Temperature dependent electrical conductivities of fruit purees during ohmic heating

Ohmic heating takes its name from Ohm’s law; the food material switched between electrodes has a role of resistance in the circuit. In this study, the apricot and peach purees were heated on a laboratory scale static ohmic heater by applying voltage gradients in the range of 20–70 V/cm. The voltage gradient was statistically significant on the ohmic heating rates for both purees (P < 0.05). The linear temperature dependent electrical conductivity relations were obtained. Bubbling was observed above 60 °C especially at high voltage gradients. The ohmic heating system performance coefficients were in the range of 0.49–1.00. The unsteady-state heat conduction equation for negligible internal resistance was solved with an ohmic heating generation term by the finite difference technique. The predictions of the mathematical model using obtained electrical conductivity equations were found to be very accurate.     

Feasibility of utilizing bioindicators for testing microbial inactivation in sweetpotato purees processed with a continuous-flow microwave system

ABSTRACT:  Continuous-flow microwave heating has potential in aseptic processing of various food products, including purees from sweetpotatoes and other vegetables. Establishing the feasibility of a new processing technology for achieving commercial sterility requires evaluating microbial inactivation. This study aimed to assess the feasibility of using commercially available plastic pouches of bioindicators containing spores of Geobacillius stearothermophilus ATCC 7953 and Bacillus subtilis ATCC 35021 for evaluating the degree of microbial inactivation achieved in vegetable purees processed in a continuous-flow microwave heating unit. Sweetpotato puree seeded with the bioindicators was subjected to 3 levels of processing based on the fastest particles: undertarget process ( F ₀ approximately 0.65), target process ( F ₀ approximately 2.8), and overtarget process ( F ₀ approximately 10.10). After initial experiments, we found it was necessary to engineer a setup with 2 removable tubes connected to the continuous-flow microwave system to facilitate the injection of indicators into the unit without interrupting the puree flow. Using this approach, 60% of the indicators injected into the system could be recovered postprocess. Spore survival after processing, as evaluated by use of growth indicator dyes and standard plating methods, verified inactivation of the spores in sweetpotato puree. The log reduction results for B. subtilis were equivalent to the predesigned degrees of sterilization ( F ₀). This study presents the first report suggesting that bioindicators such as the flexible, food-grade plastic pouches can be used for microbial validation of commercial sterilization in aseptic processing of foods using a continuous-flow microwave system.     

Effect of microwave heating of wheat grains on the browning of dough and quality of chapattis

Studies were conducted to evaluate the effect of microwave (900 W, 2450 MHz) heating of wheat grains conditioned to different moisture levels on the activity of polyphenol oxidase (PPO) and subsequent colour changes during storage of wheat dough. The results showed that microwave heating of wheat grains for 80 s decreased the PPO activity significantly ( P  ≤ 0.05) with an increase in moisture level from 12 to 18%. Whole wheat flour milled from microwave treated wheat grains (80 s, 18% MC) had 93.39% less activity of PPO. The dough balls (40 g), pressed and rolled into circular form (150.00 mm diameter and 2.00 mm thickness) and stored at 5–6 °C for 72 h. The colour of flattened circular dough was measured using Hunter Colour Meter and it was found that L -value (lightness) decreased from 64.21 to 60.23 in treated and from 64.79 to 47.93 in control samples after 72 h of storage. The results showed that the microwave heating effectively controlled the enzymatic browning in dough and improved the consumer acceptability of chapattis prepared from the same during refrigerated (5–6 °C) storage for 72 h.     

Strawberry juice colour: A study of the quantitative and qualitative pigment composition of juices from 39 genotypes

The juices from 39 strawberry genotypes were analysed for anthocyanin composition and colour characteristics. No single cultivar or breeding line possessed the innate pigment characteristics thought to confer greater colour stability on strawberry juices but the anthocyanin patterns were far more complex than hitherto reported in strawberries. Thirteen different peaks were observed during high-performance liquid chromatography of the juices and tentative identifications are proposed for two previously unidentified strawberry anthocyanins. The occurrence of pelargonidin 3-rutinoside was confirmed. Colour quality measurements derived from CIELAB L*a*b* values are reported for the fresh juices.     

