P1: SFK/UKS BLBS102-c27 P2: SFK BLBS102-Simpson 548 March 21, 2012 13:25 Trim: 276mm X 219mm Printer Name: Yet to Come Part 5: Fruits, Vegetables, and Cereals Figure 27.6 Isoprenoid biosynthetic pathway in plants HMG CoA reductase expression and activities in apple fruits are hormonally regulated (Rupasinghe et al 2001, 2003) There are two genes for HMGR in apples designated as hmg1 and hmg2, which are differentially expressed during storage The expression of hmg1 was constitutive, and the transcripts (mRNA) were present throughout the storage period In contrast, the expression of hmg2 increased during storage in parallel with the accumulation of alpha-farnesene Ethylene production also increased during storage Ethylene stimulates the biosynthesis of alpha-farnesene as evident from the inhibition of alphafarnesene biosynthesis and the expression of hmg2 by the ethylene action inhibitor 1-methylcyclopropene (MCP) Thus, biosynthesis of isoprenoids is a highly controlled process Carotenoids, which are major isoprenoid components of chloroplasts, are biosynthesised through the Rohmer pathway The precursors of this pathway are pyruvate and glyceraldehyde3-phosphate, and through a number of enzymatic steps, 1deoxy-D-xylulose-5-phosphate (DOXP), a key metabolite of the pathway, is formed NADPH-mediated reduction of DOXP leads ultimately to the formation of IPP Subsequent condensation of IPP and DMAPP are similar as in the classical mevalonate pathway Carotenoids have a stabilising role in the photosynthetic reactions By virtue of their structure, they can accept and stabilise excess energy absorbed by the light-harvesting complex During the early stages of fruit development, the carotenoids have primarily photosynthetic function During fruit ripening, the composition of carotenoids changes to reveal the coloured xanthophylls pigments In tomato, lycopene is the major carotenoid pigment that accumulates during ripening Lycopene is an intermediate of the carotene biosynthetic pathway In young fruits, P1: SFK/UKS BLBS102-c27 P2: SFK BLBS102-Simpson March 21, 2012 13:25 Trim: 276mm X 219mm Printer Name: Yet to Come 27 Biochemistry of Fruits lycopene formed by the condensation of two geranylgeranyl pyrophosphate (C20) moieties mediated by the enzyme phytoene synthase is converted to beta-carotene by the action of the enzyme sesquiterpene cyclase However, as ripening proceeds, the levels and activity of sesquiterpene cyclase are reduced leading to the accumulation of lycopene in the stroma This leads to the development of red colour in ripe tomato fruits In yellow tomatoes, the carotene biosynthesis is not inhibited, and as the fruit ripens, the chlorophyll pigments are degraded exposing the yellow carotenoids Carotenoids are also major components that contribute to the colour of melons Beta-carotene is the major pigment in melons with an orange flesh In addition, the contribution to colour is also provided by alpha-carotene, delta-carotene, phytofluene, phytoene, lutein and violaxanthin In red-fleshed melons, lycopene is the major ingredient, whereas in yellow-fleshed melons, xanthophylls and beta-carotene predominate Carotenoids provide not only a variety of colour to the fruits but also important nutritional ingredients in human diet Beta-carotene is converted to vitamin A in the human body and thus serves as a precursor to vitamin A Carotenoids are strong antioxidants Lycopene is observed to provide protection from cardiovascular diseases and cancer (Giovanucci 1999) Lutein, a xanthophyll, has been proposed to play a protective role in the retina maintaining the vision and prevention of age-related macular degeneration Anthocyanin Biosynthesis The development of colour is a characteristic feature of the ripening process, and in several fruits, the colour components are anthocyanins biosynthesised from metabolic precursors The anthocyanins accumulate in the vacuole of the cell, and are often abundant in the cells closer to the surface of the fruit Anthocyanin biosynthesis starts by the condensation of three molecules of malonyl CoA with p-coumaroyl CoA to form tetrahydroxychalcone, mediated by the enzyme chalcone synthase (Fig 27.7) Tetrahydroxychalcone