Mikal E. Saltveit

Effect of ethylene on quality of fresh fruits and vegetables

Ethylene is a naturally occurring plant growth substance that has numerous effects on the growth, development and storage life of many fruits, vegetables and ornamental crops at m ll 1 concentrations. Harvested fruits and vegetables may be intentionally or unintentionally exposed to biologically active levels of ethylene and both endogenous and exogenous sources of ethylene contribute to its biological activity. Ethylene synthesis and sensitivity are enhanced during certain stages of plant development, as well as by a number of biotic and abiotic stresses. Exposure may occur inadvertently in storage or transit from atmospheric pollution or from ethylene produced by adjacent crops. Intentional exposure is primarily used to ripen harvested fruit. The detrimental effects of ethylene on quality center on altering or accelerating the natural processes of development, ripening and senescence, while the beneficial effects of ethylene on quality center on roughly the same attributes as the detrimental effects, but differ in both degree and direction. Care must therefore be taken to insure that crops sensitive to the effects of ethylene are only exposed to the desired atmosphere. A number of techniques to control the effects of ethylene are discussed in relation to their application with commercially important fruits and vegetables. Examples of general and specific beneficial and detrimental ethylene effects are given.

Wound induced changes in phenolic metabolism and tissue browning are altered by heat shock

Wounding is one of many abiotic stresses that produce signals that propagate from injured into adjacent non-injured tissues and induce the de novo synthesis of specific wound-induced proteins. Some of these induced proteins are enzymes of phenolic metabolism, such as phenylalanine ammonia-lyase (PAL), whose increased activity leads to the accumulation of phenolic compounds (e.g., chlorogenic acid, dicaffeoyl tartaric acid and isochlorogenic acid) and tissue browning. Wounding of iceberg lettuce leaves increases PAL activity six- to 12-fold over 24 h at 10°C and leads to a three-fold increase in the total phenolic content within 3 days. There may be a hierarchical order to the plants response to different abiotic stresses. Plant tissue simultaneously exposed to a heat shock and wounding responds to the heat shock in preference to wounding by producing heat shock proteins instead of PAL. A 90 s, 45°C heat shock prevents an increase in PAL activity if administered either 4 h before or 2 h after wounding. This diversion of wound-induced protein synthesis by heat shock might be used to prevent browning in other crops that normally have low phenolic content; e.g., celery and lettuce. The persistence of the ability of a heat shock to preferentially induce the synthesis of heat shock proteins (hsps) in place of wound-induced enzymes of phenylpropanoid metabolism offers a new way to control browning in lightly processed fruits and vegetables. The design of processing lines using a heat shock to extend the shelf-life of fresh-cut lettuce will need to be modified from existing designs to take full advantage of the effect of the heat treatment.

Wound-induced phenylalanine ammonia lyase activity: factors affecting its induction and correlation with the quality of minimally processed lettuces

Abstract One of the major causes of quality loss in minimally processed lettuce (Lactuca sativa L.) is the browning of the cut pieces. Phenylalanine ammonia lyase (PAL, EC 4.3.1.5) is a key enzyme in phenolic synthesis. PAL activity increased in lettuce midrib tissue with wounding and storage in the presence or absence of ethylene. PAL activity increased 2.5 and 3-fold at 5 and 15 °C, respectively, by reducing midrib size from 2.5 × 15 to 0.5 × 1 cm. Wounding usually induced maximum levels of enzyme activity within 3 days at 5 °C and 1 day at 15 °C. Pre and postharvest factors affected the kinetics of wound-induced PAL (WI-PAL) activity and of subsequent changes in the quality of minimally processed lettuce. The rate at which WI-PAL activity increased and the maximum level attained were influenced by the duration of storage before processing. These parameters were also affected by the cultivar and type of lettuce used. Butterhead and iceberg types had the highest and lowest levels, respectively, of WI-PAL activity; romaine, green leaf and red leaf had intermediate levels. The activity of PAL 1–2 days after processing, and the slope of the induction curve between days 0 and 2, were the enzyme measurements that gave the highest correlations with processed lettuce quality attributes, including overall visual quality, leaf edge browning, and leaf surface browning.

