Characterization of polyphenoloxidase (PPO) and total phenolic contents in medlar (Mespilus germanica L.) fruit during ripening and over ripening

Phytochemistry, nutritional composition, health benefits and future prospects of Mespilus germanica L. (Medlar): A review

Mespilus germanica L., commonly known as medlar, is one of two species of the Rosaceae family. The medlar plant has a long history of use in gastronomy and healthcare. Medlar waste is used to extract hazardous heavy metals from contaminated water. The nutritional value of M. germanica fruits comes from their composition of carbohydrates, carotenoids, amino acids, organic acids, proteins, vitamins, fatty acids, and vital components. M. germanica fruit contains a high concentration of important phenolic components, which contribute to its anti-diabetic and antioxidant properties. Additionally, several studies have identified diverse biological properties of the M. germanica plant, including the cytotoxic, neurodegenerative, and antibacterial properties of its fruits and leaves. Scientists are investigating underutilized plant species to address sustainability issues in food production. This review study will provide a comprehensive examination of its chemical composition, medical applications, plant waste utilization, and potential biological activities.

Characterization of Browning, Chlorogenic Acid Content, and Polyphenol Oxidase Activity in Different Varietal Types of Eggplant (Solanum melongena) for Improving Visual and Nutritional Quality

Eggplant (Solanum melongena L.) breeding for fruit quality has mostly focused on visual traits and nutritional and bioactive compounds, including chlorogenic acid. However, higher contents of chlorogenic acid may lead to more pronounced fruit flesh browning. We examined a diverse collection of 59 eggplant accessions across five varietal types ('black oval', 'striped', 'anthocyanin-free', 'purple', and 'black elongated') to evaluate the degree of browning, polyphenol oxidase (PPO) activity, and chlorogenic acid (CGA) content. The results reveal moderate correlations among these traits, with no clear differences among the varietal types, suggesting that other factors, including genetic variation, might significantly influence these traits. Notably, 'black oval' accessions demonstrated higher browning and PPO activity, whereas 'striped' accessions showed low variability. The identification of genotypes with lower browning and higher CGA content highlights opportunities for targeted genotype selection to improve eggplant chlorogenic acid content while maintaining low or moderate browning, pointing towards the importance of genetic considerations in breeding strategies to reduce browning and enhance nutritional value.

Investigation into the role of phenolic compounds in the protection of citrus against Phyllosticta citricarpa

INTRODUCTION

The use of chemical fungicides to combat disease has made a substantial contribution to food quality and security. Nonetheless, their applications have been limited due to environmental and health concerns, unaffordability, and the fact that pathogens have acquired resistance to some of these fungicides. Alternative eco-friendly and safe control methods should be explored. The current study investigated the influence of citrus rind phenolic compounds against Phyllosticta citricarpa infection by metabolic profiling of two citrus cultivars with varying degrees of susceptibility to infection.

METHODS

Chromatographic data obtained by Ultra Performance Liquid Chromatography-Mass Spectrometry (UPLC) was subjected to multivariate data analysis to identify biomarkers associated with the tolerant cultivar. The identified biomarkers were tested in vitro against P. citricarpa.

RESULTS

Seville oranges, a tolerant cultivar, displayed higher levels of phenolic content and lower total sugar content, that are both associated with lower susceptibility to citrus black spot infection. The generated Principal Component Analysis (PCA) and Orthogonal Projection to Latent Structures-Discriminant Analysis (OPLS-DA) models gave an overview of the data set and identified components that may be responsible for the differences in susceptibility between the two cultivars. Candidate biomarkers associated with tolerance were identified as naringin, neoeriocitrin, bruteiridin, melitidin, and lucenin-2.

CONCLUSION

Naringin, a major candidate biomarker was able to inhibit the growth of the pathogen at 10 000 ppm.

