Analysis of effect of yogurt fermentation process on calcium speciation in milk by all bionics models of digestive system

Abstract: An effective method was developed for the speciation analysis of Ca in pure milk and yogurt using in vitro whole-bionics models by microwave digestion and high-resolution continuum source flame atomic absorption spectrophotometry (CS FAAS). In order to study the influences of the yogurt fermentation on speciation and bioavailability assessment of Ca in pure milk, the gastric and intestinal whole-bionic extraction solutions were obtained by pretreating pure milk and yogurt using in vitro whole-bionics technology. The suspended Ca and the soluble Ca were obtained by using 0.45 μm filtration membranes, and the inorganic Ca and the organic Ca in soluble species were gained via D101 macroporous resin. N-octanol-water absorption model was established for simulating the absorption of Ca in stomach and intestine with n-octyl alcohol as the cell membrane. Moreover, the unilamellar liposomes absorption model was developed for simulating the absorption of Ca in stomach and intestine with monolayer liposomes as the cell membrane. The effects of the fuel flow, the burner height and the mass fraction of lanthanum nitrate on the absorbance were investigated. The appropriate fuel flow, 70 L/h, the optimum burner height, 6 mm, and the suited mass fraction of lanthanum nitrate, 0.5%, were obtained by the single factor experiments. The results showed that Ca contents in pure milk and yogurt were 12.894 and 15.920 mg/g, respectively, under the optimum conditions using microwave digestion as a pretreatment method. The average recovery rate and the relative standard deviation of Ca for pure milk were 96.1% and 1.20%, respectively, indicating that the proposed determination method was accurate and reliable. The suspended Ca contents in the gastric and intestinal whole-bionic extraction solutions of pure milk were higher than those of yogurt. Except the suspended Ca, other forms of Ca contents in the gastric and intestinal whole-bionic extraction solutions of yogurt were higher than those of pure milk. The soluble Ca and the inorganic Ca in gastric whole-bionic extraction solutions of pure milk and yogurt had significant difference by t-test at 0.05 significance level. The soluble Ca and the water-soluble Ca of n-octanol-water absorption model and unilamellar liposomes absorption model in intestinal whole-bionic extraction solutions of pure milk and yogurt also had significant difference by t-test at 0.05 significance level. The partition coefficients of n-octanol-water absorption model of Ca were 4.18 and 3.97, those of unilamellar liposomes absorption model of Ca were 5.21 and 6.24, and the bioavailability assessment values of Ca were 45.40% and 66.31% in yogurt gastric and intestinal whole-bionic extraction solutions, respectively. Thus, the partition coefficients of n-octanol-water absorption model and unilamellar liposomes absorption model, and the bioavailability assessment of Ca in yogurt were higher than those in pure milk. These results showed that the Ca supplement effect of yogurt was better than that of pure milk. Therefore, the proposed models are reliable, simple and convenient with a high practical value, which will provide a scientific basis for beneficial metal elements supplement.