has the basic flavonoid structure C6-C3C6, with two phenyl groups separated by a three-carbon link Chalcone isomerase enables the ring closure of chalcone leading to the formation of the flavanone, naringenin that possesses a flavonoid structure having two phenyl groups linked together by a heterocyclic ring The phenyl groups are designated as A and B and the heterocyclic ring is designated as ring C Subsequent conversions of naringenin by flavonol hydroxylases result in the formation of dihydrokaempferol, dihydromyricetin and dihydroquercetin, which differ in their number of hydroxyl moieties Dihydroflavonol reductase converts the dihydroflavonols into the colourless anthocyanidin compounds leucocyanidin, leucopelargonidin and leucodelphinidin Removal of hydrogens and the induction of unsaturation of the C-ring at C2 and C3, mediated by anthocyanin synthase results in the formation of cyanidin, pelargonidin and delphinidin, the coloured compounds (Figs 27.7 and 27.8) Glycosylation, methylation, coumaroylation and a variety of other additions of the anthocyanidins result in colour stabilisation of the diverse types of anthocyanins seen in fruits Pelargonidins give orange, pink and red colour, cyanidins provide magenta and crimson colouration, and delphinidins 549 provide the purple, mauve and blue colour characteristic to several fruits The colour characteristics of fruits may result from a combination of several forms of anthocyanins existing together, as well as the conditions of pH and ions present in the vacuole Anthocyanin pigments cause the diverse colouration of grape cultivars resulting in skin colours varying from translucent, red and black All the forms of anthocyanins along with those with modifications of the hydroxyl groups are routinely present in the red and dark varieties of grapes A glucose moiety is attached at the and positions or at both in most grape anthocyanins The glycosylation pattern can vary between the European (Vitis vinifera) and North American (Vitis labrusca) grape varieties Anthocyanin accumulation occurs towards the end of ripening, and is highly influenced by sugar levels, light, temperature, ethylene and increased metabolite translocation from leaves to fruits All these factors positively influence the anthocyanin levels Most of the anthocyanin accumulation may be limited to epidermal cell layers and a few of the sub-epidermal cells In certain high anthocyanin containing varieties, even the interior cells of the fruit may possess high levels of anthocyanins In the red wine varieties such as merlot, pinot noir and cabernet sauvignon, anthocyanin content may vary between 1500 and 3000 mg/kg fresh weight In some high-anthocyanin-containing varieties such as Vincent, Lomanto and Colobel, the anthocyanin levels can exceed 9000 mg/kg fresh weight Anthocyanins are very strong antioxidants and are known to provide protection from the development of cardiovascular diseases and cancer Many fruits have a tart taste during early stage of development, which is termed as astringency, and is characteristic to fruits such as banana, kiwi, grape and so on The astringency is due to the presence of tannins and several other phenolic components in fruits Tannins are polymers of flavonoids such as catechin and epicatechin, phenolic acids (caffeoyl tartaric acid, coumaroyl tartaric acid, etc.) The contents of tannins decrease during ripening, making the fruit palatable Ester Volatile Biosynthesis The sweet aroma characteristic to several ripe fruits are due to the evolution of several types of volatile components that include monoterpenes, esters, organic acids, aldehydes, ketones, alkanes and so on Some of these ingredients specifically provide the aroma characteristic to fruits and are referred to as character impact compounds For instance, the banana flavour is predominantly from isoamyl acetate, apple flavour from ethyl-2 methyl butyrate, and the flavour of lime is primarily due to the monoterpene limonene As the name implies, ester volatiles are formed from an alcohol and an organic acid through the formation of an ester linkage The alcohols and acids are, in general, products of lipid catabolism Several volatiles are