Role of Pectic Enzymes in Fruit Ripening Process

Role of Pectic Enzymes in Fruit festering ProcessABSTRACTFruits argon important natural sources of valet nutrition for regulation of some(prenominal) metabolic functions and maintenance of good health. They are one of the significant commercial food commodities and their frugal value depends on several limiting f proceedingors. They easily undergo festering in a shorter period of time and produce a limited post-harvest existence. Fruit ripening is a common phenomenon occurring in yields based on which they are harvested, consumed directly or processed for various purposes. It is an intricate phenomenon involving extremely synchronized, genetic altogethery encoded and an irreversible process that results in a sequence of biochemical, physical and organoleptic alterations in a harvest-feast. This finally develops into soft, palatable and a ripe yield which possess necessary superior characteristics. On the another(prenominal) hand, excessive ripening results in spoilage of fr uits. modify is related to the changes of cell argue polysaccharides alike starch, cellulose, hemicellulose and pectins. Despite of several changes occurring in fruits such as liberation of turgor, starch degradation and loss of contents during ripening, enzyme-catalysed variations to cell wall composition and structure are considered as main reason for change intensity of fruits. Pectic enzymes are a group of enzymes responsible for pectin degradation in fruits. They are termed as pectinase that includes pectic lyase, polygalacturonase, pectic methyl radical esterase and rhamnogalacturonase. Pectic enzymes degrade pectin resulting in change intensity and agedness in fruits. Pectins are the important polysaccharides occurring in middle gill and primary cell wall. It contributes to the texture, consistence and quality of fruits. Degradation of pectin during ripening process was known to cause fruit tissue demulcent. new-made developments in molecular(a) biology nominate helped in better understanding of fruit ripening mechanism payable to enzymic activity. A review about the role of pectic enzymes on the plant cell walls to bring about salving during ripening process is discussed.Keywords Fruits, cell wall polysaccharides, pectin, pectic enzymes, ripeningINTRODUCTIONFruits are important sources of human nutrition and it is a commercially essential food commodity. Fruits are greatly specialized structures developed from a matured and pollinated ovary which have aesthetic characteristics with delicate aroma, striking colours, flavour, taste and juiciness from the edible seed vessels or receptacles (Prasanna et al., 2007). Prasanna et al., (2007) have indicated that fruits possess a wide variety of nutritional substances like vitamins (B-complex, C and K), phenolics and carotene providing higher(prenominal) energy value. They are classified as temperate, tropical and sub-tropical fruits based on their distribution in personality (Prasanna et al., 2007). When completely matured, harvesting of fruits is do. Maturity and ripening of fruits depends on their catalytic mechanism and based on the airing pattern and biosynthesis of ethylene during ripening, harvested fruits are grouped as climacteric (capable of ripening after legal separation from parent plant) and non-climacteric (unable to continue ripening when detached from parent plant) type (Prasanna et al., 2007). Fruit ripening is an intricate irreversible phenomenon which is highly synchronized and genetically encoded causing biochemical, physiological and organoleptic alterations resulting in a ripened edible fruit (Prasanna et al., 2007). Several factors knotted in ripening of fruits are shown in Figure 1.Figure 1 Factors involved in fruit ripening (Koning, 1994) found cell walls are complex structures which contain polysaccharides, proteins and enzymes. Pectin which contributes about tercet of the plant cell wall in dicots plays a crucial role in tissue development , bond certificate of cells, apoplastic porosity and so on (Almeida and Huber, 2007). Pectin is in control of maintenance of integrity of cell wall. It forms a cross-linkage between cellulose microfibrils and xyloglucans as shown in Figure 2. During ripening, pectin is greatly modified by the enzyme follow outs resulting in relaxing of cellular interconnections. This leads to loosening and hydrolysis of cell wall polymers which is the main reason for softening of fruit tissues (Gwanpua et al., 2014).Figure 1 Plant cell wall structure (Sticklen, 2008)Texture, an important quality attribute in fruits is altered leading to softening of fruits during ripening (Barret and Gonzalez, 1994). The mechanisms involved in fruit ripening were not understood clearly. It may be collectible to several factors like turgor loss, starch deprivation, loss of nutritional contents and so on (Ali et al., 2004). The major changes in texture causing softness