Changes in the Fruit Quality Parameters of Medlar Fruit (Mespilus germanica L.) after Heat Treatment, Storage, Freezing or Hoarfrost

The present study deals with the comparison of traditional fruit processing methods on medlar fruits and their effects on sugar content, organic acids, and phenolic composition in the medlar fruit variety 'Domača nešplja'. The study aimed to analyze which processing methods can be used to make technologically mature medlar fruits that are not yet suitable for consumption edible and to maintain their good chemical quality. The two major sugars in medlars are fructose (59.30 g/kg FW) and glucose (54.43 g/kg FW), and the most abundant organic acids present are malic (8.44 g/kg FW) and quinic acid (8.77 g/kg FW). A total of 38 different phenolic compounds were identified in the medlar fruits: 13 phenolic acids, 9 flavanols, 1 flavone, 3 flavanones, and 12 flavonol glycosides. To explicate: phenolic acids (532.85 mg/kg FW) and flavanols (375.21 mg/kg FW) predominated; neochlorogenic acid had the highest content among phenolic acids; and procyanidins were the most abundant flavanols. The analysis observed statistical differences in metabolite content amongst fruits treated differently (technologically ripe fruits (harvested from the three fruits), edible fruits (technologically ripe fruits stored at 8 °C for 25 days), fruits exposed to the hoarfrost (temperature -1 °C to -4 °C), fruits heated at 60 °C (3 h), and frozen fruits (at -20 °C for 2 months). The lowest levels of fructose (191.77-195.1 g/kg DW) and sorbitol (29.35-31.3 g/kg DW) were detected in the heated and edible fruits. Edible fruits had a 30% lower content of organic acids than technologically ripe fruits and a five times lower content of flavanols, whereas flavonols had an 18.7 times lower content of phenolic acids than technologically ripe fruits. Heating the fruits to 60 °C resulted in a 40% increase in total phenolic compounds in medlars. The results of the study indicate that exposure of medlar fruit to hoarfrost does not significantly affect the chemical quality of the fruit and only minimally alters the composition of sugars, acids, and phenolic compounds. The processing of medlar fruit with hoarfrost, therefore, remains the most suitable method of fruit bletting.

Effects of Dietary Medlar (Mespilus germanica L.) Extract on Growth Performance, Innate Immune Characteristics, Antioxidant Status, and Responses to Crowding Stress in Rainbow Trout (Oncorhynchus mykiss)

High stocking density is a stress factor that potentially affects physiological and immune responses. In this study, the effects of medlar (Mespilus germanica) extract (ME) supplementation on growth performance, antioxidant, immune status, and stress responses in rainbow trout (Oncorhynchus mykiss) were studied. Six hundred fish (40.19 ± 1.09 g; average fish weight ± standard error) were distributed randomly into five experimental groups (assayed in triplicates). The experimental diets were formulated as follows: 0 (T1, control), 0.5% (T2), 1% (T3), 1.5% (T4), and 2% (T4). After 60 days feeding trial, the fish were confined, and the density increased (60 kg/m3) for further 14 days. Results showed significant increases in final weight (FW), weight gain (WG), specific growth rate, and feed intake in the T4 compared to the control (P < 0.05). The feed conversion ratio (FCR) in T4 significantly decreased compared to the control (P < 0.05). Also, the treated groups showed significant improvements in catalase (CAT), superoxide dismutase (SOD), glutathione peroxidase (GPx), lysozyme (LYZ), total immunoglobulin (total Ig), respiratory burst activity (RBA), total protein, and phagocytosis (PHA) (P < 0.05). Moreover, compared with the control group, supplementation could significantly decrease glucose (GLU) and cortisol (CORT), alanine transaminase (ALT), lactate dehydrogenase (LDH), aspartate transaminase (AST), and alkaline phosphatase (ALP) (P < 0.05). After the challenge, FW and WG in all treated challenge groups were significantly improved compared to the control group (P < 0.05). FCR showed a significant decrease in all treated challenged groups compared to the control group (P < 0.05). However, malondialdehyde, CAT, GPx, SOD, LYZ, complement activity (C3 and C4), total Ig, RBA, peroxidase, and PHA in challenged treated groups were significantly increased compared to the control group (P < 0.05). All treated challenged groups showed lower ALT, LDH, AST, ALP, GLU, and CORT levels than the control group (P < 0.05). The experiment herein successfully demonstrated that dietary ME stimulated fish growth, antioxidant status, and immune responses in crowding conditions and can be recommended as beneficial feed additives for rainbow trout.

