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

A comparison of the quality of plain yogurt and its analog made from coconut flesh extract

The aim of this study was to compare the quality of plain yogurt made from cow milk (n = 10) and its plant-based analog made from coconut flesh extract (n = 14). Coconut yogurt alternatives were divided into 2 experimental groups based on differences in their color, which were noted after the packages had been opened. The first group included products with a typical white color (n = 8), and the second group comprised products with a grayish pink color (n = 6) that developed as a result of oxidative processes. In comparison with its plant-based analog, plain yogurt was characterized by higher values of lightness (L*), yellowness (b*) and chroma (C*), higher titratable acidity, a higher content of retinol and α-tocopherol, higher nutritional value of fat, and lower values of water-holding capacity (WHC) and redness (a*). Plain yogurt had lower volatile acidity than its plant-based analog with a grayish pink color. A comparison of yogurt analogs with different colors revealed that the product with a grayish pink color was characterized by a lower value of L*, and higher values of a*, b*, C*, and pH. An analysis of its fatty acid profile demonstrated that it also had a higher proportion of C14:0 and C18:1 cis-9; higher total monounsaturated fatty acids content; a lower proportion of C10:0, C12:0, and C18:2; a lower total content of polyunsaturated fatty acids (PUFA) and essential fatty acids; and a lower ratio of PUFA to saturated fatty acids. The yogurt analog with a grayish pink color had a lower total content of tocopherol isoforms than the remaining products. The yogurt analog with a white color had the highest WHC and γ-tocopherol content. Consumers should be aware of the fact that coconut yogurt alternatives may have nonstandard quality attributes. The differences between such products and yogurt made from cow milk should be explicitly communicated to consumers so that they could make informed purchasing decisions.

Salicylic Acid Spray Delays Sand Pear Fruit Senescence during Room Temperature Shelf Life by Regulating Antioxidant Capacity and Senescence-Related Genes

'Whangkeumbae' (Pyrus pyrifolia) is a variety of sand pear fruit well-known for its smooth surface and good taste. However, the fruit quality is adversely affected by postharvest ethylene production. Therefore, improving postharvest shelf life by regulating fruit senescence is critical to promoting the 'Whangkeumbae' fruit industry. Here, we investigated the effect of salicylic acid (SA) spray on fruit senescence in sand pears during room temperature shelf life. Exogenous SA reduced polyphenol oxidase (PPO) activity and malondialdehyde (MDA) content during room temperature shelf life. Additionally, SA effectively maintained the fruit skin coloration and increased the activity of antioxidant enzymes, such as superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), and ascorbate peroxidase (APX). SA treatment inhibited PpPPO1 expression and upregulated PpSOD1, PpAPX6, and PpGST2 expression. Furthermore, SA application downregulated the expression of PpACO2, PpEIN3a, PpNCED1, and PpAOC2, while upregulating PpNPR-1, PpTAR2, and PpCOMT1 during room temperature shelf life. SA treatment also influenced cell wall metabolism and modification genes by inhibiting PpPG1, PpPME2, and PpCEL3 and inducing PpPGIP1 expression. Additionally, SA treatment affected sugar and acid metabolism genes and increased the expression of PpSPS1, PpSUS1, PpSOT1, PpTMT4, PpSWEET15, and PpcyNAD-MDH, but suppressed the expression of PpcyNADP-ME. The Pearson correlation analysis indicated that PPO activity and MDA content were positively correlated with the expression of PpPPO1, PpACO2, PpEIN3a, PpNCED1, PpAOC2, PpPG1, PpPME2, PpCEL3, and PpcyNDA-MDH. Conversely, these factors were negatively associated with the activities of SOD, POD, CAT, and APX, as well as the expression levels of PpSOD1, PpPOD1, PpCAT1, PpAPX6, PpGST2, PpNPR-1, PpTAR2, PpCOMT1, PpPGIP1, PpSPS1, PpSUS1, PpSOT1, PpTMT4, PpSWEET15, and PpcyNAD-MDH. Our results reveal that exogenous SA could delay fruit senescence in sand pear fruit by regulating various biochemical and molecular mechanisms and can be used to effectively extend fruit shelf life during room temperature storage. However, further research is necessary to determine whether the fruits sprayed with SA are suitable for direct human consumption.

