Biochemical characterization and thermal inactivation of polyphenol oxidase from radish (Raphanus sativus var. sativus)

A cooperative combination of non-targeted metabolomics and electronic tongue evaluation reveals the dynamic changes in metabolites and sensory quality of radish during pickling

Pickled radish is a traditional fermented food with a unique flavor after long-term preservation. This study analyzed the organoleptic and chemical characteristics of pickled radish from different years to investigate quality changes during pickling. The results showed that the sourness, saltiness, and aftertaste-bitterness increased after pickling, and bitterness and astringency decreased. The levels of free amino acids, soluble sugars, total phenols, and total flavonoids initially decreased during pickling but increased with prolonged pickling. The diversity of organic acids also increased over time. Through non-targeted metabolomics analysis, 349 differential metabolites causing metabolic changes were identified to affect the quality formation of pickled radish mainly through amino acid metabolism, phenylpropane biosynthesis and lipid metabolism. Correlation analysis showed that L*, soluble sugars, lactic acid, and acetic acid were strongly associated with taste quality. These findings provide a theoretical basis for standardizing and scaling up traditional pickled radish production.

Citric Acid: Properties, Microbial Production, and Applications in Industries

Citric acid finds broad applications in various industrial sectors, such as the pharmaceutical, food, chemical, and cosmetic industries. The bioproduction of citric acid uses various microorganisms, but the most commonly employed ones are filamentous fungi such as Aspergillus niger and yeast Yarrowia lipolytica. This article presents a literature review on the properties of citric acid, the microorganisms and substrates used, different fermentation techniques, its industrial utilization, and the global citric acid market. This review emphasizes that there is still much to explore, both in terms of production process techniques and emerging new applications of citric acid.

Using Numerical Analysis to Develop a Retort Process to Enhance Antioxidant Activity and Physicochemical Properties of White Radish (Raphanus sativus L.) in Different-Sized Packages

Thermal processing of white radish using retort sterilization at different temperatures was investigated according to the dimension of the package. Four different samples with the same weight and volume were placed in packages with different dimensions. The degree of sterilization (i.e., F0-value) at the cold point targeted at 6 min was determined based on experimental data and heat transfer simulation. The sterilization time was considerably increased with a decrease in surface area to unit volume ratio (φ) at each temperature. The sterilization time for the sample with the highest φ (155.56) was approximately five times faster than the sample with the lowest φ (72.22) at all heating temperatures. Numerical simulation conducted with a proper heat transfer coefficient (h) showed mostly good agreement with the experimental data (RMSE < 2 °C). Changes in color and total phenolic content were higher for samples heated at higher temperatures. Hardness values of white radish samples measured for center and edge parts separately were more uniform for samples with a high φ. Results in this study suggest that optimizing heating conditions of root vegetables must consider their package dimensions to satisfy quality attributes after sterilization. Numerical simulation can be utilized as a useful tool to design the sterilization process.

Microbial Responses to the Reduction of Chemical Fertilizers in the Rhizosphere Soil of Flue-Cured Tobacco

The overuse of chemical fertilizers has resulted in the degradation of the physicochemical properties and negative changes in the microbial profiles of agricultural soil. These changes have disequilibrated the balance in agricultural ecology, which has resulted in overloaded land with low fertility and planting obstacles. To protect the agricultural soil from the effects of unsustainable fertilization strategies, experiments of the reduction of nitrogen fertilization at 10, 20, and 30% were implemented. In this study, the bacterial responses to the reduction of nitrogen fertilizer were investigated. The bacterial communities of the fertilizer-reducing treatments (D10F, D20F, and D30F) were different from those of the control group (CK). The alpha diversity was significantly increased in D20F compared to that of the CK. The analysis of beta diversity revealed variation of the bacterial communities between fertilizer-reducing treatments and CK, when the clusters of D10F, D20F, and D30F were separated. Chemical fertilizers played dominant roles in changing the bacterial community of D20F. Meanwhile, pH, soil organic matter, and six enzymes (soil sucrase, catalase, polyphenol oxidase, urease, acid phosphatase, and nitrite reductase) were responsible for the variation of the bacterial communities in fertilizer-reducing treatments. Moreover, four of the top 20 genera (unidentified JG30-KF-AS9, JG30-KF-CM45, Streptomyces, and Elsterales) were considered as key bacteria, which contributed to the variation of bacterial communities between fertilizer-reducing treatments and CK. These findings provide a theoretical basis for a fertilizer-reducing strategy in sustainable agriculture, and potentially contribute to the utilization of agricultural resources through screening plant beneficial bacteria from native low-fertility soil.