Metabolomic Profiling of Citrus unshiu during Different Stages of Fruit Development

Citrus fruits undergo significant metabolic profile changes during their development process. However, limited information is available on the changes in the metabolites of Citrus unshiu during fruit development. Here, we analyzed the total phenolic content (TPC), total carotenoid content (TCC), antioxidant activity, and metabolite profiles in C. unshiu fruit flesh during different stages of fruit development and evaluated their correlations. The TPC and antioxidant activity significantly decreased during fruit development, whereas the TCC increased. The metabolite profiles, including sugars, acidic compounds, amino acids, flavonoids, limonoids, carotenoids, and volatile compounds (mono- and sesquiterpenes), in C. unshiu fruit flesh also changed significantly, and a citrus metabolomic pathway related to fruit development was proposed. Based on the data, C. unshiu fruit development was classified into three groups: Group 1 (Aug. 1), Group 2 (Aug. 31 and Sep. 14), and Group 3 (Oct. 15 and Nov. 16). Although citrus peel was not analyzed and the sensory and functional qualities during fruit development were not investigated, the results of this study will help in our understanding of the changes in chemical profile during citrus fruit development. This can provide vital information for various applications in the C. unshiu industry.

Medlar—A Comprehensive and Integrative Review

Among fruit plants belonging to the Rosaceae family, medlar (Mespilus) can be classified as neglected or underutilized. It is a genus of two species of flowering plants: Mespilus germanica (common medlar) and Mespilus canescens. Appreciated for its specific taste and flavor, medlar also possesses biological properties (antioxidant and antimicrobial). Despite the special properties of medlar, there are few research papers on this subject. This review paper includes data not only on medlar fruits but also its leaves, bark, and bud flowers. The main identified components are presented, as well as several biological properties, morphological properties, ethnopharmacological uses, and molecular biology analyses emerging from the scientific papers published in this area.

Variation in Phytochemical, Antioxidant and Volatile Composition of Pomelo Fruit (Citrus grandis (L.) Osbeck) during Seasonal Growth and Development

Citrus fruits exhibit a high level of different phytoconstituents, of which the changes in the different parts of the fruit during ripening have not been thoroughly studied yet. Thus, in this study, we have investigated how different parts of pomelo fruit (Citrus grandis L.) are modified throughout the development of two consecutive growing seasons. In detail, the main phytochemical compounds, such as total phenolic content, total flavonoid content, antioxidant capacity, DPPH free radical scavenging activity, Ferric reducing antioxidant power (FRAP), and naringin and tannin content, were analyzed. A systematic metabolism of these compounds was found during the development of the fruit, but some pomelo tissues showed a fluctuating trend, suggesting a dependence on the different growing season. Focusing on the tissue distribution of these compounds, the fruit membrane contained the highest level of total phenolic and flavonoid content; fruit flavedo displayed the highest antioxidant capacities and FRAP activities, whereas maximum accumulation of naringin was noticed in fruit albedo. Instead, the highest DPPH free radical scavenging activity and tannin contents were found in the pomelo juice. Regarding the distribution of compounds, a possible bias pattern for the accumulation of those compounds has been noticed throughout the fruit development. From the GC-MS analysis, a total of 111 compounds were identified, where 91 compounds were common in both seasons. Overall, these results could be useful for the food processing industry as guidelines for excellent quality foods and for introducing health-beneficial products and components into our daily diets.