From polyphenol to o-quinone: Occurrence, significance, and intervention strategies in foods and health implications

Polyphenol oxidation is a chemical process impairing food freshness and other desirable qualities, which has become a serious problem in fruit and vegetable processing industry. It is crucial to understand the mechanisms involved in these detrimental alterations. o-Quinones are primarily generated by polyphenols with di/tri-phenolic groups through enzymatic oxidation and/or auto-oxidation. They are highly reactive species, which not only readily suffer the attack by nucleophiles but also powerfully oxidize other molecules presenting lower redox potentials via electron transfer reactions. These reactions and subsequent complicated reactions are capable of initiating quality losses in foods, such as browning, aroma loss, and nutritional decline. To attenuate these adverse influences, a variety of technologies have emerged to restrain polyphenol oxidation via governing different factors, especially polyphenol oxidases and oxygen. Despite tremendous efforts devoted, to date, the loss of food quality caused by quinones has remained a great challenge in the food processing industry. Furthermore, o-quinones are responsible for the chemopreventive effects and/or toxicity of the parent catechols on human health, the mechanisms by which are quite complex. Herein, this review focuses on the generation and reactivity of o-quinones, attempting to clarify mechanisms involved in the quality deterioration of foods and health implications for humans. Potential innovative inhibitors and technologies are also presented to intervene in o-quinone formation and subsequent reactions. In future, the feasibility of these inhibitory strategies should be evaluated, and further exploration on biological targets of o-quinones is of great necessity.

Monitoring tert-Butylhydroquinone Content and Its Effect on a Biolubricant during Oxidation

The use of biolubricants as a replacement for petroleum-based products is becoming more and more important, due to the current global energy and crude oil scenario. Thus, the production of biolubricants (which could take place in biorefineries) should be as efficient as possible, obtaining high-quality products with suitable viscosity or oxidation stability values to compete with oil refineries. One of the ways to produce biolubricants is through double transesterification from vegetable oils, where the role of catalysts (usually homogeneous) is vital, as they can improve the yield of the process. However, they should be removed after the chemical reaction, which is difficult once the biolubricant is obtained. Otherwise, they could act as catalysts during oxidation, contributing to a further decrease in oxidation stability and provoking significant changes. To avoid this, antioxidant addition could be an interesting choice. The aim of this work was to assess TBHQ addition in frying oil biolubricants, monitoring properties such as viscosity, acid number, absorbance or TBHQ content (through voltammetry) during oxidation. TBHQ addition (2114 mg·L^-1) kept the main quality parameters during oxidation compared to control samples. In contrast, TBHQ content decreased during oxidation (to 160 mg·L^-1), which proved its antioxidant effect.

Enzymatic browning in apple products and its inhibition treatments: A comprehensive review

Apple (Malus domestica) is widely consumed by consumers from various regions. It contains a high number of phenolic compounds (majorly hydroxybenzoic acids, hydroxycinnamic acids, flavanols, flavonols, dihydrochalcones, and anthocyanins) and antioxidant activity, which are beneficial for human health. The trends on healthy and fresh food have driven the food industry to produce minimally processed apple, such as fresh-cut, puree, juice, and so on without degrading the quality of products. Enzymatic browning is one of the problems found in minimally processed apple as it causes the undesirable dark color as well as the degradation of phenolics and antioxidant activity, which then reduces the health benefits of apple. Proper inhibition is needed to maintain the quality of minimally processed apple with minimal changes in sensory properties. This review summarizes the inhibition of enzymatic browning of apple products based on recent studies using the conventional and nonconventional processing, as well as using synthetic and natural antibrowning agents. Nonconventional processing and the use of natural antibrowning agents can be used as promising treatments to prevent enzymatic browning in minimally processed apple products. Combination of 2-3 treatments can improve the effective inhibition of enzymatic browning. Further studies, such on as other potential natural antibrowning agents and their mechanisms of action, should be conducted.

Control of Enzymatic Browning in Strawberry, Apple, and Pear by Physical Food Preservation Methods: Comparing Ultrasound and High-Pressure Inactivation of Polyphenoloxidase

Polyphenoloxidase (PPO) enzyme can be found in fruits, vegetables and crustaceans. Its activity, promoted by oxygen, causes food browning with subsequent loss of quality and limited shelf life. Foods are pasteurized with conventional and novel physical methods to inactivate spoilage enzymes, thus avoiding the addition of unhealthy chemical preservatives. Ultrasound and high- pressure processing (HPP) are non-thermal technologies capable of retaining vitamins, bioactives and sensory components of fresh fruits. Enzyme residual activity vs. processing time were plotted for strawberry, apple, and pear purees subjected to thermosonication (1.3 W/g—71 °C), HPP-thermal (600 MPa—71 °C) and heat treatment alone at 71 °C. The PPO residual activities after treatments were highly variable. TS was the most effective for inactivating PPO, followed by thermal processing. HPP-thermal did not improve the inactivation compared with thermal treatment at 71 °C. The resistance of the three fruits’ PPOs exhibited the same pattern for the three technologies: pear PPO was the most resistant enzyme, followed by apple PPO and, lastly, strawberry PPO. However, the resistance of the three PPOs to TS was lower and very similar. Given the huge variability of PPO resistance, it is important to run inactivation tests for different fruits/cultivars. The results can assist manufacturers to avoid browning during processing, storage and distribution of fruit purees, juices and concentrates.