Perforation-mediated modified atmosphere packaging. Part II. Implementation and numerical solution of a mathematical model

The present article describes the numerical implementation of a space-and-time dependent mathematical model of perforation-mediated modified atmosphere packaging for respiring commodities as Part II of a study in which the mathematical model is described in Part I. The model includes species transport of CO₂, H₂O, N₂, and O₂ through the Maxwell–Stefan equations, velocity and pressure through Darcy's law and the Navier–Stokes equations, and temperature of the gas mixture and the commodity through the energy equation. Numerical solutions of the coupled system of equations were obtained using the finite-element method. To illustrate the capabilities of the general approach, simulations of a package of strawberries have been presented, for which experimental results are available in the literature. Comparison of the measurements and the model predictions were fair, considering the uncertainty of the available information. The sensitivities of the solution to the respiration rate model, respiration rates, porosity, package aspect ratio, relative humidity, transpiration coefficients, storage temperature, CO₂ solubility and volume of ambient storage area considered were examined. The steady-state concentrations of O₂ and CO₂ in the package were found to be very sensitive to the value of respiration rate, but much less so to changes of other input parameters within the ranges considered. The steady-state commodity temperature depended on the aspect ratio of the package. Including the CO₂ solubility in the commodity did not affect the steady-state gas concentrations, but it increased the time required for the CO₂ concentration to reach equilibrium in the package. It was shown that neglecting the ambient space beyond the perforation overpredicted the steady-state O₂ concentration. Approximately 80% of the resistance to diffusion was found to occur within the perforation, with the remainder equally divided between the spaces beyond each end of the perforation.     

二氧化碳高渗透性保鲜膜包装对鸭梨采后生理与贮藏品质的影响

以微孔保鲜膜为对照,研究二氧化碳高渗透性保鲜膜包装对7个月贮藏期间鸭梨采后生理和贮藏品质的影响。结果表明,二氧化碳高渗透性保鲜膜具有较高的二氧化碳渗透系数和很高的二氧化碳/氧气渗透系数比,在保持包装袋内较低二氧化碳浓度的同时也能维持较低的氧气浓度。两种保鲜膜包装袋内的二氧化碳浓度均低于0.5%的水平;微孔保鲜膜包装袋内的氧气浓度接近于空气(20%左右),二氧化碳高渗透性保鲜膜包装袋内的氧气浓度在5%～6%之间;二氧化碳高渗透性保鲜膜包装的鸭梨贮藏期间果实的可溶性固形物和VC含量显著高于微孔膜包装(P0.05),细胞的相对电导率和丙二醛含量显著低于微孔膜包装的果实(P0.05);两种包装的鸭梨果心褐变程度无明显差异。二氧化碳高渗透性保鲜膜包装有利于延缓鸭梨采后衰老进程,保持果实的良好品质。     

吸氧包装材料对橙汁品质影响的研究

以吸氧薄膜作为橙汁的包装材料,在室温条件下贮存,研究了贮存期内橙汁理化和感官品质指标的变化。结果表明,经5周贮存后,吸氧薄膜包装袋内的橙汁VC含量、可溶性固形物含量、pH值均高于对照组;褐变指数明显低于对照组;感官品质指标评价也好于对照组。这表明吸氧薄膜可以有效清除包装顶空内氧气,阻隔外界氧的进入,在包装内形成低氧环境;减缓了橙汁中VC、糖类的降解;延缓了橙汁的褐变,有效地抑制了橙汁感官品质的下降;有利于延长货架期。     

Headspace fingerprint mass spectrometry to characterize strawberry aroma at super-atmospheric oxygen conditions

