Browning

Presence of browning or black-spot in fresh foods can adversely affect consumer acceptance. Onion has been reported to exert inhibitory activity against browning reaction. This research aimed to uncover the mechanism and identify active compounds in onion responsible for PPO inhibitors based on metabolomic approach. Onion was fractioned using different solvents, i.e n-hexane; chloroform; ethyl acetate; water, respectively. As a result, ethyl acetate fraction (EAF) of the onion demonstrated the strongest inhibition to PPO in comparison with other fractions, i.e. n-hexane, chloroform, and water. The reversible inhibitory activity of PPO by EAF occurred with presence of L-DOPA as substrate through competitive inhibition and Cu chelation in the active side of the PPO. Based on 1H-NMR (X) score plot and PPO inhibition (Y) using OPLS, NMR signals revealed that active compounds accounting for inhibition of PPO included quercetin, kaempferol, cyanidin 3.4’-di-O-β-glucopyranoside, quercetin ...

The relationship between acrylamide content and Maillard browning in French fries was studied as a function of different glucose/fructose ratios in the raw product using several colour measurement methods. An exponential correlation was found between acrylamide formation and surface colour, as evaluated by the parameters a*, delta E and Agtron. As a consequence, small differences in product colour could result in more pronounced differences in acrylamide contamination. This relationship however appeared to be dependant upon the glucose/fructose ratio of the raw material. An excess of fructose compared to glucose stimulated acrylamide formation to a higher extent than Maillard browning. The opposite effect was established with an excess of glucose. In addition, a linear relationship was found between the absorbance of aqueous French fries extracts and acrylamide content, which moreover appeared to be less affected by the addition of extra reducing sugars prior to frying. To predict acrylamide formation, sugar analysis however remains an important instrument and is complementary to surface colour measurement.

Breakfast cereals are significant contributors to the daily intake of food-derived acrylamide in Western countries. Acrylamide was determined by LC-MS in 60 commercial breakfast cereals marketed in Spain. Several SPE cartridges were evaluated for clarification of the aqueous extract. LOQ was 62 μg/kg. Acrylamide content ranged from < 62–803 μg/kg (average 292 μg/kg, and median 258 μg/kg, with an estimated acrylamide intake from breakfast cereals of 2.68 μg acrylamide/person/day. According to the German concept of minimization, a signal value 450 μg/kg was calculated. Relationships among acrylamide and some parameters of the studied samples such as type of cereal, its physical form (puffed and flaked) or certain ingredients in the formulation (proteins and dietary fibre content) were also investigated. Wheat-based cereals contained significantly higher levels of acrylamide, as did samples with higher fibre or protein content. In addition, puffed breakfast cereals also contained significantly higher levels of acrylamide. There was no significant correlation between acrylamide levels and contents of 5-hydroxymethylfurfural, furosine or cereal browning.

ABSTRACT:  We have investigated the mechanism of action of 4-hexylresorcinol (4-HR) and ascorbic acid (AA) on the polyphenol oxidase (PPO) catalyzed oxidation of phenolic substrates. Incubation of PPO with 4-HR diminishes strongly PPO activity. This effect can be erroneously interpreted, due to the high affinity of 4-HR for PPO, as irreversible inactivation of PPO. However, PPO activity can be recovered by dialysis after incubation with 4-HR. 4-hexylresorcinol is a canonical enzyme inhibitor that binds preferentially to the oxy form of PPO. It is a mixed-type inhibitor, because it influences both apparent Vmax (1.26 compared with 0.4 units in the absence and presence of 4-HR, respectively) and Km values (0.28 mM compared with 0.97 mM in the absence and in the presence of 4-HR, respectively) of PPO. AA can prevent browning by 2 different mechanisms: In the absence of PPO substrates it inactivates PPO irreversibly, probably through binding to its active site, preferentially in its oxy form. In the presence of PPO substrates, AA reduces PPO oxidized reaction products, which results in a lag phase when measuring PPO activity by monitoring dark product formation but not when monitoring O2 consumption. The simultaneous use of both 4-HR and AA on PPO results in additive prevention of browning.

ABSTRACT:  The effects of ascorbic acid (AA) and 4-hexylresorcinol (4-HR) on pear polyphenoloxidase (PPO) activity and stability have been investigated in vitro. AA does not interact directly with PPO but prevents browning by reducing oxidized substrates. The 4-HR exerts a dual role on PPO. If no substrates are present, it interacts preferably with the deoxy form of PPO inactivating it. If substrates and 4-HR are both present they compete for the catalytic site. The 4-HR behaves then as a canonical enzyme inhibitor, binding to the met form of PPO. Simultaneous addition of 4-HR and AA has synergistic inhibition or inactivatory effects depending on the presence or the absence of PPO substrates.

Pericarp browning is one of the most important factors limiting the shelf life of litchi fruit. The storage behavior of three cultivars of litchi with different shelf life were studied to comprehensively address the importance of phenolic content and energy status in delaying the development of browning in litchi fruit after harvest. Results revealed that slower changes of browning indices; higher content of EGC, EC, ECG, GCG, PA2, total phenols and anthocyanins; lower activities of ANT, LAC, PPO, POD and PAL were detected in 'Jingganghongnuo', as compared to that in 'Guiwei' and 'Nuomici'. The transcript abundant of oxidation-enzyme corresponding genes LcANT, LcLAC, LcPPO, LcPOD and LcPAL exhibited similar trends as changes of phenolics and enzyme activities in three cultivars. The energy status and the relative expression intensity of ATP metabolic-related genes LcATPb, LcSnRK2, LcAAC1, and LcAOX1 differed among the three cultivars and maintained higher levels in 'Jingganghongnuo'. Accordingly, the development of pericarp browning was significantly related to the content of phenolics, especially ECG and EC, and to the changes in ATP of litchi fruit after harvest. The comparative study on variety of litchi cultivars evident that the phenolics, energy status as well as the transcript abundant of their corresponding genes are potential indicators to mark the browning change in litchi fruit.

Chlorogenic acid (5-O-caffeoyl-quinic acid; CGA) is an ester of caffeic acid and (-)-quinic acid with many beneficial properties for human health, such as anti-oxidant, anti-inflammatory, cardioprotective, anti-carcinogenic, anti-obesity, and anti-diabetic properties. This has raised an interest for the development of new crop cultivars with increased CGA content. One of the crops with higher CGA content is eggplant (Solanum melongena). There is a wide diversity for CGA content in cultivated eggplant germplasm, which is influenced by the fruit developmental stage, storage conditions, and environmental factors. Therefore, appropriate experimental designs are required for an efficient breeding. Several strategies are proposed for breeding for high CGA content such as intraspecific variation, selection among accessions, development of hybrids and lines with good agronomic and commercial characteristics, or introgression of the high CGA trait in élite lines. Some wild relatives, like S....

Presence of browning or black-spot in fresh foods can adversely affect consumer acceptance. Onion has been reported to exert inhibitory activity against browning reaction. This research aimed to uncover the mechanism and identify active compounds in onion responsible for PPO inhibitors based on metabolomic approach. Onion was fractioned using different solvents, i.e n-hexane; chloroform; ethyl acetate; water, respectively. As a result, ethyl acetate fraction (EAF) of the onion demonstrated the strongest inhibition to PPO in comparison with other fractions, i.e. n-hexane, chloroform, and water. The reversible inhibitory activity of PPO by EAF occurred with presence of L-DOPA as substrate through competitive inhibition and Cu chelation in the active side of the PPO. Based on 1H-NMR (X) score plot and PPO inhibition (Y) using OPLS, NMR signals revealed that active compounds accounting for inhibition of PPO included quercetin