Analysis of in vitro digestibility of starches and microcapsules: evaluation of retention and release of folic acid in the fortification of foods

摘要

In the context of the increasing requirements for foods to be fortified with vitamins, particularly folic acid, the relative instability of these essential nutrients is a significant concern. Microencapsulation offers unrealised potential as a means to enhance retention, if the inherent challenges of this approach can be overcome. Currently there is a lack of effective ways to evaluate the release characteristics of microencapsulated materials. Accordingly the objectives of this study have been to investigate the microencapsulation of folic acid and to study the application of in vitro digestibility analyses as a means to establish the retention and release properties of the resultant microcapsules. Procedures for analysis of carbohydrates were validated for use in the study of digestibility and dinitrosalicylic acid reagent was used to measure reducing sugar release. This gave a reliable means of assaying degree of digestion and the results were confirmed by comparisons with HPLC analyses of component sugars. A dialysis model was adapted for evaluation as a way to analyse digestibility and the factors influencing this system were investigated. As starches are potential microencapsulation agents, the focus has been on the in vitro digestion of starch granules. The activity of a number of α-amylase preparations showed significant dependency on the presence of CaCl2 while the type of dialysis tubing used did not impart significant effects on results. In a direct comparison of α-amylases, different rates of reducing sugar were observed in the dialysis model with the animal source giving highest rate followed by bacterial and finally the fungal source. The effects on surface morphology of the granules showed similar patterns of pitting, channelling and endo-corrosion followed by complete collapse of the structure. The formulation and production of microcapsules by spray drying was investigated with focus on selected binding agents alginate (ALG) and low-methoxy pectin (LMP) in conjunction with rice starch granules. The effect of simultaneously varying the ratio and level of binding agents gave a surface plot that indicated higher folic acid retention with a decrease in LMP. As a means of strengthening the outer shell of the microcapsules, a secondary treatment with CaCl2 was applied and generally, a hardening of the microcapsule surface was observed with the environmental scanning electron microscope. The CaCl2 treatment time did not affect the folic acid loss while the ratio of binding agent Abstract vi particularly the sole presence of ALG lead to a higher loss of the core material during hardening. As a compromise between core material recovery and subsequent loss during calcium treatment, a combination of 1% of 1:1 LMP:ALG was shown to give optimum core material retention. When the in vitro digestion model was applied to the microcapsules, the release of both reducing sugars and folic acid was significantly reduced for the Ca2+ treated microcapsules as compared to untreated controls. Morphologically, both types of samples showed some collapse of structure but a more cohesive cluster was observed from Ca2+ treated microcapsules corresponding to enhanced retention of the core material. These findings demonstrate the potential of the microencapsulation strategy including calcium treatment as an effective way to retain sensitive core materials. This is the first systematic study of an in vitro digestion model as an effective means of assessing physiological release of core materials. In addition to contributing to the standardisation of in vitro digestibility procedures, the proposed model can now be adapted and extended to evaluation of capsular release of a wide range of food systems.