Yoram Fuchs

Examination of Bacteriophage as a Biocontrol Method for Salmonella on Fresh-Cut Fruit: A Model Study

The preparation and distribution of fresh-cut produce is a rapidly developing industry that provides the consumer with convenient and nutritious food. However, fresh-cut fruits and vegetables may represent an increased food safety concern because of the absence or damage of peel and rind, which normally help reduce colonization of uncut produce with pathogenic bacteria. In this study, we found that Salmonella Enteritidis populations can (i) survive on fresh-cut melons and apples stored at 5 degrees C, (ii) increase up to 2 log units on fresh-cut fruits stored at 10 degrees C, and (iii) increase up to 5 log units at 20 degrees C during a storage period of 168 h. In addition, we examined the effect of lytic, Salmonella-specific phages on reducing Salmonella numbers in experimentally contaminated fresh-cut melons and apples stored at various temperatures. We found that the phage mixture reduced Salmonella populations by approximately 3.5 logs on honeydew melon slices stored at 5 and 10 degrees C and by approximately 2.5 logs on slices stored at 20 degrees C, which is greater than the maximal amount achieved using chemical sanitizers. However, the phages did not significantly reduce Salmonella populations on the apple slices at any of the three temperatures. The titer of the phage preparation remained relatively stable on melon slices, whereas on apple slices the titer decreased to nondetectable levels in 48 h at all temperatures tested. Inactivation of phages, possibly by the acidic pH of apple slices (pH 4.2 versus pH 5.8 for melon slices), may have contributed to their inability to reduce Salmonella contamination in the apple slices. Higher phage concentrations and/or the use of low-pH-tolerant phage mutants may be required to increase the efficacy of the phage treatment in reducing Salmonella contamination of fresh-cut produce with a low pH.

Determination of mango physiological indices by near-infrared spectrometry

Abstract The objectives of the study were to evaluate the use of near-infrared (NIR) spectrometry in measuring the physiological properties of mango fruit, cv. ‘Tommy Atkins’ and to establish relationships between the nondestructive NIR spectral measurements and the major physiological properties and quality indices of mango fruit. These include softening of the flesh, total soluble solids content and acidity. Intact mango fruit were measured by reflectance NIR in 1200–2400 nm range. NIR models were developed based on multi-linear regression (MLR), principal component analysis (PCA) and partial least square (PLS) regression with respect to the reflectance and its first derivative, the logarithms of the reflectance reciprocal and its second derivative. The above regression models, related the NIR spectra to storage period, firmness, sugar content and acidity. The best combination, based on the prediction results, was MLR models with respect to the second derivative logarithms of the reflectance reciprocal. Predictions with MLR models resulted standard errors of prediction (SEP) of 1.223, 0.161, 17.14 and 37.03, and coefficients of determination of 0.9276, 0.6085, 0.8226 and 0.9380 for TSS, acidity, firmness and storage period, respectively. It was concluded that by using the NIR spectrometry measurement system, in the appropriate spectral range, it is possible to nondestructively assess the maturity factors of mango fruit.

Post-harvest retention of the red colour of litchi fruit pericarp☆

Abstract Litchi fruit colour was preserved by sulphur dioxide (SO2) fumigation, followed by a dip in dilute hydrochloric acid. The pH and the colour of the pericarp (rind) were affected by the pH of the acid and by the duration of the treatments. The fumigation inhibited polyphenol oxidase (PPO) activity, but had no effect on peroxidase (PO) activity. Methanolic extracts of red pericarp absorbed strongly at 525 nm, while extracts of brown pericarp had a low peak of absorbance (at 525 nm) even after acidification. It is suggested that when PPO activity is inhibited soon after harvest, the red colour of litchi fruit can be preserved if the pH of the rind remains acidic.

Modified atmosphere and modified humidity packaging alleviates chilling injury symptoms in mango fruit

Storage of mango (Mangifera indica L. cvs. Tommy Atkins and Keitt) fruits at 12°C caused slight chilling injury (CI) symptoms on the fruit peel, expressed as red spots around the lenticels (lenticel spotting). A modified atmosphere (5% CO2 and 10% O2) was created in 4-kg film-lined cartons by using microperforated polyethylene (PE) or Xtend ® film (XF). For ‘Keitt’ fruit, a similar atmosphere was also applied using controlled atmosphere chambers. After 3 weeks of storage at 12°C plus 1 week at 20°C, both modified and controlled atmosphere treatments were effective in reducing CI. The most effective reduction was found in fruits that were packed in the XF film. A second advantage of using XF film was the reduction in the level of sap inside the package due to the lower relative humidity in the XF film (90%) compared with that of PE packaging ( 99%).

Hot water brushing: an alternative method to SO2 fumigation for color retention of litchi fruits

Abstract Distribution of high-quality litchi ( Lychee chinensis Sonn.) fruits to global markets depends exclusively on postharvest treatments to suppress peel browning. The current standard treatment of litchi fruits in Israel includes fumigation with sulfur dioxide (SO 2 ) followed by dipping the fruits in hydrochloric acid containing the fungicide prochloraz. As part of the effort to reduce the use of potentially hazardous chemicals in agriculture, we developed a new procedure that may enable SO 2 to be avoided. Instead of fumigation, litchi fruits are sprayed with hot water while being brushed in a revolving drum, after which the fruits are subjected to hydrochloric acid treatment. Fruits that are processed in this way maintain a uniform red color for at least 35 days, without apparent deterioration in external or internal quality, or taste. Physiological studies demonstrate that polyphenol oxidase (PPO) activity is reduced by the hot water brushing (HWB) procedure as compared with controls but not to the same extent as inhibition by SO 2 treatments. In addition to its effect on PPO activity, HWB may lead to reduced pH of the pericarp, or more uniform distribution of the acid in it. This result suggests that HWB may act by bruising the external layer of the pericarp allowing the peel to be uniformly exposed to the acid which may inhibit PPO activity and maintain the anthocyanins in their red-pigmented form.

