Kin-Chor Kwok

Investigation on ultrasound assisted extraction of saikosaponins from Radix Bupleuri

Abstract Ultrasound assisted extraction (UAE) of saikosaponins from Radix Bupleuri (Bupleurum Chinense DC) was studied. The effects of various factors such as time (t), temperature (T), ultrasound power (I), particle size (S), solvent to solid ratio (R) and solvent concentration (C) on the yield of target components were investigated. The experimental results indicated that the extraction by UAE is six times faster than those by conventional thermal extraction. It is also found that fast extraction rate was obtained in the first 30min of sonication. Prolonged sonication did not give a proportional increase in the yield. For the effect of ultrasound power, the maximum yield was obtained at the power level of around 21W. Scanning electron microscope (SEM) images of the plant cells after UAE treatment were obtained to provide visual evidence of the sonication effect. The effects of particle size, solvent to solid ratio and solvent concentration on the yield are also discussed. The preliminary optimum conditions for UAE of saikosaponins were found at time of 30min, temperature of 80°C, power of 21W, particle size <0.3mm, solvent to solid ratio of 25ml/g and solvent concentration of 50%.

Effect of thermal processing on available lysine, thiamine and riboflavin content in soymilk

Soymilk was heated over a range of temperatures (90–140°C) and times (0–6 h). The available lysine, thiamine and riboflavin content of the soymilk samples were determined. There was no significant change in available lysine during a 3 h heating period at 95°C. At elevated temperatures of 120 and 140°C, optimum heat processed soymilk gave higher measured values of available lysine than did soymilk processed at 95°C. Prolonged heating at 120 and 140°C caused a decline in available lysine. Kinetic data on the thermal degradation of thiamine and riboflavin in soymilk were fitted with first-order kinetics and the kinetic parameters were determined.

Properties of tea-polyphenol-complexed bromelain

Tea polyphenols extracted from Chinese green tea were used to complex and separate bromelain from pineapple juice. The antioxidant activity of tea polyphenols and their effects on the properties of bromelain were studied. The thermal stability of bromelain was enhanced after complexing with tea polyphenols. By complexing, the activity half-life of bromelain at 60°C was lengthened from 60 to 105 min. The improved thermal stability of the tea-polyphenol-complexed (TPC) bromelain is attributed to the antioxidant activity of tea polyphenols. Iso-electric focusing showed that six iso-enzymes fractions of bromelain could complex tea polyphenols with no change in their iso-electric points. Complexing with tea polyphenols did not change the optimum temperature (60°C) for the enzyme activity but the optimum pH value range was narrowed down from pH 6.8–9.0 to around 7.8. The Michaelis constant (K) of the TPC bromelain for casein substrate was determined to be 1.42×10−5 mol/l, higher than that of free bromelain (8.06×10−6 mol/l). Tea polyphenols were found to act as a competitive inhibitor on bromelain in the hydrolysis of casein.

Kinetics of sensory quality changes in soymilk during thermal processing, by parametric and non-parametric data analyses

Soymilk was subjected to various heat treatments at 90, 120 and 140 °C for various lengths of time. The colour and flavour of the soymilk samples were evaluated by a sensory panel consisting of eight semi-trained assessors on (1) a 9-unit hedonic scale, with analysis based on the assumption that the units are equidistant, and (2) a 15-point hedonic ‘Smiley’ pictorial scale, the results being analysed non-parametrically. The proportion (%) of ratings higher than or equal to/higher than the overall median of the samples was used as a rigorous estimate of product quality. These values were used to determine the colour and flavour changes in heated soymilk and their temperature dependence. The Z-values derived from the parametric and non-parametric analyses were in close agreement, ranging from 31 to 33 °C and from 30 to 32 °C for colour and flavour respectively. © 2000 Society of Chemical Industry

Ultrasonic inactivation of soybean trypsin inhibitors

Abstract Soybean trypsin inhibitors (TI) in soymilk were treated by heat and ultrasound of 20 kHz. The influence of several factors (temperature, time of treatment, pH, ultrasonic power, soymilk concentration, and ionic strength) on inactivation of TI was investigated. The results of the experiment shown that temperature was the major factor to affect the inactivation of TI, and treatment time was the next important factor. Under the experimental conditions of temperature 80°C, ultrasonic power of 150 w, and pH 7.0 for 5 min treatment, TI in soymilk sample could be inactivated by 73%. The retained 27% TI was difficult to inactivate. This residual component is mainly Bowman-Birk inhibitor (BBI) which is extremely stable to heat as well as ultrasound of 20 kHz.