Microbial Spoilage of Fruits
SYED DARAKSHAN1, FARHANA MEHRAJ ALLAI1', ABIDA JABEEN1, SHAHNAZ PARVEEN2, and KHALID GUL3
'Department of Food Technology, Islamic University of Science and Technology, Awantipora , Jammu and Kashmir, 192122, India
2Department of Horticulture, SKUAST-K, Wadura 193201, India
institute of Agriculture and Lite sciences,
Cyeongsang National University, Jinju 660701, South Korea
Microbial spoilage is the most common cause of post-harvest loss in fruits worldwide. Many environmental factors act as driving forces for microbial spoilage. Although, fruits contain natural microbiota and their count ranges from 102 to 107 colony-forming units (CFU) per gram, but they do not cause any spoilage in fresh fruits. Microorganisms responsible for spoilage contaminate fresh fruits at various stages from harvesting to processing and this contamination goes on spreading from fruit to fruit, if it is not taken care while processing and storage. Microbial spoilage causes physiological changes in fruits by increasing respiration rate and ethylene production, whatever may be the storage condition. Rotting is the typical examples of microbial spoilage occurring in fruits during storage. A number of preharvest and postharvest factors play their role in microbial spoilage of fruits. Pre-harvest factors include pruning, training, use of fungicides, methods of harvesting, and so on while as postharvest factors include proper handling, transportation, quality of water used for processing and so on.
Fresh fruits get spoiled due to various microbial infections. Contamination can occur at any stage from ripening of fruit to consumption. The spoilage takes place due to physical, biological, and mechanical agents which increases the susceptibility to spoil and permits the infection to occur. The foremost reason for the spoilage of fruits after harvesting is washing because the surface of fruit gets moistened which serves as a mode of transfer of microorganisms. Spoilage is a result of several factors such as enzymes, mechanical damage (bruises, cracks etc.), microbial spoilage or a combination of all these. Within a spoiled fruit different population successions take place that rise and fall with the availability of different nutrients (Rawat, 2015).
Microbial spoilage does not depend only on the variety and type of fruit but also depends on the variety of microorganisms that survives in different environmental changes. It can be due to plant pathogens acting on stems, leaves, flowers, roots, or other part of fruit which is used in culinary purposes or other saprophytes which may also succeed in developing an infection. During spoilage, the chemical reactions take place that produces off-flavors and off-odors which makes the fruit unacceptable. Spoilage organisms mainly reside in water, soil, or intestinal tract of animals and spread in a wide area through water, air or by the activities of living beings especially insects. The microorganisms find their way in fruits and try to explore if temperature, humidity, and other conditions of growth are favorable. At higher temperature and other favorable conditions pathogens actively infect the host and causes serious diseases. Fruits with wet surfaces are directly penetrated by some fungi.
The harvesting of fruits generally takes place before they ripe so as to keep them fresh for a long time. Fresh fruits are being protected during the early developmental stage as the ripe fruits are prone to various fungal diseases. A major loss of spoilage is due to the spores growing on the surface of fruits, but these spores are unable to penetrate because of the onset of favorable conditions. After harvesting these fungal spores attack the surface and gets deeply penetrated into the fruit as the moisture and nutrients are available and there is no protection to microorganisms by the intrinsic factors to provide them necessaiy resistance during the developmental phase. Development of colonization and abrasion takes place more rapidly within damaged tissues of fruit. Damage such as cracks, bruising and puncture creates sites for establishment of spoilage microorganisms. Within days or weeks’ time, this lesion development can increase rapidly and infect the whole fruit (Watkins et al., 2004).
NATURAL MICROFLORA ON FRUITS
Fresh fruits have complex relation with respect to their composition. Fruits consists of 85% water, 13% carbohydrate, 0.8% proteins, 0.3% fat, 0.6% ash, and pH is 4.8 or below (Erkmen and Bozoglu, 2016). Fruits contain natural microbiota coming from extrinsic and intrinsic factors like air, soil, high humidity, high temperature, water activity (a j, and pH. In fresh fruits, aw plays an important role in survival of fungi such as lactic acid bacteria (LAB), Acetobacter and Gluconobacter, and bacteria. Souring in fruits can be caused by acetic acid and Lactic bacteria. In fresh fruits, the bacterial count ranges from 102 to 107 colony-forming units (CFU) per gram. Spoilage of fruits such as citrus fruits and apples take place by fermenting sugars with the production of CO, and alcohol using yeast genera like Saccharomyces, Candida, Torulapsis, and Hansemda.
