Shyam S. Sablani

Concentration polarization in ultrafiltration and reverse osmosis: a critical review

Abstract A primary reason for flux decline during the initial period of a membrane separation process is concentration polarization of solute at the membrane surface This can occur in conjunction with irreversible fouling of the membrane as well as reversible gel layer formation Experimental and mathematical studies have been performed by various groups to gain a better understanding of concentration polarization phenomena in ultrafiltration and reverse osmosis This article critically reviews published studies on concentration polarization in both systems It presents progress made in determination of, for example, critical or limiting flux, and recommends specific models such as surface renewal, and experimental methods such as laser-based refractometry, for quantification of the problem.

Fouling of Reverse Osmosis and Ultrafiltration Membranes: A Critical Review

Abstract Desalination by using reverse osmosis (RO) membranes has become very popular for producing freshwater from brackish water and seawater. Membrane lifetime and permeate flux, however, are primarily affected by the phenomena of concentration polarization and fouling at the membrane surface. The scope of the current paper was to critically review the literature on the fouling phenomena in RO and ultrafiltration (UF) membrane systems, the analytical techniques used to quantify fouling, preventive methods, and membrane cleaning strategies. The paper also makes specific recommendations on how scientists, engineers, and technical staff can assist in improving the performance of these systems through fundamental and applied research.

Thermodynamic and economic considerations in solar desalination

Abstract The thermodynamic efficiency of single-basin and multiple-effect solar water desalination systems was critically reviewed with special emphasis on humidification-dehumidification processes. Solar energy may be used, either directly or indirectly, to produce fresh water. The concept of using the humidification-dehumidification process in combination with the growth of crops in a greenhouse system, however, is relatively new. System economics was also covered since it affects the final cost of produced water. While a system may be technically very efficient, it may not be economic. The challenges and opportunities of solar energy were also briefly discussed. The paper closes with a summary of key factors affecting system performance and recommendations for future areas of investigation and development.

Drying of Fruits and Vegetables: Retention of Nutritional/Functional Quality

In the past, research and development in drying has focused on the process and technology and food drying was performed mainly to extended the shelf life without much importance on retaining quality attributes. Recently, however, efforts have been made to develop high-quality dried foods. This is achieved by utilizing novel drying technologies, by improving and optimizing existing drying methods, and by maximizing quality attributes such as structure, color, flavor, nutrition, etc. In an effort to highlight quality aspect of dried foods and biomaterials, a special issue of Drying Technology [2005, 23(4)] was published. The objective of this article is to present an overview of quality attributes normally considered in the drying of food and biomaterials and highlight the recent advances in drying methods for the retention of nutritional and functional properties of fruits and vegetables.

Effect of feed temperature on permeate flux and mass transfer coefficient in spiral-wound reverse osmosis systems☆

The objective of the present study was to analyze and model concentration polarization in spiral-wound seawater membrane elements. In particular, the influence of feed temperature, salinity and flow rate on permeate flow and salinity was evaluated. Membrane lifetime and permeate fluxes are primarily affected by the phenomena of concentration polarization (accumulation of solute) and fouling (i.e., microbial adhesion, gel layer formation and solute adhesion) at the membrane surface. Results show that the polymer membrane is very sensitive to changes in the feed temperature. There was up to a 60% increase in the permeate flux when the feed temperature was increased from 20 to 40°C. This occurred both in the presence and absence of solute. Surprisingly, the permeate flux appears to go through a minimum at an intermediate temperature. There was up to a 100% difference in the permeate flux between feed temperatures of 30 and 40°C. The differences were statistically significant (p<0.05). A doubling of the feed flow rate increased the permeate flux by up to 10%, but only at a high solute concentration. Membrane parameters were estimated using an analytical osmotic pressure model for high salinity applications. A combined Spiegler-Kedem/film theory model described the experimental results. The modeling studies showed that the membrane transport parameters were influenced by the feed salt concentration and temperature.

Neural networks for predicting thermal conductivity of bakery products

An artificial neural network (ANN) approach was used to model the thermal conductivity of bakery products as a function of product moisture content, temperature and apparent density. The bakery products considered in this work were bread, bread dough, French bread, yellow cake, tortilla chip, whole wheat dough, baked chapati and cup cake. Data on thermal conductivity of bakery products were obtained from the literature for a wide range of product moisture contents, temperatures and apparent densities resulted from different baking conditions. In developing the ANN model, several configurations were evaluated. The optimal ANN model was found to be a network with six neurons in each of the two hidden layers. This optimal model was capable of predicting the thermal conductivity values of various bakery products for a wide range of conditions with a mean relative error of 10%, a mean absolute error of less than 0.02 W/m K and a standard error of about 0.003 W/m K. The simplest ANN model, which had one hidden layer and two neurons, predicted thermal conductivity values with a mean relative error of less than 15%.