Is it possible to find an optimal controlled atmosphere

Every controlled atmosphere (CA) conference has included recommendations for controlled atmospheres that purport to produce the optimal storage atmosphere for specific horticultural crops. However, the natural variability in the raw material and its dynamic response to processing and storage conditions may render it impossible to identify a truly optimal storage atmosphere. Additional refinements in recommendations for the CA and modified atmosphere (MA) storage of fruits and vegetables will continue to accrue through empirical observations derived from traditional experiments in which the six components of the storage environment (i.e. duration, temperature, relative humidity, O2, CO2 and ethylene levels) are varied in well-defined steps. However, the variability inherent in biological systems and the often dynamic response of the stored commodities to changes in their storage environment may not be adequately accounted for in these types of static experiments where all variables are usually held constant for the duration of the experiment. Truly significant advances in the use of CA and MA may require the development of mathematical models that incorporate some measure of the commoditys dynamic response to the storage environment. These measurements should reflect the commoditys changing response to various storage parameters (e.g. a shifting anaerobic compensation point), and should be useful in predicting future changes in quality.

The rate of ion leakage from chilling-sensitive tissue does not immediately increase upon exposure to chilling temperatures

Exposure to non-freezing temperatures below 10 °C causes an increase in the subsequent rate of ion leakage from chilling sensitive tissue (e.g. tomato fruit). The conditions of tissue preparation and conductivity measurements necessary to accurately calculate the rate of ion leakage from excised discs of tomato pericarp tissue were determined. Under the proper conditions, the rate of leakage expressed as a percent of the total conductivity per hour was linear for the period of 30–240 min following immersion of the discs in aqueous solutions of 0.2 M mannitol. A kinetic analysis of the efflux data showed that a combination of two exponential equations of the form y= C0(1− e − K·t ), one for a ‘fast’ extra-cellular reservoir of ions (C0f, Kf; y=C0f 1− e − K f·t ), and one for a ‘slow’ cellular reservoir (C0s, Ks; y=C0s(1−e −K s·t ), fit the original data with an R 2 0.95. When measured at 25 °C after chilling, chilling at 2.5 °C for 3.5 days increased the rates of ion leakage and the values of C0f and Ks, but had no effect on C0s or Kf. This implies that chilling increased the permeability (Ks) of the cellular reservoir that allowed ions to leak out during the exposure to chilling and increase the content of C0f. However, when ion leakage was measured at temperatures from 2t o 20 °C,C0f remained unchanged and the changes in Ks paralleled those in Kf. These changes in the rate constants reflect the effects of temperature on diffusion and did not show the abrupt increase predicted by the membrane phase-transition model of chilling injury.

The visual quality of minimally processed lettuces stored in air or controlled atmosphere with emphasis on romaine and iceberg types

Five different types of lettuce (iceberg, romaine, butter, green leaf, and red leaf) were prepared as salad pieces and stored for 16 days at 5°C in air or in a controlled atmosphere (CA) (3% O2 + 10% CO?). Samples were evaluated for overall visual quality, surface and edge browning, and russet spotting. After 8 days, differences in overall visual quality between air and CA-stored samples were observed; after 12 days, air-stored samples were below the limit of salability, whereas visual quality was maintained in CA-stored pieces from all lettuce types except butter lettuce. Transferring lettuce to air for 12 h at 15°C accentuated the differences between storage atmospheres. An analysis of the various components of overall visual quality showed that surface and edge browning were the defects which most contributed to a decrease in quality. Among the lettuce types studied, the CA benefit for overall visual quality was highest for iceberg lettuce, and some differences between cultivars were observed. There was no CA benefit for butter lettuce, but CA induced surface discoloration and tissue softening. Pre-processing storage of lettuce heads at 5°C for 7 and 14 days negatively affected overall visual quality and leaf edge browning of the prepared salad pieces; this intact head storage effect was more evident on romaine than iceberg pieces.

