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Drying
Published in C. Anandharamakrishnan, S. Padma Ishwarya, Essentials and Applications of Food Engineering, 2019
C. Anandharamakrishnan, S. Padma Ishwarya
Water activity relates to the proportion of available water which facilitates the chemical reactions and microbial growth in foods. A graphical representation of the relationship between moisture content and water activity of a food is known as the moisture sorption isotherm (Figure 10.2). The shape of moisture sorption isotherm is characteristic of the chemical composition, physical structure, and nature of the water binding in the food. The moisture sorption isotherms can be plotted from adsorption (wetting or gain of moisture until equilibrium is attained with the surrounding of high RH) or desorption (drying or loss of moisture until equilibrium is attained with the surrounding of low RH). The difference between the adsorption and desorption curves is known as hysteresis (Figure 10.2). Obtaining an understanding of the moisture sorption isotherms is relevant from the perspectives of design and optimization of drying equipment.
Applications
Published in Raj P. Chhabra, CRC Handbook of Thermal Engineering Second Edition, 2017
Joshua D. Ramsey, Ken Bell, Ramesh K. Shah, Bengt Sundén, Zan Wu, Clement Kleinstreuer, Zelin Xu, D. Ian Wilson, Graham T. Polley, John A. Pearce, Kenneth R. Diller, Jonathan W. Valvano, David W. Yarbrough, Moncef Krarti, John Zhai, Jan Kośny, Christian K. Bach, Ian H. Bell, Craig R. Bradshaw, Eckhard A. Groll, Abhinav Krishna, Orkan Kurtulus, Margaret M. Mathison, Bryce Shaffer, Bin Yang, Xinye Zhang, Davide Ziviani, Robert F. Boehm, Anthony F. Mills, Santanu Bandyopadhyay, Shankar Narasimhan, Donald L. Fenton, Raj M. Manglik, Sameer Khandekar, Mario F. Trujillo, Rolf D. Reitz, Milind A. Jog, Prabhat Kumar, K.P. Sandeep, Sanjiv Sinha, Krishna Valavala, Jun Ma, Pradeep Lall, Harold R. Jacobs, Mangesh Chaudhari, Amit Agrawal, Robert J. Moffat, Tadhg O’Donovan, Jungho Kim, S.A. Sherif, Alan T. McDonald, Arturo Pacheco-Vega, Gerardo Diaz, Mihir Sen, K.T. Yang, Martine Rueff, Evelyne Mauret, Pawel Wawrzyniak, Ireneusz Zbicinski, Mariia Sobulska, P.S. Ghoshdastidar, Naveen Tiwari, Rajappa Tadepalli, Raj Ganesh S. Pala, Desh Bandhu Singh, G. N. Tiwari
Water present in a food product may be broadly classified as free water (if it is available to participate in various biochemical reactions) and bound water (if it is not available to participate in various biochemical reactions). Water activity is a measure of the amount of free water in a product. It is determined as the ratio of the vapor pressure of water in that product to the vapor pressure of pure water at that same temperature. It is also sometimes defined as equilibrium relative humidity divided by 100. Water activity is determined using a hygrometer. Most microorganisms require water for their growth and hence grow faster in high water activity foods. Accordingly, lowering water activity is one of the common techniques used to preserve many types of foods. Since most bacteria do not grow rapidly at water activity levels below 0.85, that is used as a cutoff value for foods likely to be shelf-stable with minimal or no thermal processing. Between a water activity of 0.80 and 0.85, molds are generally the organisms of concern. Between water activity values of 0.5 and 0.8, osmophilic yeasts, xerophilic molds, and halophilic bacteria are the organisms of concern.
