole slaw, and other salad products, has resulted from the popularity of salad bars.
  • Sulfite residues in meals may be linked to asthma attacks, and their concentrations in restaurant salads have been measured.

b. Developed Preservatives 

  • Normal acid fermentation occurs at room temperature in shredded, chopped, or crushed vegetables containing sugar, but instead of a clean, acid flavour from the action of lactic acid bacteria, undesirable flavours and body changes may result from the growth of coliform bacteria, bacilli, anaerobes, and proteolytic bacteria, among others.
  • The addition of salt to such substances reduces competition from unwanted microorganisms, hence promoting lactic fermentation.
  • Additionally, the salt draws juice from the veggies and promotes a more even distribution of lactic acid bacteria.
  • The acidity that can be produced depends on the amount of sugar in the vegetable, while the amount of salt and temperature govern the rate of acid generation and the types of bacteria involved.
  • In general, when the salt concentration rises, the rate of acid generation slows and the number of affected bacterial species decreases.
  • Some recipes ask for a relatively modest salt concentration at the beginning, a gradual increase while fermentation progresses, and then enough salt to prohibit further bacterial development.
  • This method is used to brine vegetables like string beans and maize.

5. Preservation by Irradiation 

  • The treatment of most vegetables with gamma rays to destroy decay-causing microorganisms, followed by storage, has resulted in discoloration, softening, or other deterioration.
  • However, irradiation has been utilised successfully to prevent the sprouting of potatoes, onions, and garlic, as well as the reproduction of insects on certain vegetables.

Preservation of Fruits And Fruit Products 

  • In general, the preservation of fruits and fruit products involves the same principles as those for vegetables and vegetable products.
  • The surfaces of healthy fruits include the natural flora as well as contaminating microorganisms from soil and water; hence, their surface flora is comparable to that of vegetables, with yeasts and moulds predominating.
  • Moreover, some fruits carry plant diseases or saprophytic spoilage microbes that may proliferate after harvesting.
  • These imperfect fruits should be separated, and rotten pieces can be removed. A small number of bacteria inhabit the interior of occasional healthy fruits.

1. Asepsis 

  • Fruits, like vegetables, are susceptible to contamination from containers and rotting fruits between harvesting and processing, and care should be made to avoid contamination as much as possible.
  • Before harvest, fruits are frequently treated with pesticides and fungicides, which may affect their flora.

2. Removal of Microorganisms 

  • Thorough washing of fruits serves to remove not only dirt and hence casual contaminating microorganisms but also poisonous sprays. 
  • Washing may be with water, detergent solutions, or even bactericidal solutions such as chlorinated water. 
  • Trimming also removes microorganisms. 
  • Clear fruit juices may be sterilized by filtration.

3. Use of Heat Fruits 

  • seldom are blanched before other processing because blanching causes excessive physical damage. 
  • Note that the fruits are in one of two groups on the basis of their pH: the acid foods, such as tomatoes, pears, and pineapples, or the high-acid foods, such as berries. 
  • A steam-pressure sterilizer is not required for most fruits, since heating at about 100 C is sufficient and can be accomplished by flowing steam or boiling water. 
  • In general, the more acid the fruit, the less heat required for its preservation. Similar principles are involved in canning fruit juices. 

4. Use of Low Temperatures 

  • A few fruits, such as apples, can be preserved for a limited time in common or cellar storage, but controlled lower temperatures usually are employed during most of the storage period of fruits.

