دانلود رایگان مقاله انگلیسی نور مرئی ابزاری جدید برای حفظ کیفیت تازه پس از برداشت کاهو - نشریه الزویر

ABSTRACT

Fresh-cut lettuces are susceptible to tissue browning and quality deterioration during post-harvest storage, even if they are kept in cold temperature. In this study we tried to counteract these undesirable physiological disorders by testing either storage under continuous light or after short treatments (2 d) with intermittent light (2 h on/2 h off) followed by storage in darkness. Two light intensities, 50 and 150 μmol m−2 s −1 , were studied. Continuous light (50 or 150 μmol m−2 s −1 ) significantly inhibited tissue browning but stimulated dehydration. However, intermittent light during 2 days minimized browning and water loss and showed a global positive residual physiological change during the following 5 d of storage in darkness. All light treatment maintained the photosynthetic capacity of fresh cut lettuces excepting for high continuous light (150C). The photosystem II efficiency was negatively affected by both the continuous and intermittent light at 150 μmol m−2 s −1 but not by the moderate intermittent light (50 μmol m−2 s −1 ). Finally, among the overall conditions tested, the short treatment (2 d) of fresh-cut lettuce by intermittent moderate level light (50 μmol m−2 s −1 ) followed by storage in darkness appeared to be the best compromise. Although not yet ideal, this treatment could maintained the product quality by reducing browning, minimizing weight loss and respiration and also keeping high level of photosynthetic capacity. Future studies in this context of visible light based post-harvest treatments are consequently promising.

4. Discussion

4.1. Light influence on cut-edge browning and water loss

We observed that high (150 μmol m−2 s −1 ) and medium (50 μmol m−2 s −1 ) continuous light intensity inhibited the cut-edge browning of fresh-cut lettuces during a 7 d storage period (Fig. 2). These results are in agreement with Zhan et al. who reported that continuous light around 35 μmol m−2 s −1 intensity decreased the browning of fresh-cut romaine lettuce and fresh-cut celery (Zhan et al., 2012, 2013c) In this study, we also experimented 2 d intermittent light treatments (2 h on/2 h off) followed by a 5 d storage in darkness (LD). The 150 LD treatment also inhibited the cut-edge browning although the salads are placed in the dark for 5 days. This result demonstrated that the effect of light was residual and was maintained in the dark. Interestingly, the lettuce treated by the 50 μmol m−2 s −1 intermittent light (50 LD) showed a slight cut-edge browning only at the end of storage (days 7). Thus, the inhibition of browning increased as the light intensity increased and the residual effect was also dependent of the light intensity. The effect of light intensity was described in model systems by Manzocco et al. (Manzocco et al., 2009). However the mechanism of action of light was not known precisely. Manzocco et al. said that visible light could inactivate polyphenol oxidase (PPO) activity due to nonreversible structural changes. Zhan et al. (2013b) have proposed that samples exposed to light could contain more ascorbic acid than those stored in darknessand inhibit the catalytic action of PPO by decreasing the pH value below the enzyme activity optimum. Several other hypotheses can be proposed: (1) light can act to inhibit PPO neosynthesis following cutting, (2) it can alter the pathways of synthesis of phenolic compounds and reduce the amounts of oxidizable phenols, and (3) it may also contribute to reducing browning due to a slight dehydration of the surface of the cuts.