esterified with ethanol giving rise to ethyl derivatives of aliphatic acids (ethyl acetate, ethyl butyrate, etc.) The ester volatiles are formed by the activity of the enzyme Acyl CoA: alcohol acyltransferase or generally called as alcoholacyltransferase (AAT) In apple fruits, the major aroma components are ester volatiles (Paliyath et al 1997) The alcohol can vary from ethanol, propanol, butanol, pentanol, hexanol and P1: SFK/UKS BLBS102-c27 P2: SFK BLBS102-Simpson March 21, 2012 13:25 Trim: 276mm X 219mm 550 Printer Name: Yet to Come Part 5: Fruits, Vegetables, and Cereals Anthocyanin biosynthetic pathway Phenylalanine Phenylalanine ammonia lyase Trans-cinnamic acid Cinnmate-4-hydroxylase p-Coumaric acid p-Coumaryl-CoA Coumaryl-CoA ligase (3) Chalcone synthase Malonyl-CoA Chalcone Chalcone isomerase Acetyl-CoA Glycolysis Naringenin Glucose Flavonol hydroxylase Dihydroquercetin Dihydrokaempferol Dihydromyricetin Dihydroflavonol-4-reductase Leucocyanidin Leucodelphinidin Leucopelargonidin Anthocyanin synthase Cyanidin Pelargondin Delphinidin 3-Glucosyl transferase Methyl transferase Anthocyanin Figure 27.7 Anthocyanin biosynthetic pathway in plants so on The organic acid moiety containing the CoA group can vary in chain length from C2 (acetyl) to C12 (dodecanoyl) AAT activity has been identified in several fruits that include banana, strawberry, melon, apple and so on In banana, esters are the predominant volatiles enriched with esters such as acetates and butyrates The flavour may result from the combined perception of amyl esters and butyl esters Volatile production increases during ripening The components for volatile biosynthesis may arise from amino acids and fatty acids In melons, the volatile components comprise esters, aldehydes, alcohols, terpenes and lactones Hexyl acetate, isoamyl acetate and octyl acetate are the major aliphatic esters Benzyl acetate, phenyl propyl acetate and phenyl ethyl acetate are also observed The aldehydes, alcohols, terpenes and lactones are minor components in melons In mango fruits, the characteristic aroma of each variety is based on the composition of volatiles The variety “Baladi” is characterised by the presence of high levels of limonene, other monoterpenes and sesquiterpenes, and ethyl esters of even numbered fatty acids By contrast, the variety “Alphonso” is characterised by high levels of C6 aldehydes and alcohols (hexanal, hexanol) that may indicate a high level of fatty acid peroxidation in ripe fruits C6 aldehydes are major flavour components of tomato fruits as well In genetically transformed tomatoes (antisense PLD), the evolution of pentanal and hexenal/hexanal was much higher P1: SFK/UKS BLBS102-c27 P2: SFK BLBS102-Simpson March 21, 2012 13:25 Trim: 276mm X 219mm Printer Name: Yet to Come 551 27 Biochemistry of Fruits Antho cyanidins 3’ OH 2’ + HO O 4’ B 1’ 5’ 6’ C A 10 OH OH Pelargonidin OCHa OH OH + ⊕ O HO OH + ⊕ O HO OH OH OH OH Peonidin Cyanidin OH OCH3 OH + ⊕ O HO OH OH + ⊕ O H OH OH OH OH OH Petunidin Delphinidin OCH3 OH + ⊕ O Anthocyanidins HO Glycosylation Galactosylation OCH3 OH OH Anthocyanins Malvidin Figure 27.8 Some common anthocyanidins found in fruits and flowers after blending, suggesting the preservation of fatty acids in ripe fruits Preserving the integrity of the membrane during ripening could help preserve the fatty acids that contribute to the flavour profile of the fruits and this feature may provide a better flavour profile for fruits Kays SJ 1997 Postharvest Physiology of Perishable Plant Products 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AB, Christofferson RE 1 986 Synthesis and processing of cellulose from ripening avocado fruit Plant Physiol 81 : 83 0? ?83 5 Bird CR et al 1 988 The tomato polygalacturonase gene and ripening specific expression... S, Bramlage WJ 1 988 Antioxidant activity in “Cortland” apple peel and susceptibility to superficial scald after storage J Amer Soc Hort Sci 113: 412–4 18 Negi PS, Handa AK 20 08 Structural deterioration... fruits and their processed products Food Biotechnol 17: 163– 182 Paliyath G, Droillard MJ 1992 The mechanism of membrane deterioration and disassembly during senescence Plant physiol Biochem 30: 789 ? ?81 2