| Natto has high nutritional value and contains a variety of functional active components, it can significantly dissolve thrombus, fall blood pressure, anti-oxidation, prevent osteoporosis and improve the intestinal environment. Traditional natto is a common food in Japan, but in China, natto is difficult to be accepted by consumers because of its poor taste and flavor, the storaging form is so limited that can only choose frozen storage, during the storage process, it can make important active ingredients such as natto kinase erosion, so the natto resources have not been widely used. To broaden the application of natto in China market, in this research, natto was acceptted as raw material, different drying methods on quality of natto powder were studied, the optimal drying conditions were determined, on this basis, the preparation progress of natto powder microcapsule was optimizated, and its physical and chemical properties and storage conditions were systematically studied.The main results of this research are as follows:(1) The comprehensive score on four drying methods of the preparation of natto powder, respectively is-0.02,-0.08, 0.02 and 0.04. It shows that vacuum freeze drying method is the best choice, followed by hot-microwave drying, but vacuum freeze drying has low efficiency and large energy consumption, the hot air-microwave drying keeps a variety of nutrients and the typical functional ingredient——natto kinase, and the drying efficiency is higher. Therefore, hot air-microwave drying technology can be applied to industrialized production, in order to improve product quality and reduce production costs.(2) The mathematical model of hot air-microwave drying is established by studying its drying characteristics:142.363exp[MR)098.7169.342888.5019(/828.76502TTTte(10)--(28), earlier period:049.0368.11(t Texp391.9754MR(10)-(28), later period.On the basis of single factor experiments, the quadratic equation between natto kinase(Y) and air temperature(A), the breaking point of moisture content(B), microwave power(C)was fitted through the response surface design:Y=289.60-13.38A+9.38B+7.75C+3.50AB+0.75AC+17.25BC-62.05A2-36.05B2-53.80C2.The optimum process parameters of hot air-microwave drying on natto was determined: the hot air temperature is 60℃, breaking point moisture content is 30%, microwave power is 265 W, the activity of natto kinase measured under this condition is 290U/g, and error between the test value and predicted value is 0.71%, verify the feasibility of test model.(3) The process conditions of spray drying to make natto powder microcapsule were studied. The results show that the optimal formula of the natto powder microcapsules are: the ratio of maltodextrin and gum Arabic is 6:1, composite wall material concentration is 0.2g/m L, the best ratio of core and wall material is 1:7, emulsifier concentration is 20%, additive amount is 2%, emulsifying temperature is 90℃:The best process conditions for spray drying: inlet air temperature is 160℃, inlet air volume is 25m3/h, the feed volume of peristaltic pump is 450 m L/h, the embedding rate of microcapsule products is 91.69% under this condition.(4) The chemical properties, microstructure and storage stability between natto powder microcapsule and the general powder were contrasted, the results show that:(1)Through the sensory test of two kinds of powders, the color of natto ordinary powder is deep yellow, grain size is larger, there is a clear phenomenon of sticking together, and the color of natto powder microcapsule is light yellow, size is smaller, powder is delicate: In terms of odor, the ammonia smell of natto powder microcapsule significantly reduced, and has a strong bean fragrance: In terms of palatability, natto ordinary powder is rough and the size is not uniform, taste slightly adhesional wetting, as well as natto microcapsule powder, the powder is uniform and has a palatable texture.(2)The bulk density change condition of powder shows that: bulk density of natto powder is increased after microcapsulation, the porosity decreases. The angle of repose of natto powder microcapsule is 26.64°, liquidity can up to the basic standard, the angle of repose of ordinary powder is 22.28°, it has good liquidity.(3)According to the determination results of amino acid composition, natto powder microcapsule contains the same kind of amino acids with ordinary powder and the content is more, the total essential amino acid is 13.57g/100 g, slightly lower than ordinary powder(14.35g/100g), it proves that microcapsulation treatment can protect main nutritional ingredients of natto powder from destruction.(4)The storage stabilities of natto powder microcapsule and ordinary powder were studied, the results show that: the natto kinase retention rate of natto ordinary powder up to is 39.8% after storage for 60 d, the value is extremely low; the natto kinase retention rate of microcapsule is greatly improved, up to 75.1%, which can effectively improve the shelf life of product. |
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