This paper reports on the relation between super-atmospheric oxygen packaging and aroma production in strawberries. Volatile compounds were extracted from the headspace of strawberries stored at 7 °C for 0, 4 and 7 days under different gas conditions. Super-atmospheric oxygen concentrations were applied in combination with or without elevated CO₂ concentrations and the volatile production was measured with GC–MS. The potential of headspace fingerprint mass spectrometry (HFMS), a rapid technique for aroma profiling, was evaluated. Canonical discriminant analysis (CDA) based on the 16 most abundant volatile compounds and 26 HFMS signals (m/z) was applied to discriminate the samples according to gas storage treatment. The results showed that after 4 and 7 days of storage under super-atmospheric oxygen concentrations (without carbon dioxide) the production of ethyl acetate was suppressed. Ethyl acetate is one of the most important off-flavors in strawberries. Samples treated with only oxygen were mainly characterized by the ester methyl hexanoate with pineapple notes. Similar results were obtained with HFMS. CDA also showed that it is possible to classify samples according to days of shelf-life and that the effect of gas conditions was smaller than the effect of days of shelf-life.     

Effect of Air Humidity on Photosynthesis and Transpiration of Soybean Leaves

The responses of photosynthesis, transpiration, respiration, leaf conductance and cuticular transpiration to water vapour pressure deficit (VPD) have been simultaneously measured in single attached leaves of soybean in order to test the mechanism in the effects of air humidity on photosynthesis and transpiration. The results of present experiments have shown that the affecting mechanism of VPD or air humidity on the photosynthesis and transpiration is by means of controlling not only the stomatal behavior, but also the cultivars behavior, and the stomatal closure at high VPD or low air humidity is a function of the cuticular transpiration, rather than the stomatal transpiration.     

谷物呼吸热值测定研究

谷物呼吸热值测定是粮食通风散热课题的核心问题.本文利用气体分析仪和恒温水槽测定谷物呼吸对氧气的消耗量和二氧化碳的生成量,作者首次提出了同时考虑有氧呼吸和无氧呼吸两因素,对呼吸方程进行配平,从而由呼吸生成物与呼吸反应物之间生成??值之差来精确计算出呼吸热. 数据处理全过程由微机完成.     

Quadrature Methods and Their Extrapolations for Solving Boundary Integral Equations of Steady Navier-Stokes Problems

This paper presents an iteration method of solving non linear boundary integral equations (BIE) of the plane Navier Stokes problem,which each step of the iteration is to solve a linear BIE of nonhomogeneous Stokes problem. Using the quadrature methods of [1],we give a new algorithm with a high order accuracy.The algorithm not only saves work,but also the accuracy can be improved by Richardson extrapolation.     

3种绿化藤本植物降温增湿效应的研究

为研究常见藤本植物对环境降温增湿效应,选择山东省3种常见的城市绿化藤本植物扶芳藤(Euonymus fortunei)、金银花(Lonicera japonica)、爬山虎(Parthenocissus tricuspidata),测定其光合速率及蒸腾强度,估算其固碳量、释氧量和降温增湿量,并量化评价3种藤本植物的降温增湿效应。结果表明：扶芳藤的固碳量、释氧量比金银花、爬山虎大,分别为7.32、5.32 g/(m²·d);金银花、爬山虎固碳量分别为6.87、5.43 g/(m²·d),释氧量分别为4.99、3.95 g/(m²·d)。3种植物的降温增湿作用最明显的是金银花,其次是扶芳藤,爬山虎最低。     

Effect of oxygen and carbon dioxide absorbers on strawberry quality

This study investigated the effect of active modified atmosphere packaging on quality of fresh strawberries. Strawberries were treated with one oxygen and two different carbon dioxide scavengers throughout storage at 4 °C for 4 weeks. The effect of active packaging was assessed by comparing gas concentrations, pH, electrical conductivity, total soluble solids, surface color, decay incidence, texture profile analysis, sensory analysis, and FT-NIR analysis values. pH values were significantly higher in the controls (package without absorbers) during storage compared with other treatments. The package headspace with CO₂ absorbers, had the lowest CO₂ accumulation and O₂ absorbers resulted in constant O₂ levels (5 kPa) during storage. Total soluble solid contents of all treatments were between 10.34% and 7.7% except for the control which had a value of 6.94% at the end of the storage. CO₂ absorbers are clearly effective for maintaining soluble solid contents. Electrical conductivity was lowest with CO₂ absorbers throughout storage and color was better maintained in all treated fruit. Firmness values of the controls were significantly lower than those of the treatments. Results of sensory evaluation showed that the controls had the lowest scores for all attributes. This study shows that the FT-NIR is a powerful-nondestructive tool for monitoring quality of strawberries and produces rapid readings during storage. In general, our study showed that the use of active packaging preserves strawberry.     