Polyphenoloxidase (PPO): Effect, Current Determination and Inhibition Treatments in Fresh-Cut Produce

Fresh-cut produce are quite popular among consumers due to their eating ease, high quality and functional content. However, some of the processing steps taking place during minimal processing (such as cutting, peeling, draining, etc.) might speed up decay, e.g., microbial growth, dehydration or browning. When it comes to the latter, polyphenol oxidase (PPO) plays an important role, being the center of many works focused on the understanding of its reaction mechanism and the application of conservative techniques. The aim of this review study was to compare recent research about the effect of PPO on minimally processed fruits and vegetables, trying to understand the way it acts, the measurement of its activity and current treatments, such as modified atmosphere packaging, washing treatments or edible coatings, among others. In conclusion, the combination of conservation techniques (that is, hurdle technology) is vital to guarantee global quality in minimally processed fruits and vegetables, including synergistic effects which will allow the use of mild treatment conditions to decrease PPO activity. However, further research is required to clearly understand PPO inhibition in trendy techniques such as irradiation.

Browning inhibition on fresh-cut apple by the immersion of liquid endosperm from mature coconuts

The application of natural antibrowning agents for fresh-cut products has been recently considered. This study manifested the efficiency of coconut liquid endosperm, coconut water (CW), from mature tall (cooking) coconut (C-CW) and yellow dwarf coconut (Y-CW) on browning inhibition of 'Gala' apple wedges during storage at 4 ± 1 °C for 9 days. The apple wedges were immersed in water (control), C-CW or Y-CW for 1 min. The visual appearance, superficial colour attributes, browning pigment concentration, total phenols concentration, polyphenoloxidase (PPO) activity and reducing antioxidant capacity of apple wedges were monitored. Moreover, antioxidant activity of both C-CW and Y-CW was also observed. Antioxidant activity of Y-CW was higher than that of C-CW. Both of the CW immersions maintained visual appearance, whiteness and lightness values as well as delayed the increased yellowness and brownness values of hypanthium (flesh) and mesocarp (core) of apple wedges. The browning pigment concentration and PPO activity were obviously lowered by both CW immersions. Total phenols concentration and antioxidant activity of C-CW and Y-CW immersed apple wedges were higher than those of control samples. In conclusion, both of the mature coconut liquid endosperms are feasible natural agent inhibiting browning incidence of fresh-cut fruits during storage.

Inhibitory Effect of Azamacrocyclic Ligands on Polyphenol Oxidase in Model and Food Systems

Enzymatic browning is one of the main problems faced by the food industry due to the enzyme polyphenol oxidase (PPO) provoking an undesirable color change in the presence of oxygen. Here, we report the evaluation of 10 different azamacrocyclic compounds with diverse morphologies as potential inhibitors against the activity of PPO, both in model and real systems. An initial screening of 10 ligands shows that all azamacrocyclic compounds inhibit to some extent the enzymatic browning, but the molecular structure plays a crucial role on the power of inhibition. Kinetic studies of the most active ligand (L2) reveal a S-parabolic I-parabolic noncompetitive inhibition mechanism and a remarkable inhibition at micromolar concentration (IC₅₀ = 10 μM). Furthermore, L2 action has been proven on apple juice to significantly reduce the enzymatic browning.

Approach to preserve phenolics in Thunbergia laurifolia leaves by different drying treatments

Different drying methods and blanching were investigated as to their effects on antioxidant and oxidase activities of Thunbergia laurifolia leaves. Results showed that oven-drying had the highest degradation of total phenolic content (TPC) and antioxidant activity at >85%, while freeze-drying had the lowest at <20%. However, inactivation of oxidase enzymes by blanching at 100°C resulted in a lesser decrease in TPC for oven-drying at 50 and 100°C (51% and 65%, respectively), indicating the importance of inactivating the oxidase enzymes for lower degradation of phenolics on drying. The high-performance liquid chromatography analysis showed that its major antioxidant, rosmarinic acid, degraded tremendously in the presence of oxidase enzymes, but only degraded slightly upon inactivation of oxidase enzymes. Hence, this work showed that by controlling the enzymatic activity, the preservation of phenolics with specific bioactivity in herbal tea leaves can be achieved. PRACTICAL APPLICATIONS: Thunbergia laurifolia leaves have been frequently consumed in the form of a tea or pill due to its medicinal properties. Processing of fresh herbal plant leaves by drying is required to preserve antioxidant phenolic compounds and quality of the plant leaves. Although the drying effects on the antioxidant properties have been studied, the factors that cause the change in properties have not been investigated in-depth. Controlling the factors that affect the phenolic content can help to preserve the beneficial antioxidants when processing the leaves by drying. The result of this study will be of relevance and beneficial to the herbal tea industry.

