In order to develop the tyrosinase inhibitors with potential application in food industry, a series of stilbenehydroxypyridinone hybrids were prepared. Among these compounds, 1h was found to possess the most potent tyrosinase inhibitory effect on both monophenolase and diphenolase activities, with IC50 values of 2.72 mu M and 15.86 mu M, respectively. The inhibitory effect of 1h on monophenolase activity was 4.6 times that of kojic acid. An inhibition kinetic assay indicated that 1h was a mixed-type and reversible inhibitor. The copper-binding and reducing ability assays, molecular docking study, intrinsic and ANS-binding fluorescence assays indicated that copper coordination and reduction is likely to be the causative mechanism for 1h-induced inhibition on tyrosinase. The results of color measurement and browning index determination indicated that treatment with 1h retarded effectively the browning of freshly-cut apples during their storage. Meanwhile, PPO and POD activities in apple slices were found to be effectively inhibited.

Macromolecular chelators have potential applications in the medical area, for instance, in treatment of iron overload-related disorders and in the treatment of external infections. In this investigation, several novel iron (III)-selective hydroxypyridinone hexadentate-terminated first and second generation dendrimeric chelators were synthesized using a convergent strategy. Their iron chelating ability was demonstrated by UV/Visible spectrometry and high resolution mass spectrometry (HRMS). The iron binding affinities were also investigated by the competition with a fluorescent iron chelator CP691. The result indicated that these dendrimers possesses a high affinity for iron with a very high pFe(3+) value, which is close to that of an isolated hexadentate unit. These dendrimeric chelators were found to exhibit inhibitory effect on the growth of both Gram-positive and Gramnegative bacteria.

In order to explore the potential of a novel tyrosinase inhibitor, namely the hydroxypyridinone derivative (5-hydroxy-1-octyl-4-oxo-1,4-dihydropyridine-2-carbaldehyde O-ethyl oxime, HPO), for shrimp preservation, six bacterial isolates from shrimp (Penaeus vannantei) (S1, S2, S3, S5, S6 and S7) were investigated. These bacteria were identified as Shewanella amazonensis, Shewanella haliotis, Microbacteriutn oxydans, Photobacterium damselae, Staphylococcus haemolyticus and Bacillus cereus. The antimicrobial effect of HPO on these bacteria was evaluated by an inhibition zone test, together with minimal inhibitory concentration (MIC) and minimal bactericidal concentration (MBC) assays. The results indicated that HPO possessed a stronger inhibitory effect than kojic acid. The antioxidant activity of HPO was evaluated by determining its 1,1-diphenyl-2-picrylhydrazyl (DPPH) and superoxide anion radical scavenging ability. The preservation effect on shrimp (Penaeus vannamei) of HPO was assessed by determining the quality indicators of the shrimp during the storage, including total bacterial count, pH value, total basic volatile nitrogen content, thiobarbituric acid reactive substances (TSARS) and by sensory evaluation. It was found that treatment of shrimp with HPO effectively extended the shelf life to 10 days at 4 degrees C. In the same assay, the shelf life of shrimps treated with 4-hexylresorcinol, kojic acid, and the control group was found to be 10, 6, and 4 days, respectively. The treatment of shrimps with HPO in combination with 0.1% citric acid further extended its shelf life to 12 days. Clearly, HPO could be applied as a shrimp preservative due to its potent anti-browning, antimicrobial and antioxidant activities.

Pectinase was immobilized onto sodium alginate/graphene oxide beads via amide bonds by using N, N'-dicyclohexylcarbodiimide/ N-hydroxysuccinimide as the activating agent. The immobilized pectinase was characterized by Fourier transform infrared spectra and scanning electron microscopy analyses. Immobilization conditions were optimized by Box-Behnken design and the response surface method. The activity of the immobilized pectinase prepared under optimal conditions reached 1236.86 +/- 40.21 U/g, with an enzyme activity recovery of 83.5%. The optimal pH of free pectinase was 4.5, while that of immobilized pectinase was shifted to 4.0. The optimal temperature of immobilized pectinase was increased to 60 degrees C, which was 10 degrees C higher than that of free form. Furthermore, the immobilized pectinase possessed a superior thermal stability and storage stability to those of free pectinase. Reusability studies indicated that the immobilized pectinase retained 73% of initial activity after six times cycles. Due to these good properties, such immobilized pectinase may find application in food industry.

In order to search for a new method for the anti-browning of food products, a novel hydroxypyridinone (HPO) derivative with a formyl group was evaluated for its anti-tyrosinase property. This compound was found to exhibit potent tyrosinase inhibition on the monophenolase activity of mushroom tyrosinase with an IC 50 value of 1.33 μM, indicating that this HPO derivative was 12-fold stronger than kojic acid (IC 50 15.89 μM). This molecule can inhibit tyrosinase via two action modes, namely copper reduction and chelation, and the formation of a Schiff’s base with the amino group at the active site of the enzyme. A synergistic effect of these two action modes to enhance the inhibitory activity was observed. This compound was also investigated for the inhibitory effect on diphenolase activity of mushroom; the inhibitory mechanism was found to be reversible and of competitive-uncompetitive mixed-type inhibition. This hydroxypyridinone was demonstrated to effectively control the browning of vegetable products.

A range of hydroxypyridinone derivatives were synthesized starting from kojic acid. Among them, 10 and 11 were found to possess the strongest inhibitory effect on monophenolase activity of mushroom tyrosinase, having IC50 values of 2.04 and 1.60 μM, respectively. The IC50 values of 10 and 11 for the inhibition of diphenolase activity of mushroom tyrosinase were determined as 13.89 and 7.99 μM, respectively. Investigation of the inhibitory mechanism of these two compounds indicated that the inhibition was reversible and of a competitive-uncompetitive mixed type. The KI and KIS values of 10 were determined to be 24.84 and 32.54 μM, respectively, and the corresponding values for 11 being 18.07 and 21.34 μM, respectively. The effect of 11 on the browning process of fresh-cut apples was evaluated by measuring the color change and browning index. The results indicated that 11 had a significant effect on controlling the browning of fresh-cut apple slices.

Bovine pancreatic lipase was immobilized onto N-succinyl-chitosan beads by using N-(3-dimethylaminopropyl)-N′-ethylcarbodiimide (EDC)/N-hydroxysuccinimide (NHS) as the activating agent. The immobilized lipase was characterized by Fourier transform infrared spectra and scanning electron microscopy. The activity of the immobilized lipase obtained under the optimized immobilization conditions was found to reach 1,480 U/g, with enzyme activity recovery of 74.5%. Compared to the free lipase, thermal stability of the immobilized lipase was greatly improved. The reusability study indicated that the immobilized lipase retained 76.6% of its initial activity after being recycled five times. A preliminary study indicated that the prepared immobilized lipase can effectively enrich the polyunsaturated fatty acid in fish oil (increasing from 45.1 to 67.7%), of which the combined docosahexaenoic acid and eicosapentaenoic acid contents increased from 16.3 to 27.1%. Practical applications To overcome the drawbacks of using free lipase, an effective method was developed by immobilization of lipase onto N-succinyl-chitosan beads using EDC/NHS as the activating agent. The immobilized lipase prepared by this method may find application in the enrichment of polyunsaturated fatty acids of fish oil.