Study On The Browning Inhibition Of Alkali Induced Heat-set Konjac Gel From Amorphophallus Bulbifer And Its Application

Konjac flour has good film-forming,thickening,gel-forming,and other functional properties,and is usually mixed with excess alkali and properly heated for deacetylation to produce heat-irreversible alkali-induced konjac gel products.However,the alkali-induced heat-set konjac gel prepared by Amorphophallus bulbifer flour（ABF）exhibits obvious browning,which greatly limits the commercial application of ABF.It is urgent to study the causes of alkali-induced heat-set Amorphophallus bulbifer gel（ABG）browning and effectively inhibit the browning in the research and application of A.bulbifer.In this paper,we investigated the variation rule of the composition of ABF on the color change of ABF during the process of deacetylation.On this basis,we compared and analyzed the methods of inhibiting browning of konjac alkali-induced gels prepared by ABF,and the effect mechanism of the inhibition method of citric-acid heat treatment on the quality of ABF was further investigated.Finally,the application potential of citric-acid heat treatment ABF was evaluated.The main research contents and results are as follows:Firstly,ABF with different degrees of deacetylation was obtained by homogeneous deacetylation reaction.The variation rule of different degrees of deacetylation on the color difference,component content,and physicochemical properties of ABF were investigated.The results showed that the browning of the alkali-induced gel prepared by ABF was a kind of non-enzymatic browning.TheΔE value and browning index increased with the increase in deacetylation degree of ABF（ΔE value increased from 5.24 to 11.98）.However,the contents of L*,h*,total phenols,total flavonoids,reducing sugars,and free amino acids decreased with the increase in deacetylation degree of ABF.In addition,the results of FTIR spectroscopy and HPLC-MS indicated that the browning products of ABF consisted of chemical components such as polyphenol oxidation products and meladox reaction products.These results showed that the oxidative dehydration polymerization of total phenols and total flavonoids under alkaline conditions was one of the main reasons for the browning of ABF in the process of deacetylation.Secondly,the effect mechanism of different methods of browning inhibition on the color difference,gel properties,and rheological characteristics of ABG were investigated.The browning of ABG was inhibited by adding inhibitors（citric acid,L-ascorbic acid,and L-cysteine）,removing prebrowning substances（citric-acid heat treatment）,and using a covering method（ABF mixed with Ti O₂-containing potato starch）,respectively.Then the color and gelation properties were studied and compared.The results showed that all five inhibition methods significantly inhibited the browning of ABG,and had significant effects on the structural properties,rheological properties,microstructure,and thermal stability of ABG.Among them,the citric-acid heat treatment method not only significantly inhibited the browning of ABG（ΔE value decreased from 25.74 to 14.68）,but also significantly improved the water-holding capacity,water mobility,and thermal stability of the gels without destroying their original structural properties.These results suggested that the citric-acid heat treatment method was the best method for inhibiting gel browning compared with other methods.Thirdly,different citric acid concentrations and treatment times were used for the treatment of ABF.The effect mechanism of the citric-acid heat treatment inhibition method on the microstructure,physicochemical properties,and rheological characteristics of ABF were further investigated.The results showed that the citric acid concentration had more significant effects on the physicochemical properties and rheological characteristics of ABF compared with the treatment time.Due to the intramolecular glycosidic bond of KGM was broken with the increase in the concentration of citric-acid heat treatment,the apparent viscosity and swelling of ABF treated with 1.2 M citric-acid heat treatment for 2 h were significantly reduced（apparent viscosity decreased from 3485.59 Pa·s to 948.66 Pa·s）.In addition,the hydrogen bond and ester bond in ABF could be partially broken by citric-acid heat treatment,and the bound polyphenols and flavonoids dissolved in ethanol could be removed,thus inhibiting the browning of the gel.Finally,the application impact of citric acid heat-treated ABF in sugar-substituted konjac jelly was investigated utilizing sucrose,xylitol,and erythritol-sucralose as typical sweeteners.The results showed that the erythritol-sucrose substitution system promoted browning of konjac jelly.In addition,the moderate sucrose addition and 3:1 xylitol substitution systems（sucrose:xylitol=3:1,w:w）were superior to the other groups in terms of appearance,kernel chewiness,gelation properties,and cooking loss of konjac jeely.And in general,the konjac jelly produced by the xylitol substitution system（sucrose:xylitol=3:1）was not significantly different from the konjac jelly produced with all sucrose addition.