Application of casein-lipid edible film emulsions to reduce white blush on minimally processed carrots

White blush on the surface of peeled carrots is a major cosmetic disadvantage in marketing this lightly processed, ready-to-eat product. The loss of quality is exacerbated by surface dehydration. To maintain good appearance, edible coatings consisting of emulsions incorporating caseinates with beeswax, stearic acid or acetylated monoglyceride were tested. All except the latter increased water vapor resistance. Sodium caseinate-stearic acid was particularly effective in ameliorating the disorder.

Phenylalanine ammonia lyase inhibitors control browning of cut lettuce

Inhibitors of the first enzyme in the phenylpropanoid pathway, phenylalanine ammonia lyase (PAL), were used to investigate the role of phenolic metabolism in browning of lettuce tissue. Excised 4 7-cm midrib segments were soaked fo r1ha t20°C in aqueous solutions of the PAL inhibitors, a-aminooxyacetic acid (AOA; 0.1‐10 mM), 2-aminoindan-2-phosphonic acid (AIP; 50‐100 mM), and a-aminooxi-b-phenylpropionic acid (AOPP; 200 mM). Browning of the cut ends and uncut surfaces was measured using a visual score, and CIE color values (L*, a*, b*). Overall browning potential was measured as the absorbance at 340 nm of an aqueous extract of the tissue. The visual scores were more highly correlated with hue angle than with the a* and b* values; there was no correlation with the L* values. Ethylene applied at 5 m ll 1 had no effect upon browning compared with the air treatments. AIP at 50 mM and AOPP at 200 mM effectively inhibited browning; AOA was less effective requiring 3‐10 mM to reduce browning. These results confirm the view that for browning to occur in lettuce PAL activity is required to form phenolics that are subsequently oxidized and polymerized.

Optimization of edible coating formulations on zucchini to reduce water loss

Abstract Zucchini ( Cucurbita pepo; melopepo ) fruit were coated with 0.5, 0.75 or 1.0% aqueous solutions of Semperfresh ™ and with different formulations of calcium caseinate-acetylated monoglyceride aqueous emulsions ranging from 2.5 to 7.0% total solids. Semperfresh ™ did not increase water vapor resistance of zucchini. Rates of respiration and ethylene production from coated and uncoated zucchini increased at the beginning of storage at 12.5 and 86% RH, but decreased rapidly thereafter. This type of response is indicative of a temporary metabolic disturbance resulting from preparation and a non-climacteric pattern of respiration. Coatings did not affect internal carbon dioxide or ethylene concentrations. Hue angle and lightness values were not significantly different for coated and uncoated zucchini. Ridge analysis indicated that a maximum water vapor resistance will result from relatively high sodium caseinate and low acetylated monoglyceride contents in edible coatings for zucchini fruit.

Chilling injury is reduced in cucumber and rice seedlings and in tomato pericarp discs by heat-shocks applied after chilling

A number of abiotic shocks (e.g. heat-shock) applied before chilling have previously been shown to increase the chilling tolerance of cucumber and rice seedlings and of tomato fruit. The subsequent rate of radicle growth after chilling of these seedlings is a sensitive measure of chilling injury as is the subsequent increase in ion leakage from chilled discs of tomato pericarp fruit tissue. Data is presented to show that a brief heat-shock applied after chilling also decreases the subsequent development of chilling injury symptoms in cucumber and rice seedlings and in tomato pericarp discs. Development of this post-chilling, heat-shock induced tolerance is optimal if applied 1 h after removal from chilling, so induced synthesis of heat-shock proteins is not excluded as a possible mechanism of action.