22nd May 2008Warwick HRI Conference Centre, Wellesbourne
Abstract and poster both entitled 'The genetic characterisation of post harvest spoilage in lettuce' was submitted. Please contact L.D.Atkinson@warwick to request copy of poster (due to unpublished results).
Genetic Characterisation of Post Harvest Spoilage in Lettuce
LAURA ATKINSON1, PAUL HAND1, JOHAN SCHUT2 and DAVE PINK1
1Warwick HRI, University of Warwick,
2Rijk Zwaan Nederland B.V., 2678 ZG De Lier, Netherlands
In today’s increasing market of “food perfection”, any alterations to the visual characteristics of a product is likely to incite an unfavourable consumer response. A prime example is pre-packed cut salads. There is an increasing demand for this product due to increasingly busy lifestyles of consumers, however, the limited shelf life of fresh processed lettuce is a major food industry concern with losses entering the £millions per annum [Soliva-Fortuny and Martin-Belloso 2003].
The shelf life of cut lettuce is regularly restricted by enzymatic browning and pinking. During storage their organoleptic characteristics are altered by the emergence of these coloured pigments [Martinez and Whitaker 1995]. In addition to causing an adverse response, there is also a product quality loss consequential of the associated decrease in content of phenolic compounds [Tomás-Barberán and Espín 2001].
The phenylpropanoid pathway synthesises polyphenols that are sequentially oxidised by polyphenol oxidase (PPO) causing enzymatic browning [Hisaminato et al., 2001]. PPO catalyses the hydroxylation of monophenols to o-diphenols: monophenol monooxygenase or tyrosinase (EC126.96.36.199): and the oxidation of o-diphenols to o-diquinones; (EC188.8.131.52) [Webb 1992]. This is followed by the non-enzymatic formation of brown pigments known as melanines [Joslin and Pointing 1951; Martinez and Whitaker 1995]. PPO is the main agent responsible for enzymatic browning, although a synergistic effect between PPO and peroxidases (POD) is possible [Bouwens et al.,1999; Tomás-Barberán and Espín 2001]. POD activity may result in oxidative actions that involve non-specific hydrogen components in food causing discolouration [Padiglia et al., 1995].
Phenylalanine ammonia-lyase (PAL) is the rate-limiting enzyme of the phenylpropanoid pathway normally induced upon plant tissue wounding, therefore increasing biosynthesis of polyphenols for oxidation [Peiser et al., 1998; Hisaminato et al., 2001]. Cell disruption leads to decompartmentalisation involving a cascade of events, including PPO activation and promotion of PAL synthesis. PPO activation and both PAL and POD induction has been observed in six lettuce cultivars following cutting. However, there appeared to be no simple correlation between browning susceptibility and any of the parameters [Cantos et al.,2001].
Anthocyanin pigments are secondary phenolic metabolites from the phenylpropanoid pathway, which also have a negative quality influence on produce due to the associated red colour. Cyanidin, 3-glucoside and 3-malonylglucoside are thought to induce the red colour observed in L. sativa [Brouillard et al., 1997]. Interestingly Kubo et al.,(1988) has reported high levels of PAL activity are fundamental for anthocyanin development. Although research by Ju et al.,(1995) suggests anthocyanin synthesis may occur independently of PAL activity under conditions of plentiful precursors.
The prevention of discolouration in pre-packed processed lettuce is presently achieved by combinations of various post harvest treatments including modified atmospheric packaging (MAP). Effectively all fresh processed products are by necessity handled in MAP to receive the necessary commercial post harvest life span [Brecht et al., 2003]. A genetic engineering approach would allow regulation of the pathways, avoiding treatment use [Peiser et al., 1998; Tomás-Barberán and Espín 2001]. However, since widespread public opinion opposes this type of modification in Europe, exploitation of natural allelic variation is the preferred option for lettuce breeding companies.
The current project has evaluated the genetic variation for post harvest discoloration in a diversity set of lettuce accessions drawn from the lettuce plant genetic resources collection maintained in the Genetic Resources Unit at Warwick HRI. Included in this diversity set were the parents of the Warwick HRI mapping population; the ‘heritage’ cv Iceberg (syn Batavia blonde a bord rouge) first grown in 1870 and the cv Saladin (syn. Salinas) bred in the 1970s and an important variety in the pedigree of modern iceberg type cultivars. The parental lines showed significant variation and the mapping population is currently being grown in the field to assess its phenotype for quantitative trait analysis.
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