油桃果皮红色素的微波提取工艺及特性研究

对油桃皮红色素的微波提取及理化特性进行了研究,结果表明,该色素为水溶性多酚类花色素,在不同的pH环境下显示不同的颜色;其耐热性、耐光性、耐酸、耐糖、耐还原性较好;但耐氧化性、耐碱性较差;Fe3 对色素有破坏性。研究还表明,该色素的提取宜在酸性介质中进行,酸性条件下其综合稳定性较优,有较高的利用价值。     

Variations of colour,polyphenol oxidase and peroxidase activities during the production of low temperature dried pasta in various durum wheat genotypes

This research was conducted to determine the effect of milling and pasta making on yellow pigment content, soluble brown pigment content, polyphenol oxidase (PPO) activity and peroxidase (POD) activity in semolina and pasta derived from ten durum genotypes. Results showed that pasta colour was impacted more by the milling than by pasta making. The loss of yellow pigment content, POD activity, PPO activity and soluble brown pigment content due to milling was 1.3×, 4.4×, 6.2× and 17.5×, respectively, greater than losses due to pasta making. Dry pasta contained 62%, 31%, 16% and 25% of the original yellow pigment content, brown pigment content and PPO activity and POD activity in the grain, respectively. Semolina yellow pigment content had a positive effect on pasta colour while semolina protein content, ash content and speck count had negative effects. These results indicate the importance of selecting genotypes that have high yellow pigment content and excellent milling qualities.     

微波杀菌对果汁品质的影响

微波杀菌作为一种新型的杀菌技术,其利用微波的热效应和非热效应的双重作用抑制或消除微生物,不仅能够提高果汁的灭菌速率,还能有效地提升果汁的安全性、稳定性和新鲜度,使果汁的品质与原汁更接近,因而受到人们广泛的关注。目前,微波杀菌是果汁杀菌领域的研究热点,亦是极具工业化应用前景的杀菌技术之一。因微波参数对果汁的杀菌效果及品质存在较大的影响,所以在选择微波参数时需要针对不同的果汁通过试验确定最优的杀菌方案。本文介绍了微波杀菌的机制,综述了微波杀菌对果汁微生物的影响,微波频率、微波功率、微波时间对微波杀菌效果的影响及微波杀菌对果汁理化指标和感官指标的影响,并对微波杀菌未来发展趋势进行了展望,以期为后续微波杀菌的技术发展提供参考。     

In situ inactivation of polyphenol oxidase in mamey fruit (Pouteria sapota) by microwave treatment

Polyphenol oxidase (PPO) is the enzyme responsible for quality loss in most fruits and vegetables. Quality loss is mainly because of oxidative chemical reactions which generate the darkening of tissues. Mamey fruit (Pouteria sapota) after harvesting suffers a rapid quality decay trough activation of PPO. However, PPO may be inactivated in situ by chemical or thermal treatment. In food processing, microwave treatment (MT) has been used recently as an alternative for PPO inactivation. In this study, it was observed that mamey fruit pulp subjected to a gently MT resulted in a higher PPO activity as the generated heat induced in situ the increase in PPO activity. In contrast, PPO was completely inactivated after long MT by using a high microwave power. Temperature in mamey pulp after MT reached a maximum of 79 °C; although PPO was active up to 60 °C. PPO was completely inactivated when conventional blanching treatment was performed but required a higher temperature (92 °C/300 s). The optimum energy intensity (E(opt)) for PPO inactivation by MT was 0.51 kJ/g or 937 W/165 s. Under this condition, the remaining PPO activity was inversely proportional to energy intensity (E). Interestingly, MT resulted in a negligible damage in microstructure of mamey pulp, although blanching treatment resulted in large damaging effects on tissue organization and shape. Therefore, MT is proposed as an effective way to completely inactivate PPO without causing any significant damage to fruit tissues and shape; as preservation of color, flavor, and taste would be favored.     

Peroxidase and polyphenol oxidase in long-term frozen stored papaya slices. Differences among hermaphrodite and female papaya fruits

The effects of freezing and frozen storage on hermaphrodite and female papayas (cv Sunrise, solo group) polyphenol oxidase (EC 1.10.3.1; PPO) and peroxidase (EC 1.11.1.7; POD) were evaluated. The freezing process produces a significant increase in both enzymatic activities (22–11%, PPO and 13%, POD) depending on the kind of papaya fruit. During frozen storage, the soluble PPO activity shows a continuous increase up to 9 months of storage only in hermaphrodite tissues. This same sample also showed a significant activation of soluble POD at 3 months of storage. Female frozen samples maintained a continuous decrease in soluble POD activity during storage, while PPO activity showed some increase up to 12 months. Isoenzyme pattern of PPO in freshly frozen papaya tissues showed an intensification of the most cationic forms, D and E (R_f=0·55 and R_f=0·64, respectively) and a disappearance of band B (R_f=0·28). This band B was not regenerated during frozen storage. However, female tissues only showed a continuous inactivation of bands D and E through storage. POD isoenzyme pattern showed different changes depending on the kind of papaya fruit. In both frozen papayas a new isoenzyme (R_f=0·39) form appeared at three months of storage. From this date, hermaphrodite frozen samples lost this isoenzyme together with original the most anionic POD form (R_f=0·58), increasing the intensity of the only remaining form (R_f=0·24). © 1998 SCI.     

