| High hydrostatic pressure processing is an emerging tochnology that is being embraced by the food industry due to the benefits it offers as it is effective in inactivating microorganisms and to some degree enzymes, while flavour, colour and other phytochemicals are minimally affected. Also, ultrasound is gaining application in many food processing procedures. This study employed high hydrostatic pressure, ultrasound, and heat singly and in combination, and their effects on mulberry juice anthocyanins, inactivation of enzymes and microorganisms, as well as colour parameters were investigated. In the first study, the effect of high hydrostatic pressure treatment at200,400, and600MPa for20min respectively, on anthocyanin composition of mulberry juice were investigated. Anthocyanins were identified and characterised using high-performance liquid chromatography (HPLC), electrospray ionisation mass spectrometry (ESI/MS), and literature data. Cyanidin-3-O-glucoside (55.56%) and cyanidin-3-O-rutinoside (44.44%) were detected in the untreated sample, while two new anthocyanins [pelargonidin-3-O-coumaroylglucoside (0.46%) and delphinidin-3-O-coumaroylglucoside (5.8%)] were identified in the sample treated at200MPa for20minutes. One new anthocyanin, delphinidin-3-O-coumaroylglucoside (5.38%), was detected in the juice treated at400MPa for20minutes. At600MPa for20min, no new anthocyanins were detected.In the second study, the effects of four independent variables (temperature, heating time, pressure, and pressurizing time) on three response variables [%anthocyanin retained,%residual activities of polyphenol oxidase (PPO) and peroxidase (POD) enzymes] of mulberry juice were studied using central composite design response surface methodology. Design-Expert software was used to determine optimum levels of the independent variables and mathematical models generated to predict the outcome of response variables. In this study, temperature had the greatest effect on all the response variables. The synergistic effect of temperature and pressure had significant effect (p<0.05) on the per cent anthocyanin retained and per cent residual PPO activity. The prediction of the desirability model, based on95%confidence in the range of the independent variables, gave optimal treatment conditions of83.39℃,2.38min,480.00MPa, and21.67min, respectively for temperature, heating time, pressure, and pressurizing time. At these levels, the corresponding response variables were91.68%,44.69%and20.17%for per cent anthocyanin retained, and residual activities of PPO and POD, respectively. The desirability index obtained was0.741.In the third study, the effects of ultrasonic treatment on microorganisms, anthocyanins, colour, and enzyme inactivation of mulberry juice were examined. Ultrasound frequencies of22,24, and26kHz for10,20, and30min (full factorial), at constant power of60W, and pulse durations of10s on and3s off were used to treat mulberry juice. Total plate count, mould and yeast populations were determined using plate count agar and oxytetracycline glucose agar, respectively. Anthocyanin content, residual enzyme activities of PPO and POD, as well as colour brightness, a*, b*, hue angle, chroma, and total colour difference were determined. Total plate count, yeast and mould populations were reduced to<10cfu/ml for all ultrasound treatment regimes. Again, ultrasonication was found to have significant effect (p<0.05) on the anthocyanins of all treated samples, with the one treated at24kHz for30min retaining the highest per cent of94.40±0.56%and that treated at22kHz for10min having the least value of70.22±0.11%. Comparatively, ultrasonication significantly reduced polyphenol oxidase activity as compared to POD activity. Maximum inactivation of PPO activity (45.04%) was at22kHz for10min. Colour brightness was significantly (p<0.05) increased, as well as the hue and chroma for most of the samples in comparison with the control, indicating greater luminance of treated samples. The sample treated at24kHz for20min had highest values for brightness, hue, chroma, and total colour difference respectively as6.8±0.01,150.96±1.70,9.02±0.08, and9.12±0.13, making it the most attractive.The fourth study focussed on the optimization of ultrasonic and high hydrostatic pressure conditions and determining their effect on quality parameters of mulberry juice using the Box-Behnken response surface methodology design. Activity of PPO was significantly affected by the linear terms of ultrasonic frequency and pressure, as well as the interaction between ultrasonic frequency and pressurizing time. POD was significantly affected by the interaction between ultrasonic time and pressurizing time as well as the quadratic term of ultrasonic time. Yeast and bacteria were inactivated to levels≤10cfu/ml. Model desirability prediction at95%confidence, gave optimal manosonication levels as25.00kHz,15.35min,376.36MPa, and10.01min for ultrasound frequency, ultrasound time, pressure, and pressurizing time, respectively. At these optimum levels, the predicted% anthocyanin retained, activities of PPO and POD, colour brightness, hue angle, and chroma were83.08%,45.55%,53.93%,7.59,112.09°, and11.74, respectively. The overall desirability index was0.639.In the fifth study, the effects of HHP treatments at200,400, and600MPa, for10,20, and30min on residual activities of PPO and POD, colour, and microbial populations of mulberry juice were investigated with reference to untreated mulberry juice. Lower pressure levels were found to inactivate PPO and POD than at higher pressure levels. Also, POD was observed to be inactivated to a greater extent than PPO. Anthocyanin content was better retained at lower pressure levels than at higher levels, and this correlated positively with residual PPO and POD activities in treated samples. Total viable count as well as yeast and mould counts of HHP treated mulberry juice samples were<10cfu/ml. The colour parameters of brightness, hue angle, chroma, and total colour difference were differently affected, with sample treated at200MPa/30min having colour characteristics similar to the control. Colour degradation was linked to destruction of anthocyanins, known to give colour to berry fruits, by residual activities of PPO and POD.The study therefore provides useful insight into suitable levels of the applied technologies in this work (HHP, ultrasonication, and heat) and optimal processing conditions of the synergistic effect of manothermal and manosonication on mulberry juice. At the applied levels mulberry fruits could be processed into a functional fruit juice. |
PDF Full Text Request