Factors important for the shelf-life of minimally processed lettuce

Research output: Book/ReportPh.D. thesisResearch

  • Karla Michelle Deza Durand
The minimally processed vegetable industry has been increasing rapidly due to change in lifestyle. Both women and men work outside home and have less time to cook and need more convenience and time saving products, which also present fresh and healthy characteristics. Iceberg lettuce (Lactuca sativa L.) is one of the most popular fresh-cut vegetables. Although an increase in the number of mixed salads in retail food chains is evident, their short shelf-life due to rapid browning and off-odour is a problem that need research. Therefore, the aim of this PhD project was to investigate factors important for the shelf-life of minimally processed iceberg lettuce and to propose a new methodology to measure browning in cut lettuce.
Browning has been pointed out as the main factor limitating shelf-life in cut lettuce. The problem becomes complex because browning of cut lettuce is difficult to measure. A novel method using image analysis for the measurement of browning in minimally processed lettuce was developed and presented in paper I. The method used a flatbed scanner for image acquisition, colour dye patches for colour correction, and colour thresholding to quantify the browning, that was expressed as brown area fraction. Cut lettuce was stored at 5°C for 6 days and plus 1 day at room temperature (day 7). Changes in browning were assessed at 2, 6 and 7 days of storage using image analysis. The result showed an increase in browning as time and temperature of storage increased. It was concluded that this technique can be used for measuring the browning in cut lettuce.
Few studies are done on the formation of volatiles in cut lettuce. Temperature of storage and methods of preparation that minimized quality loss are highly desirable. As such, cutting direction and storage temperature were investigated to elucidate their influence on aroma formation and respiration rate in minimally processed lettuce, are presented in paper II. Lettuce was cut longitudinal and transverse to the mid-rib and stored at 6 and 10°C for 4 and 5 days. Changes in respiration rate were analyzed through the storage time, and aroma analysis was carried out after 4 and 5 days of storage in January and March, respectively. Respiration rate increased with increasing storage temperatures. Aroma formation was also influenced by storage temperature. Higher storage temperature allowed the increase of -longipinene, 2-methylbutanal and 3-methylbutanal. Transversal cut to the rib was strongly related with volatiles of lipoxygenase (LOX) pathway i.e. cis-3-hexenal, cis-3-hexenol and trans-2-hexenol, meanwhile longitudinal cut enhanced the formation of volatiles from other metabolic routes. Therefore, it was concluded that transversal cut cause a more severe damage to the tissue than longitudinal cut based on aroma production of LOX volatiles.
It has been indicated that cultivar, season, packaging and storage time influence the type and concentration of volatile compounds, browning, chemical constituents and texture in vegetables. As part of this project, a more integrated study was undertaken for first time in lettuce to our knowledge. The study took into account the influence of cultivar, season, packaging and storage time. In order to achieve this, iceberg lettuce cultivars Platinas, Diamantinas and Morinas were harvested from June to September 2009. Lettuces were minimally processed and stored under three different treatments: two passive modified atmosphere packaging (MAP) built up by films of different permeabilities, F1 (OPALEN 65 AF) and F2 (OPP/PE-L 2040 AF), and storage in air. All packages were stored at 5°C. Gas composition, volatile compounds, physicochemical constituents, PPO activity and browning were assessed at 1, 5, 8 and 11 days of storage in packaged lettuce, whereas in air stored samples only at 1 and 5 days of storage. Additionally, respiration rate was only assessed in air stored samples and GC-O analysis was undertaken after 1 and 11 days of storage for cultivar Morinas packaged in passive MAP F2 in September. The study was divided into three papers, paper III, IV and paper V. The study in paper III was limited to analyze the volatiles compounds as a function of packaging and storage time and was
used as basis for a more comprehensive analyses as presented in paper IV.
Paper III revealed that packaging and storage time had an influence on the volatiles of cut iceberg lettuce allowing the formation of desirable aroma but also on the development of off-odors. This result indicates that extremely low O2 and high CO2 conditions found in the passive MAP F1 and F2 after 11 days of storage enhanced the formation of volatiles of anaerobic metabolism such as ethyl acetate and 2,3-butanedione.
In paper IV 52 volatile compounds were identified and of these 21 potent odorants were shown to contribute to the aroma of cut lettuce. Among them elemene, caryophyllene, - selinene and 2,3-butanedione, enhanced under anaerobic conditions and likely to be offodours.
In August high production of these odorants probably compromised the quality in
terms of odour. The findings suggest that most of the potent odorants enhanced their
relative area under anaerobic conditions built up in the passive MAPs during the storage time, and are likely to produce off-odour. Levels of odorants such as 2,3-butanedione, elemene, caryophyllene and -selinene were significantly enhanced under anaerobic conditions after 11 days of storage, being significantly higher in passive MAP F1. Regarding the cultivars, Morinas and Diamantinas was the less tolerant to high CO2 resulting in significantly higher amount of 2,3-butanedione.
Paper V was related to browning and other physicochemical characteristics of minimally processed lettuce such as soluble sugars, organic acids, chlorogenic acid, pH, polyphenol oxidase activity and firmness. Gas chromatography mass spectrometry (GC-MS) method was developed for the analysis of soluble sugars, organic acids and chlorogenic acid in cut lettuce. Our results showed that season and storage time mainly influence over physicochemical characteristics of packaged and air stored cut lettuce, and in less degree the cultivar. In June, fructose, glucose, sucrose, malic acid and firmness were kept high under anaerobic conditions. Differences in the content of O2 and CO2 between the passive MAPs and air stored samples demonstrated to influence the formation of browning and other physicochemical characteristics as storage time increased. It was concluded that browning was remarkable controlled in passive MAPs samples, irrespective of season and cultivar due to extremely low O2 and high CO2 conditions, however, after 11 days of storage, this condition favored tissue softening, decreased of sugars and malolactic fermentation, mainly in passive MAP.F1.
Original languageEnglish
PublisherDepartment of Food Science, University of Copenhagen
Number of pages71
Publication statusPublished - 2013

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