Effects of treated industrial wastewaters and temperatures on growth and enzymatic activities of duckweed (Lemna minor L.)

The efficacy of the removal of contaminants from wastewater depends on physico-chemical properties of pollutants and the efficiency of treatment plant. Sometimes, low amounts of toxic compounds can be still present in the treated sewage. In this work we considered the effects of contaminant residues in treated wastewaters and of temperatures on Lemna minor L. Treated effluent waters were collected, analyzed and used as duckweed growth medium. In order to better understand the effects of micropollutants and seasonal variation, the plants were grown under ambient conditions for seven days in summer and winter. Relative growth rate, pigments and phenolic compounds concentrations were determined, as well as the activities of catalase (CAT), ascorbate peroxidase (APX), guaiacol peroxidase (G-POD) and polyphenol oxidase (PPO). The pollutant concentrations varied in the two seasons, depending on the industrial and municipal activities and efficiency of treatments. Treated waters contained heavy metals, nitrogenous and phosphorus compounds, surfactants and hydrocarbons. Compared to the control, duckweed growth of treated plants decreased by 25% in summer, while in the winter due to the lower temperatures and the presence of pollutants was completely impeded. The amounts of photosynthetic pigments of treated plants were not significantly affected in the summer, while they were higher than the control in the winter when the effluent had a high nitrogen amount. High CAT activity was registered in both seasons. Treated plants had significantly lower APX activity in the summer (53%) and winter (59%) respect to the controls. The observed inhibition of the peroxidase activities in the exposed plants, confirms the controversy existing in the literature about the variability of enzymatic response in stress condition.

Functional and Rheological Properties of Piñuela (Bromelia karatas) in Two Ripening Stages

The physicochemical characteristics and the activity of the polyphenol oxidase from piñuela fruit juices were determined at two ripening stages. The antioxidant capacity was evaluated by the superoxide anion yield. The ripening stage showed greater ability to inhibit ( $O_2^{. -}$ , 35.3 %). The inhibition of superoxide dismutase was higher for both ripe (88.29 %) and unripe (95.94 %) states. The rheological behaviour of the juice was satisfactorily described using Herschel-Bulkley model (R2 > 0.99). The concentration effect on the rheological parameters was described by the potential law model, and the temperature effect on the viscosity was described based on the Arrhenius equation, finding activation energy values from 11.94 and 17.80 kJ/mol. These results make Bromelia karatas L. a promissory fruit due to their content of secondary metabolites and its antioxidant activity, which could be associated to the presence of phenolic compounds, specifically flavonoids. Variations in these metabolites could also account for structural changes, physicochemical properties, the integrity protection of the fruit against adverse and an alternative to food products.

Comparative analysis of some biochemical parameters of argan pulp morphotypes (Argania spinosa (L) Skeels) during maturity and according to the continentality in Essaouira region (Morocco)

Argania spinosa (L.) Skeels is an endemic forest tree for Morocco. The phytochemical compounds evaluation of four different morphotypes of their fruit pulps was investigated. The total content of sugar, protein and phenolic compounds were monitored during three different stages of maturation in the semi-continental (Mejji) and littoral regions (R'zwa). Total sugars, proteins, phenolics increased up to the ripe stage of all argan fruit morphotypes in the two regions. Spherical shape had higher sugar and protein content than other morphotypes. A significant difference (p < 0.05), was demonstrated by Pearson's test, between the different morphotypes at three stages studied for all the phytochemicals compounds. Likewise, ANOVA test established that the variation of this compounds was influenced by the stage of maturation and/or region of development and/or their interaction according to fruit shape. Results from this study revealed that the increase of these parameters level take place for the most part during the last stages of maturity which synchronize with fruit softening. Furthermore, our results showed information about the richness of argan fruit pulp in carbohydrates compounds and secondary metabolites as the possibility of their contribution in nutritive forage value especially at ripe stage.

