Effect Of Presoaking By Ultra-high Pressure On The Quality Of Parboiled Rice

Soaking is a crucial step in processing of parboiled rice,which aims to make the rice fully absorb water and create necessary conditions for the starch gelatinization in the cooking process.Rice husks and bran layers,due to their compact structure,are unable to let the water molecules to penetrate into rice grain interior.The pigment,vitamins and aroma and other low-molecular substances lose seriously in traditional heat soaking by destroying the covalent bond.While soaking in normal temperature is inefficient,and produced chalky rice and a large amount of waste water.Ultrahigh pressure,as a kind of non-thermal processesing,can improve the water absorption rate and equilibrium moisture content,hardly change covalent bond of food nutrients.So it can effectively keep nutrient components and flavor in foods,improve texture and quality of product.Therefore,this research introduces ultrahigh pressure as a preprocessing step into soaking processing of parboiled rice.The optimum conditions of processing process of parboiled rice were determined with the single factor and the RSA response surface method.And the effects of ultra-high pressure on retrogradation properties,nutrition and in vitro digestion of parboiled rice were studied.The main results were as follows:(1)Setting ultrahigh pressure presoaking as predetermined conditions,through analyzing each factor such as soaking temperature,soaking time,cooking temperature,cooking time,drying method as well as drying temperature,key factors were defined for the response surface analysis with broken rice rate as dependent variables.When presoaking at 300 MPa for 15 min,the optimal preparation conditions for parboiled rice were: 40 ? water bath soaking 37 min,110 ? cooking 8 min,37 ? hot air drying.(2)After ultra-high pressure presoaking treatment,the leached amylose of parboiled rice obviously decreased,the water distribution changed,and the combined water content decreased,which was beneficial to retard the aging of parboiled rice.In addition,ultra-high pressure treatment can reduce the hardness of parboiled rice and improve the taste.When the pressure of presoaking treatment is more than 200 MPa,the hardness decreased significantly.After ultra-high pressure treatment,the viscosity of the gelatinization curves of parboiled rice generally decreased and presented atypical characteristics at the pressure above 200 MPa.Through differential scanning and Fourier infrared spectroscopy research,it was found that starch and fatty acids formed amylolipid complex in parboiled rice under ultrahigh pressure presoaking.And with the increase of pressure,the complex increased and the enthalpy of gelatinization decreased significantly.(3)Ultra-high pressure pretreatment can accelerate the absorption of water from rice and shorten the reheat cooking time of parboiled rice;With the increase of pressure,the brightness value L of parboiled rice increased.With the increase of soaking pressure and the extension of holding time,the enzyme activity in paddy decreased,the retention rate of phenols and ketones increased,and the antioxidant activity increased.With the increase of pressure and holding time,the hydrolysis rate of starch decreased significantly,while the content of resistant starch increased,indicating that ultra-high pressure,as presoaking step of parboiled rice,can play the role of slow release of glucose in the process of digestion.