a. Chilling 

  • Each fruit has its own optimal temperature and relative humidity for chilling storage; even varieties of the same fruit may differ in their requirements. 
  • Fruits have been treated with various chemicals before or during storage to aid in their preservation. 
  • Thus hypochlorites, sodium bicarbonate, borax, propionates, biphenyl, o-phenylphenols, sulfur dioxide, thiourea, thiabendazole, dibromotetrachloroethane, and other chemicals have been recommended. 
  • Fruit also has been enclosed in wrappers treated with chemicals, e.g., sulfite paper on grapes, iodine paper on grapes and tomatoes, or borax paper on oranges. 
  • Waxed wraps, paraffin oil, paraffin, waxes, and mineral oil have been applied for mechanical protection. 
  • There has been considerable research on the combination of the chilling storage of fruits with control of the atmosphere of the storage room. 
  • This control may consist merely of regulation of the concentrations of oxygen and carbon dioxide in the atmosphere or may involve the addition or removal of carbon dioxide or oxygen or the addition of ozone. 
  • Controlled-atmosphere (CA) storage implies the altering of various gases from normal atmospheric concentration. 
  • Usually this is done by increasing the CO2 concentration and decreasing the O2 concentration. A related term, modified atmosphere (MA), is defined similarily, but MA storage is usually used to describe CA conditions which are not accurately maintained or conditions where the air is initially replaced with gas but no further measures are taken to keep the gas atmosphere constant. 
  • “Gas storage” means CA or MA storage. Under certain circumstances only one gas is used, e.g., packaging a product in 100 percent N2 ; this type of storage would more precisely be referred to as nitrogen gas storage. Several conditions of CA storage are compiled in Table.
  • The optimal concentration of carbon dioxide and oxygen and proportion of these gases varies with the kind of fruit and even with the variety of fruit. 
  • Although carbon dioxide storage has been employed chiefly with apples, it can be used successfully with pears, bananas, citrus fruits, plums, peaches, grapes, and other fruits. 
  • Ozone in concentrations of 2 to 3 ppm in the atmosphere has been reported to double the storage time of loosely packed small fresh fruits, such as strawberries, raspberries, currants, and grapes, and of delicate varieties of apples. 
  • Ethylene in the atmosphere is used to hasten ripening or produce a desired color change and is not considered preservative, although a combination of this gas and activated hydrocarbons has been suggested for the preservation of fruits.
Conditions of Controlled-atmosphere Storage for Several Fruits and Vegetables
Conditions of Controlled-atmosphere Storage for Several Fruits and Vegetables

b. Freezing 

  • The surfaces of fruits contain the natural surface flora plus contaminants from soil and water. Any spoiled parts that are present will add molds or yeasts. 
  • During preparation of fruits for freezing, undesirable changes may take place, such as darkening, deterioration in flavor, and spoilage by microorganisms, especially molds. 
  • Washing the fruit removes most of the soil microorganisms, and adequate selection and trimming will reduce many of the molds and yeasts involved in spoilage. 
  • With proper handling there should be little growth of microorganisms before freezing. Some fruits are frozen in large drums (up to 50 lb); it would be mandatory to cool the fruit before filling the container to ensure that the product is frozen quickly. 
  • The freezing process reduces the numbers of microorganisms but also usually causes some damage to the fruit tissues, resulting in flabbiness and release of some juice. 
  • During storage in the frozen condition the physical changes occur as well as a slow but regular decrease in numbers of microorganisms. 
  • Yeasts (Saccharomyces, Cryptococcus) and molds (Aspergillus, Penicillium, Mucor, Rhizopus, Botrytis, Fusarium, Alternaria, etc.) have been reported to be the predominant organisms in frozen fruits, although small numbers of soil organisms, e.g., species of Bacillus, Pseudomonas, Achromobacter*, etc., survive freezing. 
  • Yeasts are most likely to grow during slow thawing. Numbers of viable microorganisms in frozen fruits are considerably lower than in frozen vegetables. 
  • Large numbers of mold hyphae may be indicative of the freezing of inferior fruit that included rotten parts. 
  • The numbers of microorganisms in frozen fruit juices depend on the condition of the fruit, the washing process, the method of filtration, and the opportunities for contamination and growth before freezing. 
  • There may be from a few hundred to over 1 million organisms per milliliter present in the juice at the time of freezing. 
  • The inclusion of rotten parts of the fruit increases the numbers of organisms markedly. The washing process, especially the kind of solution used for washing, has a considerable influence on the numbers of organisms, since those on the surface of fruits are difficult to remove. 
  • Numbers can build up in the washing solution, on moist surfaces of the washed fruit, and in the juice itself before freezing. 
  • In the plant, too, there is an opportunity for the addition of organisms from the equipment. The freezing process markedly reduces numbers, but added sugar or increased concentration of the juice has a protective effect against killing. 
  • The decrease in numbers of organisms during storage in the frozen condition is slow but is faster than in most neutral foods. 
  • The kinds of organisms are chiefly those of soil, water, and rots, together with the natural surface flora of the fruit. Prominent usually are coliforms, enterococci, lactics, e.g., Leuconostoc and Lactobacillus species, Alcaligenes, and yeasts. 
  • Since coliform bacteria, mostly of the Enterobacter aerogenes type, form part of the natural flora of fruits, they are present in both fresh and frozen fruit juices. 
  • The use of decayed fruit for the juice increases the numbers of coliforms, but these organisms decrease during storage. 
  • Because coliforms normally are present, there are objections to the use of the presumptive test for coliforms to indicate sanitary quality of the juice. It has been suggested that tests be made for the fecal coliform, Escherichia coli, or Streptococcus faecalis. 