Microorganisms contaminate fresh fruits at various points and by number of ways; initially from pre-harvest, harvest and post-harvest (Heard, 1999). Molds contaminating fruits include Penicillium, Aspergillus, Mticor, Alter- naria, Cladosporium, and Botrytisspp. Hanseniaspora, Saccharomyces, Kloeckera, Pichia, Candida, and Rhodotonda are the common genera of yeasts that are mainly associated with the contaminating fruits (Burnett and Beuchat, 2000; Tournas, 2005). After harvesting, the fruits are cleaned by washing that removes the surface microbial contamination but during processing like cutting, slicing, chopping, dicing, and mixing not only increases microbial load but damages surface of fruit and cellular composition that leads to outflow of fluids and nutrients (Heard, 2002).
Environmental condition is another critical factor that contaminates fresh fruits by pathogenic microorganism when it makes contact with windblown sand, insects, annuals, humans, and poorly sanitized equipment or through harvesting equipment, transportation, storage, and distribution (Sothornvit and Kiatchanapaibul, 2009). If equipment is not properly sanitized, it can contaminate fruits with pathogens for example, Geotrichum candidum. Bacteria or fungi utilize nutrients that are exuded from plant tissue for growth and development for example, Enterobacter and Klebsiella and these microorganisms like Enterobacter cloacae, Citrobacterfreundii, Aeromonas hydrophila, and Klebsiella sp are capable of causing human illness. During processing the tissues get damaged from which water is oozed out from fruits which again became the way for survival of microorganisms like bacterial population that grows on the surface of fruit when free moisture is available (Beattie and Lindow, 1999).
MECHANISM OF MICROBIAL SPOILAGE
Fruits are made up of different types of cells and each cell has its own specific function. Plant cell has rigid cell wall comprised mainly of polysaccharides, cellulose, hemicelluloses, pectins, lignins, and some proteins. The cell wall is permeable to water and other solutes. The plant’s skin or peel called epidermal cell (outer layer of plant) that develops a waxy cuticle which consists of hydrophobic surface and protects the underlying tissues against the water loss and invasion of microbial attack (Lequeu et al.,
2003) . If the surface of plant is exposed to natural environment that leads to physical damage in some way like external stress before or after harvest due to weather condition, insect infestation, rodents, or polymers which are degraded by extracellular lytic enzymes which releases water and other intracellular constituents that are used as nutrients for growth of microorganism for example. Pseudomonas fluorescens (Mercier and Lindow, 2000). Acidity present in fruit also acts as a natural barrier against many bacterias.
Physiological changes in plant also result in creating lesions to external tissues for example, dehydration of fruits that result in the separation of cells, cracks, and facilitating the microbial attack to the internal tissues. In this case, fermentable carbohydrate is utilized by microbes and produces metabolites that cause unacceptable changes in terms of color, aroma, flavor, and texture. Proteolytic enzymes are produced by few microorganisms that have tendency to denature the cell structure; once the cell structure is degraded the nutrients are released that are taken up by the pathogens to increase microbial activity.
Extracellular pectinases and hemicellulases are produced by some fungi which are particularly responsible for fungal spoilage (Miedes and Lorences,
2004) . Enzymes like cellulases, phosphatase, dehydrogenaese, pectinases, and proteases are the microbial enzymes that are liable for degradation of fruit tissue. Depolymerization of pectin chain is caused by pectic enzymes. Ester group from pectin chain is hydrolyzed by pectin methyl esterase (PME) with the production of methyl alcohol. Several pathogens and non-plant pathogens produce PME such as Erwinia carotovora, Pemcillium citrinum, Monilinia fructicola, Botrytis cinerea, and Flavobacterium, respectively. The length of pectin chain is reduced by chain-splitting pectinases such as Pectinlyase (PL) and Polygalacturonase (PG). Pectin chain is cleaved by Polygalacturonase by hydrolyzing the two molecules of galacturonic acid that are linked together. Depolymerization of pectin chain taken place by Pectinlyase by hydrolyzing [3-linkage. Pectinlyase and Polygalacturonase both are endopectinases that have an active function on the degradation of middle part of pectin chain. This degradation of pectin chain results in softening of tissues and liquefaction of pectin that result in pathogens to initiate infection.
Besides microbiological proliferation, the metabolic rates are also increased by ethylene production and respiration rate (Laurila and Ahve- nainen, 2002; Surjadinata and Cisneros-Zevallos, 2003). Another challenging factor is storage of fruits in cold storage. Pathogens already exists at the wound site on fruit, and if this damaged product is not culled out from the batch it can cause a serious problem to whole lot and can cause considerable fruit loss as two lesion microbes, P expansion and B. cinerea actively act on to it and ultimately produces wound, and cross-contaminate adjoining fruit whether it is packed in boxes, bins, or palletized. It all depends upon storage period (tune) and conditions, and if it is not cautiously maintained then these microbes can have a severe effect on the stored fruits, for example, apples if not properly stored in large CA stored rooms, then extensive blue mold infestation can take place (Watkins et al., 2004).