Preparation and improvement of dry jujube flavor through Maillard reaction
Creators: name: Xiao, Qing
Publisher: name: No Publisher Supplied
Year: 2024

description: Jujube (Ziziphus jujube Mill.), also known as Chinese dates, have been consumed as a traditional food for thousands of years. Apart from its abundant nutritional ingredients and well-known functional effects, jujube also contributes to the unique flavor in human’s cuisines. Drying fresh jujube fruits could reduce water activity and elongate the shelf life for further use and storage, so most Chinese jujubes are consumed in the dried form. Besides, drying would concentrate flavor precursors within jujube fruits and potentially generate more aroma compounds. Maillard reaction is considered to happen during this thermal treatment and plays an important role in producing aromas with baking and roasting characters of dry jujube. In this thesis, we centered on the correlations between non-volatile flavor precursors and Maillard-type volatile compounds, and controlled the production of desirable flavors while unraveling the underlying chemical mechanisms inherent in reaction cascades.Firstly, we determined the content of protein and free amino acids within the dry jujube fruits. Given that the limited amount of protein quantified, we chose to utilize flavourzyme to hydrolyze it to release more oligopeptides and free amino acids as the potential flavor precursors in Maillard reaction. In terms of the results of GC-MS, Maillard-type volatiles obtained with enzyme treatment showed significant difference to the untreated group, such as pyrazines and furaneol. The formation of furans, pyrrolizines and azepines were more influenced by thermal reaction than enzymatic hydrolysis. In which due to the abundance of pyrrolizines and azepines formed, we considered proline as an important flavor precursor for dry jujube. In addition, asparagine and proline were found as major amino acids of the dry jujube sample, both before and after enzyme treatment in free amino acid analysis, which suggested their promising potential to produce N-containing aroma heterocyclics in Maillard reaction, such as pyrazines, pyrrolizines and azepines.Secondly, in order to further investigate the contribution of major amino acids to dry jujube flavor, we utilized model reactions as the primary methodology approach and compare the concentration of Maillard-type volatile compounds between jujube aqueous extracts and the model reaction of asparagine, proline with glucose. Concentration of pyrazines, furans and furaneol formed in the model reaction was equivalent to that of dry jujube, but pyrrolizines and azepines derived from proline were displayed as the predominant group of compounds among overall volatile products.Moreover, to explore the gap between the formation of proline-specific compounds and other Maillard-type volatiles, we developed a series of model reactions involving asparagine, aspartic acid, proline with glucose to investigate the influence of deamidation, and further promote the generation of pyrazines by introducing lysine to the reaction system. Quantitative results of model reaction products demonstrated that pyrazines were not significantly increased in under deamidation while proline-specific compounds had a rapid enhancement at the same time. With excellent ability to form pyrazines, lysine did help to increase the formation of pyrazines, but still far fewer than pyrrolizines and azepines. It was assumed that proline would preferentially react with α-dicarbonyl compounds in Maillard reaction cascades with lower activation energies.Lastly, to exploit the potential of asparagine as the flavor precursor for dry jujube, we studied amadori rearrangement products of asparagine with glucose (Asn-Glc-ARP) through its formation, characterization and stability. With the study on the effect of reaction temperature, pH values and reaction time, the ideal reaction condition for accumulation of Asn-Glc-ARP was determined at 100 ℃ for 40 min under pH 7. Asparagine was prone to degrade from Asn-Glc-ARP in alkaline pH values within lower temperature range while in acidic environment with high temperatures, deamidation of Asn-Glc-ARP to Asp-Glc-ARP (amadori rearrangement products of aspartic acid with glucose) displayed as the dominant pathway. The structure of Asn-Glc-ARP was proposed as 1-deoxy-1-L-asparagino-D-fructose after separation and purification. Also, the content of Asn-Glc-ARP within dry jujube fruit was quantitated at 8.1 ± 0.5 mg/g.