Cultivar characterization of stone fruits for their minimal processing

Due to their physicochemical properties, minimal processing of fruits is challenging because of their quick quality loss. Therefore, several preservative steps are needed, such as washing treatments, modified atmosphere packaging, cultivar selection, etc. The latter is an important pre-harvest condition due to the fact that many functional compounds and enzymes are dependent on the cultivar. The aim of this study was to compare the minimal processing adaptation of different stone fruits (nectarine, peach, and plum), focusing on visual and functional quality. The fruit underwent minimal processing in a clean room (temperature and relative humidity were kept at 8 °C and 70 %, respectively) and was washed during 2 min into 2 % w/v ascorbic acid, 1 % w/v citric acid, and 1 % w/v calcium lactate solution. To sum up, nectarine and peach offered a better adaptation, and medium-season cultivars had higher quality maintenance. Nevertheless, late cultivars should not be ruled out before hand on account of their remarkable functional content.

Selected biochemical properties of polyphenol oxidase in butter lettuce leaves (Lactuca sativa L. var. capitata) elicited with dl-β-amino-n-butyric acid

The study concentrated on changes in certain biochemical parameters of polyphenol oxidase (PPO) from lettuce leaves caused by dl-β-amino-n-butyric acid (BABA) elicitation. PPO from control plants demonstrated the highest affinity toward catechol, whereas PPO from BABA-elicited lettuce showed the highest affinity to 4-methylcatechol. The optimum temperature for enzymes from control plants was 35°C, whereas from plants elicited with 1mM BABA this was 25°C. PPO from plants elicited with BABA was also more sensitive to the tested inhibitors than PPO from control plants. l-Cysteine was the most effective inhibitor. Native gel stained for PPO activity in control samples showed two isoforms. However, in BABA-treated lettuce three bands visualising PPO activity were observed. The information obtained in this study will be valuable for the development of treatment technology and storage conditions to control undesirable browning reactions in elicited lettuce.

A review of the chemistry of the genus Crataegus

Since the 1800s, natural health products that contain hawthorn (Crataegus spp.) have been used in North America for the treatment of heart problems such as hypertension, angina, arrhythmia, and congestive heart failure. Traditionally, Native American tribes used hawthorn (Crataegus spp.) to treat gastrointestinal ailments and heart problems, and consumed the fruit as food. Hawthorn also has a long history of use in Europe and China for food, and in traditional medicine. Investigations of Crataegus spp. typically focus on the identification and quantification of flavonoids and anthocyanins, which have been shown to have pharmacological activity. The main flavonoids found in Crataegus spp. are hyperoside, vitexin, and additional glycosylated derivatives of these compounds. Reviewed herein are the botany, ethnobotany, and traditional use of hawthorn while focusing on the phytochemicals that have been reported in Crataegus species, and the variation in the described chemistry between individual species.

