Effect Of Coagulants,Alkali And Heat Treatment On The Quality Of Semi-dehydrated Tofu

As a traditional food in China,the processing process of semi-dehydrated tofu（SDT）involves grinding,boiling,adding coagulant,pressing dehydration,forming and other steps,while marinated SDT also needs to go through alkali treatment,marinating and other processes.The production experience shows that adding coagulant,alkali treatment and hot sterilization are the key processing processes which be closely related to the quality of SDT.The mixing of coagulants potentially overcome the defect of single coagulant,but the gel effect of mixed coagulants is not clear yet.In the process of marinated SDT,the alkali treatment is often used to improve its surface characteristics and make it easier to color and more flexible.Traditional alkali treatment uses plant ash solution（main component K₂CO₃）,while existing Na₂CO₃,NaHCO₃ and other weak alkali solutions,but the current process mainly relies on production experience,and the quality of the product is not stable.Hot sterilization is the main preservation measure of pre-packaged SDT.Currently,the quality of production is deteriorated due to heat treatment,there are few studies on the texture change rule of SDT during heat treatment,and the quality of SDT in heat treatment should be studied.Therefore,this paper starts from adding coagulant,alkali and heat treatment are three key processing processes affecting the quality of SDT,explores the law of their influence on the quality of SDT,to provide support for revealing the formation of SDT texture,and give reference for the technical improvement of adding coagulant,alkali and heat treatment.The main conclusions are as follows:（1）The gel properties of soybean curd and the effect of MgCl₂ and lactic acid mixed coagulant on the quality of SDT were studied.The results showed that 0.20%（w/v）lactic acid could form tofu with high gel strength,when it was mixed with 0.14%（w/v）MgCl₂ at different mass ratios（3:7,5:5,7:3）,the prepared tofu showed higher gel strength and water holding capacity than the tofu prepared by single coagulant,and the5:5 mixed sample group reached the maximum value.The results of synergistic analysis showed that the mixed coagulants had synergistic effects on the gel strength,water holding capacity and gel storage modulus of tofu.It was found that the magnesium chloride-lactic acid mixed coagulant made the soybean protein in the gel form more disulfide bonds,and the hydrogen bond and hydrophobic interaction were enhanced,showing a more continuous and dense gel network structure.Compared with SDT prepared with single coagulants,the color difference value of SDT prepared with 5:5and 7:3 mixed coagulants were more than 7,the texture was softer and tender,and the sensory score was between SDT prepared with single coagulant.The color difference value of SDT prepared with 3:7 mixed coagulant was lower than 3,texture was the most similar,sensory score was the highest,and the mixing of coagulant significantly increased yield,water content and dry base protein content of SDT.（2）When SDT was used as the research object,a certain concentration of K₂CO₃solution was prepared as lye,and the treatment temperature is 90℃,the effects of alkali concentration（0～0.5%,w/v）and time（1～5 min）on the quality of SDT were investigated.The results showed that at the same alkali treatment time,with the increase of lye concentration,SDT became softer and tender in texture,whiter in color,and increased in water content and water holding capacity.The results also showed that the macroscopic quality of SDT in 0.5%-5 min treatment group was degraded.Under the same concentration of lye,with the increase of treatment time,the hardness of SDT gradually decreased,the brightness value,and the water content and water holding capacity increased.Further study found that the lye only acted on the surface of SDT during the alkali treatment,which was characterized by swelling and dissolution of the surface layer,and showed a concentration and time dependence,in which concentration had a more significant effect.The results of low-field NMR analysis showed that the increase of lye concentration made more unfrozen bound water and freezable free water transform into freezable bound water in surface layer,while the binding power of the surface layer to water decreased gradually with the increase of treatment time.The results of Raman spectroscopy and surface hydrophobicity showed that with the increase of lye concentration,theα-helix ratio of soybean protein in surface layer increased first and then decreased,the ratio of random coil decreased first and then increased,the ratio ofβ-sheet decreased,the ratio ofβ-turn increased,and the surface hydrophobicity increased.With the increase of treatment time,theα-helix and random coil increased,β-sheet decreased,and surface hydrophobicity increased,but the surface hydrophobicity decreased in 0.5%-5 min treatment group.Scanning electron microscopy showed that the holes on the surface and cross section of SDT became more and more with the increase of lye concentration and treatment time.（3）The effects of moisture content（80%,70%,65%,50%）and heating temperature（80℃,100℃,121℃）on the quality of SDT that was taken as the research object after heat treatment（20 min）were investigated.Research showed that at121℃,80%moisture content of SDT hardness increased the most,with reduce of the moisture content,SDT increased hardness degree was increased first and then decreased,the formation of the hole reduced,the loss of L*value and increase of a*,b*value increased,the loss of water content decreased,the loss of water holding performance increased,but dry pretreatment can alleviate the loss of water holding capacity.At 65%moisture content,with the increase of temperature,the damage degree of texture,color,water content and water holding capacity of SDT increased.There were no holes at80℃,fewer holes at 100℃,and more holes at 121℃.The results of low-field NMR showed that the transformation of non-freezing binding water to freezable binding water was less due to the decrease of water content,and the increase of temperature made the water transfer in SDT more active.The results of Raman spectroscopy and surface hydrophobicity showed that with the decrease of water content,the decrease ofα-helix ratio and increase ofβ-sheet ratio increased,surface hydrophobicity increased,of whichβ-sheet was dominant in 50%moisture content of SDT,whileα-helix was dominant in high moisture content（80%,70%,65%）of SDT.With the increase of temperature,the decrease ofα-helix ratio and increase ofβ-sheet ratio increased,surface hydrophobicity increased,but the increase of surface hydrophobicity slowed down at 121℃.The results showed that the loss of ionic bond,hydrogen bond and hydrophobic interaction decreased with the decrease of moisture content,the formation of disulfide bond increased,and the temperature increase showed the same influence trend.Scanning electron microscopy showed that the gel structure of SDT became more compact and the pore size was smaller with the decrease of moisture content and the increase of temperature.To sum up,adding coagulant,alkali and heat treatment are important operation units of SDT processing.The physicochemical properties and gel structure of SDT were regulated and the quality of SDT was further improved by mixing coagulants,selecting proper alkali treatments,setting appropriate moisture content of SDT,and reducing heating temperature.