5. Drying 

  • The numbers of microorganisms in dried fruits are comparatively low and that spores of bacteria and molds are likely to be the most numerous. 
  • An occasional sample may contain high numbers of mold spores, indicating that growth and sporulation of molds has taken place on the fruit before or after dehydration. 
  • Alkali treatment, sulfuring, blanching, and pasteurization reduce numbers of microorganisms.

6. Use of Preservatives 

  • The chemicals have been applied to fruits chiefly as a dip or spray or impregnated in wrapper for the fruits. 
  • Among substances that have been applied to the outer surfaces of fruit are waxes, hypochlorites, biphenyl, and alkaline sodium o-phenylphenate. 
  • Wrappers for fruits have been impregnated with a variety of chemicals including iodine, sulfite, biphenyl, o-phenylphenol plus hexamine, and others. 
  • As a gas or fog about the fruit, carbon dioxide, ozone, and ethylene plus chlorinated hydrocarbons have been tried. 
  • Sulfur dioxide and sodium benzoate are preservatives that have been added directly to fruits or fruit products. 
  • Most of the chemical preservatives mentioned have been primarily antifungal in purpose. Green olives are the only fruits which are preserved on a commercial scale with assistance from an acid fermentation. 
  • Locally, other fermented fruits sometimes are prepared, such as fermented green tomatoes and Rumanian preserved apples. In all these products the lactic acid fermentation is of chief importance.

Spoilage of Vegetables and Fruit

  • Raw vegetables and fruits may deteriorate as a result of physical reasons, the operation of their own enzymes, microbial action, or a combination of these agents.
  • Mechanical damage caused by animals, birds, or insects, or by bruising, wounding, bursting, cutting, freezing, desiccation, or other maltreatment, may boost enzymatic activity or promote the entry and proliferation of microbes.
  • Previous damage by plant diseases may render the edible portion of a plant unfit for ingestion or allow saprophytes to proliferate and cause rotting.
  • Contact with rotting fruits and vegetables may result in the transfer of organisms that cause rot and increase waste.
  • During harvesting, transport, storage, and marketing, improper environmental conditions may promote rotting.
  • The majority of the discussion that follows will focus on microbial deterioration, although it should always be remembered that plant enzymes remain active in raw plant foods.
  • If oxygen is present, plant cells will continue to respire as long as they are alive, and hydrolytic enzymes will continue to function after cell death.
  • The suitability of foods for ingestion is somewhat determined by their ripeness. If the intended level of ripeness is significantly exceeded, the meal may be deemed inedible or even ruined.
  • A ripe banana, with its black exterior and brown, mushy interior, is an example. Vegetable and fruit diseases may develop from the growth of an organism that takes its nourishment from the host and typically damages it, or from bad environmental conditions that create anomalies in the vegetable or fruit’s functions and structures.
  • The following discussion will focus mostly on diseases produced by pathogens and decompositions induced by saprophytic organisms, despite the impossibility of making a clear difference between these two types of organisms.
  • However, non-organism-caused diseases should be highlighted since they may be confused with organism-caused diseases due to their resemblance in appearance.
  • Brown heart of apples and pears, blackheart of potatoes, black leaf speck of cabbage, and red heart of cabbage are examples of nonpathogenic illnesses.
  • No attempt will be made to address changes caused by plant pathogens growing on plants or parts of plants used for food prior to harvest; instead, we will consider microbial changes that may occur during harvesting, grading, packing, transportation, storage, and handling by wholesaler and retailer, although some of these changes may begin prior to harvest.
  • Only a general overview of the topic, from the perspective of a food microbiologist rather than a plant pathologist, is possible due to space constraints.