Analysis of Condensation in Porous Wall Exposed to Hot Humid Climate

According to the work of Motakef and EI-Masri, the wall was divided into dry-wet-dry zones. Based on the heat and moisture transfer through porous media, considering the vapour condensation as a vapour sink, water source and heat source, coupled heat and moisture equations for wet zone were established. Closed-form analytical expressions for the condensation rate, moisture content and the time when critical moisture content value reached are obtained. It is indicated that the condensation rate exhibits a strong dependence on the temperature difference of the two sides of wet zone, and the maximum condensation rate value is reached at the boundary of wet zone close to lower temperature.     

四季豆贮藏保鲜技术的研究

研究四季豆在低温及PVC袋包装条件下的贮藏效果,测定其在贮藏过程中呼吸强度、水分含量、维生素C、还原糖、叶绿素含量的变化以及PPO、POD活性的变化,并且分析锈斑生成原因。结果显示,四季豆为呼吸跃变型蔬菜,9℃为四季豆适宜贮藏温度,随着贮藏时间的延长,水分、VC、还原糖、叶绿素的含量下降,PPO、POD活性上升;锈斑的形成与CO2有关;在35天之内,四季豆商品率可保持在92%左右。     

Minimally processed potatoes: Part 2. Effects of high oxygen partial pressures in combination with ascorbic and citric acid on loss of some quality traits

Sliced potatoes were stored in flexible packaging under different oxygen partial pressures (10, 55 and 100 kPa) after dipping treatments with ascorbic and citric acid at different concentrations. The levels of oxygen, ascorbic and citric acid were modulated according to a Central Composite Design. The response surface methodology allowed an assessment of the effects of these variables and their interactions on the respiration rate in a closed system, on the carbon dioxide accumulation rate and the volatile metabolites production inside flexible pouches. The results showed that the respiration rate did not increase in direct linear proportion to the oxygen partial pressure and there was no significant difference in respiration between 55 and 100 kPa, even though the respiration rate was higher at these super-atmospheric oxygen levels than at 10 kPa. Citric acid did not affect the respiration significantly, while the respiration rate increased with the increase in ascorbic acid concentration. However, at the highest level of ascorbic acid tested (5%), the respiration rate decreased. During storage in a high barrier plastic pouch, a higher CO₂ accumulation rate was generally observed under 55 kPa than under 10 and 100 kPa. High oxygen partial pressures (55 and 100 kPa) did not stop the production of hexanal but they had an inhibitory effect on the anaerobic volatiles production.     

比较食品包装纸透气性的简易方法

介绍一种快速测定食品包装纸透气性的自制简易装置,并利用该装置与氧气／二氧化碳测定仪器结合使用。分别对6种不同种类包装纸的透气性进行比较试验：试验结果表明,使用该简易装置测定食品包装纸透气性具有经济实惠、操作方便、试验结果重复率高等特点。     

氮肥基追比对滴灌冬小麦旗叶光合特性的影响

(目的)为探索小麦滴灌条件下氮肥合适的基追比对冬小麦光合作用的影响,(方法)设置4种不同基追比滴灌处理研究小麦花后旗叶光合特性。(结果结论)研究表明,与对照相比基追比为4:6的处理能显著提高小麦旗叶的SPAD值和叶面积指数,同时提高了小麦的净光合速率(Pn),促进旗叶对胞间二氧化碳(Ci)的同化,增加了气孔导度(Gs),降低了胞间二氧化碳的浓度,提高了蒸腾速率(Tr),保持了相对较高的光合能力。     