Purification and characterization of polyphenol oxidase from rape flower

The purification and partial enzymology characteristics of polyphenol oxidase (PPO) from rape flower were studied. After preliminary treatments, the crude enzyme solution was in turn purified with ammonium sulfate, dialysis, and Sephadex G-75 gel chromatography. The optimal conditions and stability of PPO were examined at different pH values and temperatures. Subsequently, PPO was also characterized by substrate (catechol) concentrations, inhibitors, kinetic parameters, and molecular weight. Results showed that the optimal pH for PPO activity was 5.5 in the presence of catechol and that PPO was relatively stable at pH 3.5-5.5. PPO was moderately stable at temperatures from 60 to 70 °C, whereas it was easily denatured at 80-90 °C. Ethylenediaminetetraacetic acid, sodium chloride, and calcium chloride had little inhibitive effects on PPO, whereas citric acid, sodium sulfite, and ascorbic acid had strongly inhibitive effects. The Michaelis-Menten constant (K(m)) and maximal reaction velocity (V(max)) of PPO were 0.767 mol/L and 0.519 Ab/min/mL of the crude PPO solution, respectively. PPO was finally purified to homogeneity with a purification factor of 4.41-fold and a recovery of 12.41%. Its molecular weight was 60.4 kDa, indicating that the PPO is a dimer. The data obtained in this research may help to prevent the enzymatic browning of rape flower during its storage and processing.

Individual astringency responsiveness affects the acceptance of phenol-rich foods

Sensory responses greatly vary between individuals, and individual sensory experiences influence eating behaviour. Three groups responding differently to phenolic astringent stimuli (Low Responding, LR, n=20, Medium Responding, MR, n=37 and High Responding, HR, n=20) were identified from a population of 77 subjects, based on the maintenance vs fluctuation of salivary characteristics after repeated stimulation of the masticatory and taste/somatosensory systems. The effect of LR, MR and HR status on perceived astringency and liking for phenol-containing apple, grape and carrot juices spiked with increasing tannic acid (TA) concentrations was examined. TA induced a greater increase of perceived astringency in HR, compared to MR and LR subjects. A decrease in liking for spiked juices was found in HR and to a lesser extent in MR and LR subjects. No significant differences were found comparing MR and LR groups for both astringency intensity and liking data. Liking for and familiarity with 37 food items, as well as preference for 14 phenol-rich foods and beverages, each paired with a less astringent counter-product, were also examined. An internal preference map was computed on liking scores and product subgroups were identified. An effect of LR/HR status was found for two food subgroups consisting of coffee without sugar, tea without sugar, raw chicory and milk chocolate, tea with sugar, coffee with sugar. LR subjects rated the products with the most astringency higher and those with the least astringency lower than did HR subjects. LR subjects also rated their familiarity with highly astringent products higher than did HR subjects. Thus, individual differences related to the physiological salivatory response to oral stimulations affect responses to astringent stimuli and can influence the overall acceptability of phenol-rich food items.

Polyphenol oxidase activity of two olive (Olea europaea L.) cultivars as an early criterion of Mn toxicity

BACKGROUND

The time course of polyphenol oxidase (PPO) activity in the leaves of two olive cultivars (Picual and FS-17) irrigated with nutrient solutions differing in Mn concentration (0, 2 and 1280 micromol L(-1)) was studied under hydroponic conditions to determine whether PPO activity could be used as an early criterion of Mn status of olive plants, and to elucidate whether genotypic differences exist between the two olive cultivars studied, concerning the effect of Mn concentration on PPO activity.

RESULTS

In all the Mn treatments, PPO activity was greater in Picual than in FS-17. Under excess Mn (1280 micromol L(-1)), PPO activity gradually increased with time, starting from day 30 of the experiment in both cultivars, and this increase preceded the appearance of Mn toxicity symptoms. In contrast, in the other two Mn treatments (0 and 2 micromol L(-1)) PPO activity increased and afterwards decreased during the experiment, but the trend was not clear. In the 1280 micromol L(-1) treatment, PPO activity linearly increased (R = 0.8836 for Picual and 0.943 for FS-17) with the increase of Mn concentration in the leaves of both cultivars. In the 1280 micromol L(-1) Mn treatment, PPO activity was negatively related with Fe and Zn concentrations in the leaves, and positively in the 0 and 2 micromol L(-1) Mn treatments with the Ca, Mg and K concentrations.

CONCLUSION

From the differential time course of PPO activity in the three Mn treatments (0, 2 and 1280 micromol L(-1)), it is concluded that periodic measurements of PPO activity in the leaves of the olive cultivars Picual and FS-17 can be used for the early detection of Mn toxicity (before the appearance of symptoms).