General Types of Microbial Spoilage 

The most prevalent or prevalent type of rotting varies not just by fruit or vegetable type, but also to some extent by variation. (2) Saprophytic organisms, which may be secondary invaders after action of a plant pathogen or may enter a healthy fruit or vegetable, as in the case of various “rots,” or grow on its surface, as when bacteria multiply on moist, piled vegetables. Occasionally, a saprophyte will replace a pathogen, or a succession of saprophytes may be responsible for the deterioration. Consequently, coliform bacteria may proliferate as secondary invaders and be present in significant numbers in fruit and vegetable juices if decaying goods are present. Although different types of breakdown and bacteria are predominant in the deterioration of each fruit and vegetable, several general categories of microbial spoilage are more prevalent than others. The most prevalent types of food deterioration are as follows:

  1. Bacterial soft rot: Erwinia carotovora and related species, which are pectin fermenters, cause bacterial soft rot. Additionally, Pseudomonas marginalis, Clostridium, and Bacillus species have been isolated from these decays. It results in a drenched appearance, a soft, mushy texture, and frequently a foul odor.
  2. Gray mold rot: Gray mould rot, produced by Botrytis species such as B. cinerea, derives its name from the mold’s grey mycelium. It thrives in conditions of high humidity and high temperature.
  3. Rhizopus soft rot: Rhizopus soft rot, produced by Rhizopus species such as R. stolonifer. The resulting rot is frequently soft and mushy. The cottony mould growth with tiny black sporangia spots frequently covers vast quantities of food. 
  4. Anthracnose: Colletotrichum lindemuthianum, C. coccodes, and other species commonly cause anthracnose. The imperfection consists of spotted leaves, fruits, or seedpods. 
  5. Alternaria rot: Alternaria decay, brought on by Alternaria tenuis and other species. Early on in the growth of the mould, patches become greenish-brown, which eventually transform into brown or black spots. 
  6. Blue mold rot: The decay induced by Penicillium digitatum and other forms of blue mould. Massive numbers of mould spores are responsible for the bluish-green hue that gives the rot its name. 
  7. Downy mildew: Downy mildew is caused by Phytophthora, Bremia, and other genera. Molds form white, fuzzy colonies.
  8. Watery soft rot: Watery soft rot, caused mostly by Sclerotinia sclerotiorum, primarily affects vegetables. 
  9. Stem-end rots: Fruit stem ends are susceptible to stem-end rots produced by fungi of several genera, such as Diplodia, Alternaria, Phomopsis, Fusarium, and others. 
  10. Black mold rot: Aspergillus niger produces black mould decay. The layperson’s term for the dark brown to black masses of mould spores that characterise rot is “smut.” 
  11. Black rot: Black rot, frequently caused by Alternaria species but sometimes occasionally by Ceratostomella and other genera.
  12. Pink mould rot: Pink mould rot induced by Trichothecium roseum with pink spores.
  13. Fusarium rots: Fusarium rots, a variety of rot forms caused by Fusarium species.
  14. Green mould rot: Green mould rot, typically caused by Cladosporium species but occasionally by other greenspored fungi, such as Trichoderma.
  15. Brown rot: The brown rot is primarily caused by Sclerotinia (Monilinia fructicola) species.
  16. Sliminess or sourness: Sliminess or sourness generated by saprophytic bacteria in moist, heated vegetables that have been heaped.

Fungal rots of fleshy fruits, such as apples and peaches, typically manifest as brown or cream-colored patches with mould mycelia growing in the tissue beneath the surface with aerial hyphae and spores appearing later.

Some varieties of fungal decay manifest as “dry rots,” in which the affected area is dry, brittle, and frequently discoloured. The decay of luscious fruits may lead to leaking. The composition of the fruit or vegetable impacts the sort of decay most likely to occur. Thus, bacterial soft rot is more prevalent in vegetables that are not very acidic, and it is restricted to vegetables that are not extremely acidic among fruits.

Molds are the most prevalent cause of deterioration since the majority of fruits and vegetables are somewhat acidic, rather dry on the outside, and low in B vitamins. Some fruits and vegetables support a wide range of spoilage microbes, whereas others support a relatively small number.

The Chief Market Diseases of Several Vegetables and Fruits.
The Chief Market Diseases of Several Vegetables and Fruits.
The Chief Market Diseases of Several Vegetables and Fruits.

The potential that spoilage organisms will enter is also a significant factor in determining the likelihood and type of spoiling. Entrance is facilitated by mechanical damage, plant diseases, and improper handling. Importantly, underground plant portions such as roots, tubers, and bulbs, as found in radishes, beets, carrots, and potatoes, are in direct touch with moist soil and become infected from that source.

Strawberries, cucumbers, peppers, and melons may be grown in direct touch with the soil surface. Leaves, stems, and flowers, such as those of lettuce, the greens, cabbage, asparagus, rhubarb, and broccoli, are especially susceptible to contamination by plant pathogens or damage by birds and insects, as are the leaves, stems, and flowers of the majority of fruits, regardless of whether they are typically classified as vegetables or “fruits.”

