| Chinese steamed bread(CSB), a traditional staple food in China, has been appreciated by the consumers due to its unique flavor, taste and eating-quality. Although lots of recent studies have reported the process optimization and quality improvement of CSB, very little work has been done on the mechanisms of CSB quality formation, especially the internal mechanism of the effect of gluten cross-linking on CSB quality. In this study, cross-linking of gluten during the production of CSB was investigated, and the effect of gluten cross-linking on CSB quality were further studied.Firstly, the cross-linking and polymerization rule during the production of CSB was investigated. SE-HPLC results indicated that the SDS extractability of gluten protein(SDSEP) decreased significantly(p<0.05) during mixing. After mixing, SDSEP increased and then decreased again, indicating depolymerization of gluten to some extent. During steaming of CSB, SDSEP decreased dramatically(p<0.05). Especially, reduction in glutenin extractability of dough steamed for 10 min was 95.4%, while the gliadin extractability decreased slowly, indicating that glutenin polymerized faster than gliadin. RP-HPLC results showed that the gluten subunits changed without significance during dough processing, except mixing. During steaming of CSB, high molecular weight glutenin subunits(HMW-GS) and B/C-low molecular weight glutenin subunits(B/C-LMW-GS) in the unextractable gluten increased significantly(p<0.05), indicating the increased cross-linking degree of glutenin subunits. Meanwhile, the α-gliadin and γ-gliadin in the extractable gluten decreased significantly(p<0.05), indicating the polymerization of gliadin subunits.Secondly, the changes of gluten microstructure, free sulfhydryl(SH) contents and glutenin macropolymer(GMP) physical properties during the production of CSB were determined. During the dough processing, a coarse and heterogeneous gluten network transformed into more homogeneous and continuous network, with the wrapped starch granules. The free SH contents increased slightly while the GMP properties changed significantly. The contents of HMW-GS in GMP decreased while the wet weight and particle size first decreased and then increased. The loss and recovery in G′ and G″ of GMP indicated the weakening of the GMP gel. During steaming of CSB, the free SH contents decreased siginificantly(p<0.05), indicating the increase in the degree of disulfide(SS) cross-linking. All these led to the formation of a continuous and three-dimensional gluten network, which maintaining better viscoelasticity to hold the gas and to wrap the gelatinized starch.Thirdly, the impact of gluten polymerization degree on CSB textural characteristics was studied, as well as specific volume, microstructure and the gas cell distribution of CSB. The results of CSB textural characteristics showed that with the increase of gluten polymerization degree, the hardness, gumminess and chewiness of CSB increased significantly(p<0.05), the springiness and resilience of CSB first increased and then decreased while the cohesiveness changed without significance. When the polymerization degree of gluten decreased, the specific volume decreased slightly. Meanwhile, uneven distribution of gas cell in CSB was observed, as well as the lager size and decreased quantity of gas cell in CSB. The internal microstructure of CSB became loose, nonuniform, coarse and unordered, and the whole structure of CSB became coarse and weak. When the polymerization degree of gluten increased, the specific volume of CSB decreased significantly(p<0.05). The internal microstructure of CSB changed into ordered, homogeneous and tight, accompanied with large amount of smaller, evener gas cell. The whole structure of CSB became compact and tight.Finally, the internal mechanism of the effect of gluten cross-linking on CSB quality was evaluated. The results indicated that glutenin acted as a predominant role in determining the polymerization/depolymerization of gluten during dough processing, whereas gliadin was the key facter which determined the polymerization degree of gluten during steaming. The redox regents changed the free SH contents in CSB dough significantly(p<0.05), thus hindered or facilitated the SH-SS exchange reaction, leading to the changes in the cross-linking degree among B/C-LMW-GS, HMW-GS, α-gliadin and γ-gliadin. All these led to the transformation of gluten network, accompanying with the alteration in the capacity of gas holding and starch wrapping. And therefore, the CSB quality changed. |