where collectible to enzyme-catalysed changes in the cell wall assembly and composition, cell wall polysaccharides (pectin and cellulose) solubilization partially or completely (Waldron and Faulds, 2007). It has been examine by Rodriguez and Restrepo (2011) that, Pectic constituents is an important determinant of the firmness and consistency of the fruits (Van Buren, 1991). Pectinesterase (PE), polygalacturonase (PG) and Pectateliase (PL) are major pectic enzymes responsible for softening of fruit tissues during ripening (Rodriguez and Restrepo, 2011). Rodriguez and Restrepo (2011) have also indicated that, have action of these enzymes add to the final texture of fruit and when these enzymes undergo excessive reactions with pectin, it results in notorious softening and provides perfect conditions for microbial outpouring.Gwanpua et al., (2014) study that softening is one of the significant changes occurring in climacteric fruit ripening. They were more(prenominal) prone to microbial infections and physical damages and may adverse during prolonged warehousing. Fruit firmness determines its economic value and considerable effects were taken to control softness of fruits due to enzymic action, metabolic changes and changes in storage conditions (Gwanpua et al., 2014). Softening associated with the ripening mediated by the pectic enzyme action on plant cell walls of antithetical fruits is discussed in this study.2. Pectic enzymesPectinases are commonly referred to as pectic enzymes. They act upon pectin constituents of cell wall intercellular substance resulting in softening of fruits. Polygalacturonase (PG), Pectin methyl esterase (PME), Pectin lyase (PL) are some of the major pectic enzymes classified based on their type of action involved in degradation of pectins. The modes of action of these enzymes are as shown in symbol 2.Figure 3 Mode of action of pectolytic enzymes (Van Rensburg and Pretorius, 2000).PME causes de-esterification by catalysing the pectin methyl group hydrolysis (Prasanna et al ., 2007). They were known to usually attack on the methyl ester linkages of a galacturonate entity which lies next to non-esterified galacturonate component. This process acts as a precondition for polygalacturonase activity (Prasanna et al., 2007). Rodriguez and Restrepo (2011) interested that pectin methyl esterases (PME) were found to alter pectin in courting of low methoxy pectin and in polygalacturonic acid by hydrolysis of methyl ester bonds in esterified carboxyls. It was observed that the softening process could be controlled by the control of this enzyme action in fruits like pear, grape, citrus, love apple, strawberry, arracacha, potato, papaw tree, kiwi and guava (Rodriguez and Restrepo, 2011 Prasanna et al., 2007).PG, referred as Pectolytic glycanase is an essential hydrolytic enzyme playing an important role in pectin termination (Prasanna et al., 2007). They act on pectic acids and -1, 4- glycosidic bonds are hydrolysed between the residues of galacturonic acid in g alacturonans. They are categorized as endo-PG and exo-PG (Prasanna et al., 2007). Prasanna et al., (2007) studied that Endo-PG act at random in pectin de-polymerization and exo-PG discharges galacturonic acid by catalysing the glycosidic bonds hydrolysis from the non-reducing end of the pectic substances. It is recognized that middle lamella dissolution in the course of fruit ripening were done by PG.PL undergoes mechanism of -elimination of the glycosidic bonds (Prasanna et al., 2007). They also indicated that PL acts an endo-enzyme which acts randomly during cleavage of esterified galacturonate units. PL was known to act at methyl-ester bonds leading to beta-elimination of glycosidic bonds by formation of double bonds in galacturonic acids (Rodriguez and Restrepo, 2011). Rodriguez and Restrepo (2011) undertake that pectin lyase (PL) enzyme were responsible for cell wall degradation resulting in softening of banana and strawberries. The changes in pectin due to these enzymes act ion were as shown in Figure 4.Figure 4 Action of Pectolytic enzymes on pectin (Prasanna et al., 2007)2.1. Role of pectic enzymes in softening during ripening in disparate fruit varietiesPectic enzymes play an important role in ripening of fruit by degradation of pectic substances found in fruits. Several researches were conducted in different fruit varieties and in their respective juices to understand the role of pectic enzymes in softening of fruit tissues and their intend use in preparation of fruit juices to enhance mashing of fruits, for clarification of fruit juices and also to increase the release of flavours from fruits (Whitaker, 1984). Invitro-studies of some selected fruits and their pectic enzymic activity is discussed.Besford and Hobson (1972) studied the pectic enzyme activity in tomato fruit and cogitate that action of PG/ PME or transelimination by pectic transeliminase were not responsible for tomato