| Summary: | 碩士 === 國立臺灣大學 === 園藝暨景觀學系 === 106 === Rice proteins are considered as valuable plant proteins due to their unique nutritional and hypoallergenic properties, high yield, and low cost. The key limitation of rice proteins is their poor solubility. Many studies have attempted to use enzymatic modification to improve the functional properties of rice proteins, but only few has been done on the lowering of bitterness of rice protein hydrolysates. Therefore, different treatments for rice protein hydrolysates of higher solubility and lower degree of bitterness of rice proteins were evaluated. There were three parts in this study. First, protease were selected and protein solubility was used as an indicator to determine the appropriate enzyme formula to reduce the bitterness of rice protein hydrolysates. Second, the relationship between rice protein and its bitterness were analyzed. Third, other physico chemical approaches to remove bitterness from hydrolyzates.
In the first part, single commercial protease was used to hydrolyze rice proteins. The treatment that produced hydrolysates with their bitterness intensity less than 5 points (the highest score: 10 point) and solubility higher than 20% were selected. The results indicated that commercial proteases such as Neutrase®, FF104, MF101, PHT, and 51FP were suitable for hydrolyzing rice protein. Bitterness intensity of major samples were as follows: RPH_A > RPH_N = CRPH > RPH_A + PP = RPH_N + PP = RP. Hydrolysates with exopeptidase after a single step of commercial protease could reduce the bitterness to zero, the same intensity as rice protein without hydrolysis.
In the second part, the factor resulting in bitter hydrolysates were analyzed. It was found that bitter peptides existed in the supernatants. The bitterness was not related to amino acid composition. After liquid chromatography/mass spectrometry analysis, the results showed that the amount of hydrophilic peptides was inversely correlated to the bitterness intensity, for example: RPH_A (8.8%) < RPH_N (10.8%) < RPH_A+PP (17.1%). The content of hydrophobic peptides were positively correlated to the bitterness intensity: RPH_A (29.1%) > RPH_N (20.6%) > RPH_A+PP (10.6%). In terms of molecular weight, the molecular weight of RPH_N was greater than that of RPH_A, following by RPH_A+PP. Both RPH_N and RPH_A+PP were less bitter than RPH_A, indicating that molecular weight was irrelevant to bitterness inrensity.
The third part of this study focused on the other physico chemical approaches to remove bitterness from hydrolyzates, including deamidation, activate carbon adsorption, and incorporation of β- cyclodextrin. Since the treatment with acid deamination before hydrolysis could not reduce the bitterness of RPH_A, the use of activate carbon or β- cyclodextrin significantly reduced the bitterness of RPH_A and RPH_N.
This study took water solubility and bitterness as the indexes for the estimation of functional property. In the evaluation of suitable commercial proteases for rice protein hydrolysates, treatment with exopeptidase after a single commercial protease could reduce the bitterness to zero level. In addition, treatment with activated carbon adsorption or β-cyclodextrin also reduced the bitterness of rice protein hydrolysates. In this study, we have verified that the hydrophobicity of peptides in the hydrolysates was one of the main factors that influenced the intensity of bitterness. The physico chemical method used in the third part was probably an appropriate application to remove or mask the hydrophobic structure of peptides. The contribution of this study was to provide various approaches to reduce bitterness of rice protein hydrolysates, and present more flexible choice of alternatives and formulas for industry. Furthermore, produce high quality rice protein hydrolysates as formulas with good fuctional properties and consumer-acceptable flavor may increase its added value and potential.
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