规模化养猪场废水和废气污染控制研究

研究污染治理措施对养猪场废气和废水中污染物浓度的影响,探讨治理措施的可行性,以期为同类型养猪场的污染防治提供参考。采用活性炭吸附和水膜除尘分别处理恶臭气体和锅炉燃烧废气;采用厌氧发酵和CASS工艺处理废水。在活性炭吸附塔进气口和出气口分别采集气体样品a和b,测定气体中氨(NH₃)和硫化氢(H₂S)的数值;在水膜除尘器进气口和出气口分别采集气体c和d,测定气体中二氧化硫(SO₂)和烟尘的浓度。在沼气池进水口采集2 L水样,记作水样A;经过沼气池厌氧发酵20、30、40天后,分别在沼气池出水口采集2 L水样,记作水样B、C、D;在CASS污水处理池出水口采集2 L水样,记作水样E;测定化学需氧量(COD_cr)、5日生化需氧量(BOD₅)、悬浮物(SS)、氨氮(NH₃-N)和总磷(TP)的数值。每次取3个平行样。结果表明：b中氨的强度显著低于a;b中硫化氢的强度极显著低于a;d和c的SO₂浓度无显著性差异;d的烟尘浓度极显著低于c。水样B、C、D、E中的化学需氧量、5日生化需氧量、悬浮物的浓度极显著低于A。水样E中,氨氮和总磷的数值极显著低于A、B、C和D;A、B、C、D之间无显著性差异。活性炭吸附处理对氨作用显著,对硫化氢作用极显著;水膜除尘处理对二氧化硫无明显影响,对烟尘作用极显著;单纯的厌氧发酵处理对化学需氧量、5日生化需氧量和悬浮物作用显著,对氨氮和总磷不显著。厌氧发酵和CASS工艺联合处理对化学需氧量、生化需氧量、悬浮物、氨氮和总磷作用极显著。除二氧化硫外,处理后各污染物浓度均能够达标。     

25℃条件下不同水分玉米对储藏环境中气体浓度变化的影响研究

通过对3种不同水分(偏低水分11.8%、安全水分13.3%和偏高水分16.3%)的玉米在25℃条件下,储藏密闭环境内N2、O2和CO2浓度变化的测定,研究密闭储藏环境条件下玉米粮粒周围环境气体成分浓度的变化规律。20L规模的实验室研究结果表明:不同水分玉米在密闭储藏时环境中N2浓度随时间的变化均不大;不同水分玉米在密闭储藏时环境中O2浓度与储藏时间呈负相关,偏低水分和安全水分玉米储藏环境中O2浓度随时间变化趋势基本无差异,偏高水分玉米储藏环境中O2浓度在各时间段均明显低于偏低水分和安全水分玉米,并且这种差异达极显著水平;不同水分玉米在密闭储藏时环境中CO2浓度与储藏时间呈正相关,其中储藏环境中CO2浓度在各时间段的大小为:偏低水分玉米安全水分玉米偏高水分玉米,且偏低水分和安全水分、安全水分和偏高水分间的差异达到显著水平,偏低水分和偏高水分间的差异达极显著水平;不同水分玉米密闭储藏环境中CO2的累积量在有氧呼吸阶段随耗O2量增加而增加,在无氧呼吸阶段CO2累积量和耗O2量无关。通过对玉米密闭储藏环境中气体浓度以及耗氧量和CO2累积量随时间的变化趋势线进行回归分析,得到25℃条件下不同水分玉米密闭环境中N2、O2和CO2的回归方程以及耗氧量和CO2累积量的回归方程,利用相应回归方程,可获得玉米在密闭储藏时环境中不同储藏时间段的气体浓度,为气调储藏时玉米粮粒自呼吸的合理利用提供基础技术参数和数据模型。     

柳州市主城区河岸植被三维绿量及生态效益分析

通过对柳州市主城区河岸植被三维绿量及生态效益进行定量化研究,为城市河岸绿地景观生态环境建设提供一些科学依据与参考借鉴,为实现“自然-人类-水体”的可持续发展奠定理论基础。在逐株实测的基础上,运用绿量方程计算植被三维绿量,通过三维绿量能换算出植物产生的生态效益。结果表明：主城区河岸植被中提供绿量最大的是常绿乔木,占总量的78.76%;生态效益的产生仍以常绿乔木最为突出,在释氧固碳量方面,常绿乔木占总量的86.68%,在吸收SO2、夏季蒸腾、滞尘量方面,常绿乔木占总量的78.77%,而灌木和草本的绿量值及生态效益值则非常小。植物多样性、植物配置模式、植物养护管理等均为河岸植被三维绿量大小及生态效益产生的主要影响因素。     