Effect of ultrasonic treatment on enzyme activity and bioactives of strawberry puree

The aim of this study was to evaluate the impact of ultrasound (US) at different frequencies (20, 370, and 583 kHz) and power levels (35 and 48 W) on the residual activity (RA) of peroxidase (POD) and polyphenol oxidase (PPO) in strawberry puree. Total anthocyanin content (TAC), total phenolic content (TPC), ferric ion reducing antioxidant power (FRAP) and trolox equivalent antioxidant capacity (TEAC) were also assessed. Results were compared with untreated, thermally treated at 40 °C (control) and pasteurized (90 °C) strawberry puree. POD and PPO RA were significantly (P < 0.05) reduced, whilst there was a significant (P < 0.05) increase in TAC (5%–19%) in all US-treated samples in comparison with the untreated samples and the controls. US at 20 kHz (35 W) increased significantly (P < 0.05) TPC (9%) and FRAP (6%) in strawberry puree, whereas the effect of 583 kHz (48 W) on these parameters was insignificant (P > 0.05). Pasteurization inactivated POD and PPO, however, decreased dramatically TAC (14%), TPC (17%) and FRAP (9.5%) in strawberry puree. These findings suggest that US is a promising novel non-thermal food technology that can be tailored to improve the quality of strawberry puree by inactivating enzymes responsible for food deterioration whilst maintaining the content of bioactive compounds.     

冷破碎工艺对猕猴桃果浆品质的影响

以海沃德、华优、红阳三个不同品种的猕猴桃为原料,利用冷破碎专利设备制取猕猴桃果浆。采用化学方法分析其营养特性指标(包括可滴定酸、总糖和还原糖、果胶、蛋白质、维生素C及多酚含量),同时与经传统工艺制取的果浆相比较,探讨冷破碎技术的优劣。实验结果表明,冷破碎工艺使海沃德、华优、红阳猕猴桃果浆酸度分别提高了69.9%、105%、34.2%,多酚及多酚氧化酶含量下降了49.4%、30.5%、43.8%和42.3%、48.3%、51.7%;总糖和还原糖变化不大,果胶、蛋白质含量变化显著。所以,采用冷破碎工艺获得的果浆比较好地保留了猕猴桃原果的营养品质,从防止营养品质劣变的角度看,冷破碎工艺是值得推广应用的果品加工先进技术。      

微波加热钝化黑米糠过氧化物酶

采用微波加热法,以过氧化物酶残余活力为指标,通过单因素和正交实验得出微波钝化黑米糠过氧化物酶的最佳条件为:黑米糠含水量26%、加热时间105s、加热功率900W,该优化条件下过氧化物酶残余活力为0.5%。旨在为黑米糠工业化开发提供基础科学依据。      

肥城桃果实多酚氧化酶酶学特性的研究

以肥城桃果实为试材,分离纯化了导致酶促褐变的多酚氧化酶（PPO）,并对其性质进行了分析。结果表明,以叔丁基邻苯二酚和绿原酸为底物,加入十二烷基磺酸钠（SDS）后使PPO活性分别增大19倍和12倍,催化效率分别提高11.5倍和17倍,PPO最适pH为6.5,2-羟基-2,4,6-环庚三烯酚酮显著抑制PPO活性。部分变性SDS-PAGE显示PPO在表观分子量分别为49ku和50ku有两条活性条带,等电聚焦IEF显示可溶性PPO和膜结合PPO都含有两个酸性范围的条带,其p I为5.7和5.8,膜结合PPO还有一条p I为5.4的条带。结合western杂交在膜结合PPO的3种同工酶中,只有2种能被抗体识别;全变性SDS-PAGE和western blot发现存在一条分子量为60ku的多肽。      

草莓冻藏过程中多酚氧化酶、过氧化物酶及维生素C的变化研究

本文研究速冻草莓在-12、-18、-40℃三种温度下十四周贮藏中可溶性多酚氧化酶（Polyphenol Oxidase，简称PPO）、过氧化物酶（Peroxidase，简称POD）及维生素C的变化，结果表明：不同温度下冻藏的草莓维生素C含量均呈下降趋势：冻藏初期可溶性多酚氧化酶、过氧化物酶活性上升，后期显著下降。     