Effects of Process Conditions of Intermittent Drying on Quality of Food Materials
Published in M. Azharul Karim, Chung-Lim Law, Intermittent and Nonstationary Drying Technologies, 2017
Chung-Lim Law, Nghia Duc Pham, Sami Ghnimi, A.M. Nishani Lakmali Abesinghe, Mohammad U.H. Joardder, Tony Petley, Scott Muller, M. Azharul Karim
Drying is a popular preservation method used to inhibit the development of microbial organisms and delay the onset of some deteriorative biochemical reactions in biomaterials such as food and agricultural products. There are two major effects of drying toward the preservation of biomaterials: The reduction of surviving microorganisms to a safe level, which inhibits deterioration and the reduction of water activity (aw) that avoids microbial growth; The minimization of destructive degradative reactions causing deterioration. Water activity contributes to chemical and biochemical reactions and cellular and biological matrices in foods (Chirife and Fontana 2008). A high water activity facilitates the growth of microbiological organisms and biochemical reaction in food material, which can reduce the shelf-life time of products. Microbiologically safe water activity is shown in Table 5.4.
Analytical theory study on latent heat coefficient of grain water vaporization
Published in Drying Technology, 2021
Water activity is a physical quantity representing the state of water existence, it has nothing to do with the kind of material in which water exists, while it is equal to the relative humidity of the medium when the system is in equilibrium with the surrounding medium. Therefore, no matter what kind of material moisture exists in, as long as the water activity and vaporization temperature are the same, the latent heat coefficient of vaporization is the same. In Figure 10, the latent heat coefficients of vaporization of paddy and wheat at the same temperature and activity are quite different, and the deviation is non-linear, which not only indicates that the previous model itself has defects, but also shows that the calculation coefficients deviate from the actual situation. Therefore, there will be huge deviation when it is used to evaluate situations other than test conditions, so there are also great limitations when it is used as an approximate calculation expression.
Nondestructive quality evaluation of banana slices during microwave vacuum drying using spectral and imaging techniques
Published in Drying Technology, 2018
Yuan-Yuan Pu, Ming Zhao, Colm O’Donnell, Da-Wen Sun
Moisture content influences water activity which affects food products’ quality and stability. The drying uniformity of different dehydration techniques can be evaluated using moisture distribution maps of dried samples. Figure 8 shows moisture distribution maps of banana slices at different drying stages. For total microwave heating times of 0, 6, 12, 18, and 21 min, the mean moisture content in each drying batch was 75.3, 62.6, 39.4, 11.2, and 5.2%, respectively. Samples in each batch had similar moisture distribution patterns, demonstrating the uniform drying characteristics of MVD process. However, some moisture variation was observed within individual samples. This could result from different tissue composition within sample slices (demonstrated by the moisture distribution maps of fresh-cut banana slices (0 min)) and a higher temperature at the center of heated samples (demonstrated in Figure 4.) Moisture evaporation was accelerated by the increased temperature as shown in Figure 8. At the end of drying, a minimal moisture variation was observed in the final dried banana slices (21 min) and the mean moisture content was reduced to 5.2%.
Use of dielectric properties measurements at microwave frequencies for real-time monitoring of water activity in almonds
Published in Journal of Microwave Power and Electromagnetic Energy, 2022
Water activity of water-containing materials including foods, agricultural products, and pharmaceuticals is a critical parameter that is used to determine their shelf life and conditions for safe storage. In food systems it is defined as the ratio of the vapor pressure of the water food system (p) to the vapor pressure of pure water (p*) under the same conditions. Unlike water content, which is a measure of the amount of water, water activity is a measure of the water available and thus it reflects the water binding status in a given material. The higher the water activity the “freer” the water. It is often used to predict growth of bacteria, yeast, and mold. Conventional methods (Prior 1979) for measuring water activity are destructive, time consuming and not applicable in dynamic situations where it is continuously changing. In addition, sampling a small amount of material for water activity determination is not always representative when large amounts are involved. Recently, direct correlations were established between dielectric properties measured at a single microwave frequency and water activity (Trabelsi 2021). These correlations constitute the basis for developing indirect methods and sensors for real-time and nondestructive determination of water activity. In this study, these correlations were tested to track changes in water activity of almond kernels (Butte/Padre and Nonpareil) placed in a chamber where the humidity was kept very high. Results are shown for the dielectric constant (ε’), the dielectric loss factor, (ε”), and the loss tangent (ε”/ε’) at 10 GHz over a time period of about 15 days. Performance of these correlations in predicting water activity was assessed through computation of the standard errors of calibration (SEC).