The nature of the spoiling will depend on the affected product and the organism. When food is soft and juicy, the rot is likely to be soft and mushy, and there may be some leakage. Some types of spoilage organisms, however, have a drying impact, resulting in dry or leathery rots or discoloured surface patches.

In certain situations, the mold’s mycelial growth is predominantly subterranean and just a small rotten patch is visible, as is the case with the majority of apple decay. In other types of deterioration, the mould mycelium is visible on the surface and may be coloured by spores. The identification of a form of fruit or vegetable decay enables the use of preventative measures against that type of decay.

Spoilage of Fruit and Vegetable Juices 

  • Juices may be extracted directly from fruits or vegetables, from macerated or crushed material containing a substantial amount of pulp, or by water, as in the case of prune juice.
  • These juices can be used in their natural concentrations, concentrated through evaporation or freezing, and conserved through canning, freezing, or drying.
  • Juices squeezed or extracted from fruits are more or less acidic, depending on the product, with pH ranging from around 2.4 for lemon or cranberry juice to 4.2 for tomato juice, and all contain sugars, with concentrations ranging from about 2% in lemon juice to nearly 17% in some grape juice samples.
  • Although moulds can and do form on the surface of such fluids when exposed to air, the high moisture content favours yeasts and bacteria, which proliferate more quickly.
  • Which of these will predominate in low-sugar and low-acid juices will depend more on temperature than composition.
  • The removal of particles from juices through extraction and sifting increases the oxidation-reduction potential and promotes yeast growth. Most fruit juices are sufficiently acidic and contain enough sugar to promote yeast growth at temperatures between 15.6 and 35 degrees Celsius.
  • Deficiency in B vitamins inhibits certain microorganisms. Therefore, it is normal for raw fruit juices stored at room temperature to undergo alcoholic fermentation by yeasts, followed by the oxidation of alcohol and fruit acids by film yeasts or moulds growing on the surface if exposed to air, or the oxidation of alcohol to acetic acid if acetic acid bacteria are present.
  • The sorts of yeasts that grow depend on the prevalent yeasts in the juice and the temperature, although the first fermentation is typically carried out by wild yeasts, such as apiculate yeasts, which produce only moderate amounts of alcohol and significant amounts of volatile acid.
  • At temperatures close to the extremes of the given range (15.6 to 35 C), unpleasant yeasts are more likely to develop than flavor-producing yeasts.
  • At temperatures above 32,2 to 35 degrees Celsius, lactobacilli would likely thrive and produce lactic and volatile acids because yeasts cannot survive at these temperatures.
  • At temperatures below 15.6 C, wild yeasts may grow, but as the temperature falls closer to freezing, bacteria and moulds are more likely to proliferate than yeasts.
  • Film yeasts and moulds can lower the acidity of a wine by developing on its surface. In addition to the typical alcoholic fermentation, fruit juices may undergo the following microbial changes:
    • The lactic acid fermentation of sugars, predominantly by heterofermentative lactic acid bacteria like Lactobacillus pastorianus*, L. brevis, and Leuconostoc mesenteroides in apple or pear juice, and by homofermentative lactic acid bacteria like Lactobacillus arabinosus*, L. leichmanii*, and Microbacterium.
    • By lactic acid bacteria, such as Lactobacillus pastorianus*, malic acid is converted to lactic and succinic acids, quinic acid to dehydroshikimic acid, and citric acid to lactic and acetic acids.
    • Leuconostoc mesenteroides, Lactobacillus brevis, and L. plantarum produce slime in apple juice, while L. plantarum and streptococci produce slime in grape juice.
  • Generally speaking, vegetable juices have pH values between 5.0 and 5.8 and contain sugars but are less acidic than fruit juices.
  • Vegetable juices also provide a plethora of growth nutrients for microorganisms, hence promoting the healthy growth of the finicky lactic acid bacteria.
  • Acid fermentation of the unpasteurized juice by these and other acid-forming bacteria is a likely cause of spoiling, however yeasts and moulds can also thrive.
  • Due to their enhanced acidity and sugar concentration, fruit and vegetable juice concentrates promote the growth of yeasts and acid- and sugar-tolerant Leuconostoc and Lactobacillus species.
  • Typically, these concentrates are packed in jars and then heated or frozen. Important microorganisms that could cause spoiling are killed by heat processing, and their growth is prevented by freezing.

References

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