白蚁堆释放温室气体在不同林型中的研究

为了研究土地利用变化后西双版纳地区原始林和橡胶林中白蚁堆释放温室气体的差异,以西双版纳地区的常见白蚁种土垅大白蚁(Macrotermes annandalei)和云南大白蚁(Macrotermes Holmgren)为例,研究其季节性的甲烷和二氧化碳的变化情况。结果表明,甲烷和二氧化碳的释放量在雨季是干季的3.5倍。西双版纳地区大面积的橡胶种植造成土地利用变化,从而造成温室效应。笔者从昆虫对于土地利用变化后对温室效应响应的角度,解释了土地利用变化后温室气体排放量的增加加速了全球变暖。研究中忽略了白蚁堆自身的材料释放的甲烷和二氧化碳的季节性变化。白蚁种群的季节性变化可能是导致白蚁堆甲烷和二氧化碳排放的季节性变化的因素。此结果需要结合白蚁堆本身的气体排放和去除其他影响白蚁堆气体排放的因素来进一步得到证明。土地利用变化后,同种白蚁甲烷的释放量在不同林型中的差异不显著,同种白蚁在不同林型中的二氧化碳释放量显著。土地利用变化后白蚁堆释放气体呈上升趋势。     

Hexanal reduces infection of tomatoes by Botrytis cinerea whilst maintaining quality

Hexanal vapour and intact tomatoes were used as models to assess the opportunities for control of Botrytis cinerea rots by controlled release of organic vapours. Hexanal vapour concentrations in the ranges 5–270 μL L⁻¹ were applied continuously or as a single dose at the start of storage. The postharvest microbiological, physiological and quality attributes of control and hexanal treated tomatoes were investigated during storage for 7 days at 20 ± 1 °C and ∼99% RH. Continuous hexanal exposure effectively suppressed grey mould with the minimum inhibitory concentration (MIC) being 40–70 μL L⁻¹; the single-dose treatment showed minimal antifungal activity. During continuous exposure at the MIC the fruit respiration rate was increased ∼50% and reddening was slowed. No clear trend was observed in ethylene production and treated fruit did not differ from the controls in firmness or mass loss. The controlled release of low concentrations of hexanal vapour into a packaging headspace appears a feasible mechanism for prolonging tomato storage life.     

Response of Photosynthesis and Water Relations of Rice (Oryza sativa) to Elevated Atmospheric Carbon Dioxide in the Subhumid Zone of Sri Lanka

The objective of the present paper is to determine the response of the physiological parameters related to biomass production and plant water relations in a standard Sri Lankan rice (Oryza sativa) variety (BG‐300) to elevated CO₂ (i.e. 570 µmol/mol). During two seasons, rice crops were grown under three different experimental treatments; namely, at 570 µmol/mol (i.e. ‘elevated’) and 370 µmol/mol (‘ambient’) CO₂ within open top chambers, and at ambient CO₂ under open field conditions. Leaf net photosynthetic rate in the elevated treatment increased by 22–75 % in comparison to the ambient. However, the ratio between intercellular and ambient CO₂ concentrations remained constant across different CO₂ treatments and seasons. CO₂ enrichment decreased individual leaf stomatal conductance and transpiration rate per unit leaf area, and increased both leaf and canopy temperatures. However, the overall canopy stomatal conductance and daily total canopy transpiration rate of the elevated treatment were approximately the same as those achieved under ambient conditions. This was because of the significantly greater leaf area index and greater leaf–air vapour pressure deficit under CO₂ enrichment. The leaf chlorophyll content increased significantly under elevated CO₂; however, the efficiency (i.e. photochemical yield) of light energy capture by Photosystem II (i.e. F_v/F_m) in chlorophyll a did not show a significant and consistent variation with CO₂ enrichment.