Effect of hot water brushing, prochloraz treatment and waxing on the incidence of black spot decay caused by Alternaria alternata in mango fruits

Abstract A combined hot water spray and fruit brushing (hot water brushing–HWB) treatment for 15–20 s was developed for reducing the incidence of postharvest disease caused by Alternaria alternata and improving mango fruit keeping quality. The efficacy of the hot water treatment was tested over a range of temperatures from 48 to 64°C, in combination with prochloraz treatment and fruit waxing. Hot water brushing of fruits significantly reduced decay (infected area) development by A. alternata. But after storage for 3 weeks at 12°C and another week at 20°C, the reduction of disease incidence by HWB and prochloraz treatment (900 μg ml−1) was more effective than by HWB alone. HWB treatment for 15 s improved colour development and was more effective than the common, commercial 5 min dip treatment at 55°C. The combination of fruit HWB and waxing yielded high quality fruits with less decay development.

Partial purification and some properties of polyphenol oxidase extracted from litchi fruit pericarp

Abstract Litchi (Litchi chinensis Sonn.) fruit peel polyphenol oxidase (PPO) was partially purified 21 fold by ammonium sulfate fractionation and gel filtration. Pyrogallol, catechol, and 4-methylcatechol were good substrates for the enzyme; with no activity observed with chlorogenic acid, p-cresol, resorcinol, or tyrosine. The optimal pH for PPO activity was 7.0 with 4-methylcatechol, with the enzyme being most stable at pH 7.4. The enzyme was relatively temperature stable with maximum activity at 70 °C and requiring a little less than 10 min at 90 °C for 50% loss of activity. The Km and Vmax for the enzyme, with 4-methylcatechol, were 10 mM and 1.47 × 104 units/min per mg protein, respectively. The enzyme was not activated by SDS. Reduced glutathione, l -cysteine, tropolone, thiourea, FeSO4, and SnCl2 markedly inhibited PPO activity, whereas MnSO4 and CaCl2 enhanced PPO activity. Data obtained in this study might help to better understand and control commercially, litchi fruit peel browning.

The Elicitation of Ethylene Biosynthesis by a Trichoderma Xylanase Is Not Related to the Cell Wall Degradation Activity of the Enzyme.

A [beta]-1,4-endoxylanase (EIX) isolated from Trichoderma viride elicits plant defense responses in certain tobacco (Nicotiana tabacum L.) cultivars in addition to its xylan degradation activity. It was not clear whether elicitation occurs by cell wall fragments released by the enzymic activity or by the xylanase protein interacting directly with the plant cells. We used protoplasts isolated from tobacco leaves to test whether the cell wall is required for the stimulation of ethylene biosynthesis by EIX. Protoplasts of tobacco (cv Xanthi) responded to treatment with the EIX, as indicated by an increased production of ethylene and the loss of protoplast viability. Protoplasts prepared from ethylene-pretreated leaves produced more ethylene and had higher rates of cell death in response to EIX than protoplasts prepared from nonethylene-treated leaves. Protoplasts of an EIX-insensitive cultivar of tobacco (Hicks) were insensitive to high concentrations of EIX. The addition of a crude cell wall preparation to protoplasts during incubation with EIX did not enhance the induction of ethylene biosynthesis by nonsaturating as well as saturating concentrations of EIX. These data indicate that the xylanase activity of EIX is unrelated to the elicitation of ethylene biosynthesis through the production of some cell wall fragment, since the protein per se appears capable of eliciting ethylene biosynthesis in protoplasts.

Combining yeasts or a bacterial biocontrol agent and heat treatment to reduce postharvest decay of 'Gala' apples.

‘Gala’ apples were treated after harvest with heat (38°C for 4 days), and then wound-inoculated with the pathogen Penicillium expansum and the antagonist Pseudomanas syringae, or one of two yeast antagonists, to reduce postharvest decay. After storage for 7 days at 20°C or 3 months at 1°C, the least decay was found on fruit where wounds had been allowed to cure by heat treatment (38°C) or cold storage (1°C) for 4 days before inoculation with the pathogen. Addition of any of the antagonists before or after heat treatment further reduced the number and size of the lesions. The highest lesion incidence occurred on apples wounded after heat treatment followed by inoculation with the pathogen. Addition of the yeast antagonists to these fresh wounds reduced the fruit decay as well. While the heat treatment is phytosanitary in that it significantly reduces the pathogen population on the apple surface, it provides little residual protection. The residual protection from the antagonists adds to the control provided by the heat treatment. Published by Elsevier Science B.V.

Purification of polyphenol oxidase and the browning control of litchi fruit by glutathione and citric acid

Polyphenol oxidase (PPO, EC 1.10.3.2) was purified to homogeneity from litchi peel yielding a single protein with a molecular weight of about 75.6 kD by Sephadex G-100 gel filtration, and a 108-fold purification of PPO achieved. The enzyme was determined to be composed of two similar subunits. Glutathione, L-cysteine and citric acid suppressed PPO activity markedly, whereas ascorbic acid and n-propyl gallate showed a little inhibition. Moreover, the effect was enhanced by the addition of citric acid. On the basis of the inhibition of PPO activity in vitro, the use of 10 mmol l −1 glutathione and 100 mmol−1 l citric acid was found to give good control of the browning of litchi fruit, and an 80–85% inhibition of PPO activity was observed. It is suggested that application of glutathione in combination with citric acid may slow down the browning of litchi fruit.