| Whole wheat food is an important part of whole grain foods. Its production and consumption have attracted increasing attention and interests in the world. However, certain amount of anti-nutritional factor phytate would appear in whole wheat foods due to the addition of wheat bran. Phytate not only affects the utilization of protein, starch and fat, but also inhibits the uptake, transfer and bioavailability of mineral elements such as calcium, iron and zinc. Consequently, it is one of the crucial reasons of causing or intensifying calcium iron zinc deficiency. Therefore, exploring wheat bran with low phytate content is of importance to improve its mineral bioavailability, to fulfil its nutritional value and functional properties, to increase the popularity of consumers and to promote the whole wheat food.First of all, from a mechanical perspective, investigated the effects of milling, brushing and ultrasonic washing on phytate content of wheat bran were investigated. Results showed that aleurone cell content with abundant phytate could be removed from wheat bran by teeth roller milling and ultrasonic washing(TU), while coarse smooth roller milling, smooth roller milling and brushing had little effect. Then the phytate content could be dropped by 62.98%. Through morphological observation, component analysis and principle analysis of milling and ultrasonic washing, teeth roller mill has almost no effect on the outer pericarp and intermediate layer because they had a certain amount of water with good toughness, but it could obviously destroy the surface of aleurone cells with thick and hard wall. Thus, the ultrasonic washing was allowed to achieve separation of the aleurone cell content from the wheat bran.During the germination of wheat kernel, the aleurone cells underwent programmed cell death, synthesized and secreted a series of hydrolases, including endogenous phytase which could enzymolyze phytate. According to this phenomenon, from a biological perspective, this study induced programmed cell death in aleurone cells of wheat bran and forced them to secrete and synthesize endogenous phytase. After that, the phytate in wheat brans was enzymolyzed. Through the optimization of storage time, process conditions(solid-liquid ratio, incubation temperature and incubation time) and enzyme activator(metal ion, gibberellin and hydrogen peroxide), the optimal technical conditions were wheat bran stored for 1 d incubated at solid-liquid ratio of 1:1, 55 °C for 80 min, then the degradation of phytate could reach to 70.09%. While, the addition of enzyme activator would inhibit the enzymatic hydrolysis of phytate. Through the morphologic observation of aleurone cells, cell autolysis was found in aleurone layer of wheat bran after incubation. This result validated that the activation technical of endogenous phytase was based on inducing programmed cell death in aleurone cells, promoting the synthesis and secretion of endogenous phytase and then achieving the purpose of enzymatic hydrolysis of phytate in wheat brans.Then, from a physical chemistry perspective, steam flash explosion(SFE) technology was applied for promoting the hydrolysis of phytate in wheat brans. The results suggested that steam pressure and residence time had significant impact on phytate degradation in wheat brans, while particle size had limited influence on it. Considering the energy consumption, the optimal conditions of SFE on phytate degradation in wheat brans were particle size of 14-20 mesh, steam pressure of 2.5 MPa, residence time of 60 s, and then the phytate could decrease by 86.76%. Afterwards, the thermal stability of phytic acid and effect of p H, polysaccharide, inorganic phosphorus, molecular bond, hydrolyzates and organizational structure on wheat bran by SFE were analyzed. Results showed that hemicellulose hydrolysised under high temperature and high pressure and produced acids which benefit to phytate acid hydrolysis. Meanwhile the aleurone cell structure was destroyed and the phytate was exposed. Then the P-O-C bonds in phytate molecule were broken and the phytate decomposed into low inositol phosphate with no anti-nutritional effects. In addition, steam pressure and residence time had positive correlation to wheat bran color(ΔE). And the color change of wheat bran associated with hemicellulose hydrolysates formed dark matter by Maillard and Caramelization reaction and lignin formed red chromophore during SFE.To research the changes of edible quality, phytate degradation, calcium iron zinc bioavailability, amino acid, dietary fiber and volatile compounds of treated wheat bran were investigated. Results showed that the best method of reducing phytate content in wheat brans was SFE, followed AEP and TU. Caco-2 cell model showed that the treatment of the highest calcium iron zinc bioavailability was SFE, followed AEP and untreated. Besides, phytate content in wheat brans was negatively related to calcium iron zinc uptake, transport and bioavailability. By TU treatment on wheat bran, hydrolytic amino acids and free amino acids content decreased significantly(decreased by 46.15% and 58.20%); soluble dietary fiber(SDF) content was slightly decreased(from 4.47% to 3.25%), and insoluble dietary fiber(IDF) and total dietary fiber(DF) content relatively raised(from 50.70% to 70.91 and from 55.17 to 74.17 respectively). By AEP treatment on wheat bran, hydrolytic amino acids and free amino acids content clearly decreased(decreased by 22.07% and 50.26%); SDF content(6.17%) had a little increase, while IDF(48.20%) and DF(54.36%) had no noticeable change. By TU treatment on wheat bran, both hydrolytic amino acids and free amino acids content declined(by 11.08% and 97.22%), SDF content(7.55%) increased, while IDF and DF content had different degrees of decline(decreased by 32.72% and 40.27%). TU and AEP treatment had no significant effect on wheat bran odor; however, after SFE treatment, numerous volatile compounds of acetic acid(relative content 20.36%) and aldehyde(relative content 35.79%) compounds appeared.At last, in order to explore the influences of reducing phytate in wheat brans on whole wheat flour, storage stability of whole wheat flour, the rheological properties of whole wheat dough, content of free sulfhydryl groups, texture properties and sensory evaluation of whole wheat steamed bread were investigated. During the 90 d storage, the changes of polyphenol oxidase(PPO) content, fatty acid(FA) addition and reducing sugar(RS) reduction in whole wheat flour pretreated by TU, AEP and SFE were all less than untreated whole wheat flour. Besides, these whole wheat flour could significantly improve the water absorption, softening degree, resistance to extension and ratio R/E of dough, but had little effect on development time and decrease the stability, farinograph index area and extensibility dramatically. Untreated, TU and AEP pretreatment had little effect on free sulfhydryl groups of whole wheat dough, while SFE pretreatment increased it by 85%. All kinds of whole wheat steamed breads had high firmness, cohesiveness and chewiness, low adhesiveness, springiness and resilience. Besides, there was no obvious sensory difference among untreated, TU and AEP treated whole wheat steamed breads. Instead, SFE treated whole wheat steamed bread had a low sensory evaluation. In summary, TU, AEP and SFE pretreatment could improve the storage stability to varying degrees. But they could not improve the rheological properties, decrease free sulfhydryl groups of whole wheat dough, or increase texture properties and sensory evaluation of whole wheat bread. However, they performed similar to untreated whole wheat flour. |
PDF Full Text Request