Effects of Extraction Conditions on Banana Peel Polyphenol Oxidase Activity and Insights into Inactivation Kinetics Using Thermal and Cold Plasma Treatment

The objective of this work was to characterize banana peel polyphenol oxidase (PPO) and to study the inactivation kinetics during thermal and cold atmospheric pressure plasma treatment. Since varietal differences in enzyme characteristics are a well-known phenomenon, 'Prata' banana peel PPO was characterized, and PPO activity and thermal stability of the peel PPO of the two dessert banana cultivars 'Cavendish' and 'Prata' were compared to identify the cultivar better suited for industrial food applications. A crude extract obtained from the peels of the Brazilian banana variety 'Prata' revealed highest PPO activities (46.0-55.2 nkat/mL) at 30-40 °C in a range of pH 6.0-6.5 after addition of 0.5 g/g_sample polyvinylpyrrolidone and 0.5% (v/v) Triton X-100 during extraction. 'Cavendish' PPO activity was four times higher. Banana peel PPO exhibited the highest affinity towards dopamine (K_M = 0.94 mM). Thermal inactivation of 'Prata' and 'Cavendish' PPO was achieved at 90 °C after 5 and 15 min, respectively, whereas cold plasma treatment did not decrease PPO activity below 46% of the initial enzyme activity. The inactivation behavior of PPO could successfully be described by a two-fraction model indicating at least two types of isoenzymes with different thermal stability. The overall high thermal stability was mainly attributed to membrane-bound PPO. The results may help to prevent enzymatic browning of banana peels and thereby facilitate their valorization as food ingredients.

The Influence of Supercritical Carbon Dioxide on Graham Flour Enzyme Polyphenol Oxidase Activity

Graham flour is a form of whole wheat flour made by grinding the endosperm and is thus also the most nutritious. Generally, the enzyme polyphenol oxidase (PPO) catalyzes two different reactions in the presence of molecular oxygen: the hydroxylation of monophenols to ortho-diphenol and the oxidation of o-diphenol to o-quinone. The purpose of the work was to inactivate PPO activity to extend the shelf life of graham flour and at the same time preserve all the of its high-quality properties. The influence of supercritical CO₂ (scCO₂) treatment on PPO activity in graham flour was investigated. First, graham flour was exposed to scCO₂ conditions, then the proteins were extracted, and in the last step the concentration of total proteins and the specific activity of the PPO enzyme were determined by spectrophotometric assay. PPO activity decreased with an increase in treatment pressure. Furthermore, the flour quality characteristics that meet all needs for wheat end-use products after scCO₂ treatment have been preserved. No major changes in the structure of the granulate or shape of the flour particles were observed. A slightly reduced value of the moisture content in scCO₂-treated graham flour also implies an extension of the shelf life.

Valorisation of the green waste parts from turnip, radish and wild cardoon: Nutritional value, phenolic profile and bioactivity evaluation

The recovery of bio-wastes to obtain high added value compounds is of great interest for the pharmaceutical, medicinal and food industries. Therefore, the aerial parts of turnip (Brassica rapa L.), radish (Raphanus sativus L.) and leaf blade of wild cardoon (Cynara cardunculus L. var. sylvestris (Lamk) Fiori) were characterized regarding their nutritional composition, as also their content in soluble sugars, organic acids, fatty acids, and tocopherols. Furthermore, their hydroethanolic extracts and infusion preparations, were profiled regarding individual phenolic compounds by HPLC-DAD/ESI-MS and their antioxidant, antibacterial and hepatotoxic activities were evaluated. Regarding the nutritional content, wild cardoon revealed the best results, however it was radish and turnip that showed higher values for organic acids and phenolic compounds. The hydroethanolic extract and infusion preparation of wild cardoon stood out for its antioxidant and antibacterial activity. Overall, the hydroethanolic extracts seemed more effective (regarding antioxidant and antibacterial activity) than the infusions. Total phenolic acids proved to be strongly correlated with the antioxidant and antibacterial (against Morganella morganii) activities. This study showed that the discarded parts of these plants can be used as an important natural source of valuable nutrient content and new and safe bioactive compounds, beneficial for human health. Moreover, the extraction of those compounds from underused parts of turnip, radish and cardoon could be used to preserve foods, avoiding artificial additives and thus, contributing to the development of new natural ingredients.

Aggregation and Conformational Changes in Native and Thermally Treated Polyphenol Oxidase From Apple Juice (Malus domestica)

This study investigated the effects of heat treatment after purification on dissociation, aggregation, and structural modification of polyphenol oxidase (PPO) activity from apple (Malus domestica) juice. PPO activity at the 70°C for 10 min was still activated and drastically decreased since 20-60 min with catechol and pyrogallol as substrate. Moreover, spectral results of fluorescence and circular dichroism (CD) indicated that increasing temperature for shorter and longer durations can cause reorganization of the secondary structure of PPO and demolished the native configuration of PPO respectively. Compared with native PPO, all thermally treated PPO showed reduced activity with gradually increasing particle size shift toward section III of some fully assembled proteins treated at 70°C for 10 min (2,670 nm). Polyacrylamide gel electrophoresis (PAGE) analysis also exhibited the increase in protein content at the 70°C for 10 min with molecular size 35 kDa (7.7 ± 0.016c). Hence, thermally treated juice subjected to purification at high temperature for a short time could induce the aggregation of protein and is not really effective for PPO inactivation. For PPO, higher degree of long duration can induce the inactivation of the enzyme after processing.