ripening. Almedia and Huber (2008) have indicated that softening of tomato fruit during ripening were not associated with the PG activity eventhough they remained active in ripened fruit. Enzymic activities of other fruits are studied comparing with tomato as model fruit (Ali et al., 2004).Pires and Finardi-Filho (2005) have extracted PE and PG from Peruvian carrots to understand the pectic enzyme activities in carrot and found that pectinases along with amylases would act together to bring about spoilage in tubers. Owino et al., (2004) have studied about the alterations in fig fruits during ripening due to changes in cell-wall polysaccharides. They have concluded that soft differences in both receptacle and drupelets were observed in pectic polymers compared to the hemi-cellulosic substances. This indicates that ripening of fig is associated with modifications in pectin. Several studies were done to understand ripening of strawberries genetically as it softens rapidly (Pombo et al., 2009 Bustamante et al., 2006). It was observed that cell wall degradation were due to pectin dissoloution by pectic enzyme activities in strawberries which enters ripening phase quicker overlapping with the senescence.Manrique and Lajolo (2004) discussed about the cell-wall modifications in papaya fruit during postharvest ripening. They found that several enzymic actions are involved in papaya fruit during the entire period. at that place were notable changes in pectic polymers during papaya ripening which were mainly due to PG activity and probably due to PME as there were charge density modifications in polymers (Manrique and Lajolo, 2004). Avocado fruit undergoes pectin depolymerisation in contrast to other fruits and it was observed that it was the only fruit in which huge amounts of pectin were readily solubilizing under milder conditions (Pesis et al., 1978 Huber at al., 2001). In case of apples, exo-PG, PME and PL activity were observed to cause changes in cell wall polysaccharides (Goulao et al., 2007 Oritz et al., 2011). Studies rel ated to cell wall modification during ripening of apples reveals that exo-PG, PME and PL activity influences softening of apples (Goulao et al., 2007). Studies by Gwanpua et al., (2014) in jonagold apples have shown that increment of PG activity occurred late during ripening and PME activity did not influence the ripening process.Cheng et al., (2011) observed that banana fruit which enters softening during ripening were associated with pectic hydrolytic activity and it was mainly due to the combined effects of PG and PME activity. Abu-Goukh and Bashir (2003) have observed that softening of guava fruit and mango were mainly due to PG and cellulose activity during ripening which resulted in drop in firmness of the fruits. as well in case of peaches, softening is due to depolymerisation of pectin by endo-PG activity resulting in solubilization of cell-wall polymers (Yoshioka et al., 2011).3. CONCLUSIONPectin polymers form a gel-like matrix which provides cell-wall integrity contributi ng to the firmness of fruits. It stabilizes the texture and colour of the fruit based on which perkiness of fruits is determined. Commercial value of fruits is based on these quality attributes. Several researches conducted on different types of fruits indicate that pectic enzymes have an important role in softening of fruit tissues during ripening. They were known to cause pectin solubilization in cell-wall matrices by different modes of action like lysis, de-esterification and glycanase activity. Research studies conducted on selected types of fruits under in-vitro conditions indicated that Polygalacturonase activity is predominant in just about all-types of fruits compare to pectin methyl-esterase and Pectic-lyase activities. Pectic enzymes were not the only factor for softening of fruits during ripening as it is influenced by several other factors like changes in pH, temperature, harvesting period, storage conditions, physical damage, microbial attack, presence of sugars, othe r hydrolytic, energising and amylase enzymes and so on. Better understanding of mechanism of pectic enzymes on primary cell-walls of fruits would enhance the control of softening and ripening process. This in turn, will improve the quality of fruits extending its shelf-life. Recent advancements favoured the isolation of pectic enzymes commercially which is use intentionally for accelerating the maturation of fruits, added in production of fruit juices to easily crush the fruits, to solubilize the complex cell-wall matrices, to release all nutrients, to control haze formation in juices, to enhance flavour and colour characteristics of fruit juices, and so on. Recent developments in molecular biology have paved a way for genetically understanding the pectic enzyme action on primary cell-walls in fruits. Henceforth, pectic enzymes play a determining role in softening associated with ripening of fruits.