奇异果芦荟悬浮饮料的研制

研究了以奇异果和芦荟为主要原料的新型复合悬浮饮料的制作工艺,对加工过程中稳定剂的选择进行了探讨,并运用正交试验确定了产品的最佳配方和生产工艺,研制出风味良好的复合悬浮饮料。     

猕猴桃果脯微波干燥工艺研究

采用单因素试验和正交试验,探讨微波干燥方式、微波功率、铺盘质量、干燥温度、送料速度、间歇时间等影响因子对猕猴桃果脯干燥效果的影响。结果表明:采用间歇式干燥,铺盘质量8 kg/m~2微波干燥机的微波输出功率8.25kW,干燥温度80℃,送料速度4 min/次,间歇时间控制在3 min以内,所得产品品质较好。     

外源过氧化氢酶对液态发酵红茶色素的影响

为得到优质速溶红茶,本研究以低档绿茶为原料,对其水提液添加外源过氧化氢酶(CAT)进行液态发酵酶促转化成红茶茶汤,探讨液态发酵工艺对红茶色素生成量的影响,结果表明:参照优质红茶色素含量标准,CAT酶促转化的最优条件为:加酶量0.25 U/m L,反应温度45℃,反应时间2 h,p H6。CAT与多酚氧化酶(PPO)存在双酶协同作用,绿茶水提液先用PPO处理后(酶添加量0.05 U/m L,温度40℃,p H5,时间1.5 h)再用CAT处理,合成红茶色素的效果比单一酶源好。      

A general method for the instrumental assessment of the colour of tomato fruit during ripening

The colour of subjectively-chosen tomato fruit of three contrasting cultivars at each of five stages of ripeness were measured on a tri-stimulus colour difference meter. The values were introduced into a formula previously developed for calculating objective limits for each ripening stage, but satisfactory separations were not achieved. When the colour components for areas on the side-walls of tomatoes were introduced into a revised formula, good discrimination was achieved between classes of fruit from fairly ripe to fully ripe. Similar measurements carried out on areas near the stylar scar and used with an alternative formula led to efficient separation of less ripe fruit into distinct classes. It is suggested that the revised formula can be applied to any cultivated tomato line except colour mutants.     

Effect of Storage on the Physico‐Chemical and Antioxidant Properties of Strawberry and Kiwi Leathers

Strawberry and kiwi leathers were used to develop a new healthy and preservative‐free fruit snack for new markets. Fruit puree was dehydrated at 60 °C for 20 h and subjected to accelerated storage. Soluble solids, titratable acidity, pH, water activity (a_w), total phenolic (TP), antioxidant activity (AOA) and capacity (ORAC), and color change (browning index) were measured in leathers, cooked, and fresh purees. An untrained panel was used to evaluate consumer acceptability. Soluble solids of fresh purees were 11.24 to 13.04 °Brix, whereas pH was 3.46 to 3.39. Leathers presented an a_w of 0.59 to 0.67, and a moisture content of 21 kg water/100 kg. BI decreased in both leathers over accelerated storage period. TP and AOA were higher (P ≤ 0.05) in strawberry formulations. ORAC decreased 57% in strawberry and 65% in kiwi leathers when compared to fruit puree. TP and AOA increased in strawberries during storage. Strawberry and Kiwi leathers may be a feasible new, natural, high antioxidant, and healthy snack for the Chilean and other world markets, such as Europe, particularly the strawberry leather, which was preferred by untrained panelists.     

Kinetics of colour change of bamboo shoot slices during microwave drying

The effect of microwave power on colour change kinetics of bamboo shoot slices was investigated during microwave drying. Colour changes were quantified by tri‐stimulus Hunter L (whiteness/darkness), a (redness/greenness), and b (yellowness/blueness) system. These values were also used for calculation of total colour change (ΔE), chroma, hue angle, and browning index (BI). Microwave drying as expected changed colour parameters because of browning. The values of L and b decreased, while values of a and ΔE increased during drying. Mathematical modelling of colour change kinetics indicated that L, b, chroma and BI could be defined using a first‐order kinetic model, while a, ΔE and hue angle could be defined using a zero‐order kinetic model. Considering together colour deterioration and quality of dried bamboo shoot slices at higher power reveals the need of process standardisation for getting good quality product.