Migration of Chemical Compounds from Packaging Polymers during Microwave, Conventional Heat Treatment, and Storage

Polymeric packaging protects food during storage and transportation, and withstands mechanical and thermal stresses from high-temperature conventional retort or microwave-assisted food processing treatments. Chemical compounds that are incorporated within polymeric packaging materials to improve functionality, may interact with food components during processing or storage and migrate into the food. Once these compounds reach a specified limit, food quality and safety may be jeopardized. Possible chemical migrants include plasticizers, antioxidants, thermal stabilizers, slip compounds, and monomers. Chemical migration from food packaging is affected by a number of parameters including the nature and complexity of food, the contact time and temperature of the system, the type of packaging contact layer, and the properties of the migrants. Researchers study the migration of food-packaging compounds by exposing food or food-simulating liquids to conventional and microwave heating and storage conditions, primarily through chromatographic or spectroscopic methods; from these data, they develop kinetic and risk assessment models. This review provides a comprehensive overview of the migration of chemical compounds into food or food simulants exposed to various heat treatments and storage conditions, as well as a discussion of regulatory issues.

Comparison of Water Blanching and High Hydrostatic Pressure Effects on Drying Kinetics and Quality of Potato

Abstract Drying kinetics and quality parameters of potato cubes were evaluated as affected by high pressure processing and hot water blanching. The potato cubes in 1% citric acid solution as immersion medium were pressure treated at 400 MPa for 15 min. Hot water blanching was conducted in boiling water for 3 min. Drying kinetics and quality parameters (i.e., rehydrability, texture, color and apparent density) were assessed for the high pressure–treated and water-blanched samples and for dehydrated and rehydrated samples. Drying rates were found to be higher (p < 0.05) in the initial period of drying for the pressure treated samples. The Page model was found to better fit drying data of the thermally treated samples, and the two-terms model better described the drying behavior of high pressure–treated samples. High pressure–treated samples had a similar rehydrability to thermally treated samples. It was found that pressure–treated samples had a hardness value close to that of fresh samples, whereas thermal treatment resulted in a softer texture. After rehydration, samples of both treatments returned their texture before drying. The total color difference for the thermally blanched samples was higher (p < 0.05) than for pressure–treated samples before drying and after drying. High pressure–treated and dried potato cubes had a color close to that of fresh potato cubes. High pressure–treated and air-dried samples were found to have higher (p < 0.05) apparent density than thermally treated samples.

An Artificial Neural Network Model for Prediction of Drying Rates

Abstract Drying rate data were generated for training of an ANN model using a liquid diffusion model for potato slices of different thicknesses using air at different velocities, humidities and temperatures. Moisture content and temperature dependence of the liquid diffusivity as well as the heat of wetting for bound moisture were included in the diffusion model making it a highly nonlinear system. An ANN model was developed for rapid prediction of the drying rates using the Page equation fitted to the drying rate curves. The ANN model is verified to provide accurate interpolation of the drying rates and times within the ranges of parameters investigated.

Effect of thermal treatments on phytochemicals in conventionally and organically grown berries

BACKGROUND Consumer demand for organic foods is increasing despite a lack of conclusive evidence of nutritional superiority of organically grown produce. The objective of this investigation was to evaluate the effects of thermal treatments on phytochemicals in conventionally and organically grown berries. Two cultivars of conventionally and organically grown red raspberries and blueberries were analysed for total anthocyanins, total and specific phenolic compounds and total antioxidant activity. Fresh berries were thermally processed into cans and juice/puree with and without blanching, and the changes in phytochemicals were monitored. RESULTS Total anthocyanin and phenolic contents of berries were not influenced by the agricultural production system. Total antioxidant activity of berries was also not influenced by the production system, but antioxidant activity varied significantly between cultivars. After canning, total anthocyanins decreased by up to 44%, while phenolic contents and antioxidant activity of both berries generally increased by up to 50 and 53% respectively. The level of changes in phytochemicals during berry puree/juice processing was influenced by blanching and type of berries. CONCLUSION Phenolic contents and antioxidant activities of berries increased while total anthocyanins decreased during canning. Blanching prior to puree/juice processing improved the retention of phytochemicals in blueberries.