One of the oldest methods of food preservation is the reduction of water content in foods. Sun and fire drying, salting of animal flesh, and sugaring of fruit in prehistoric times simulated natural drying processes such as fruit drying on trees. Foods with a high water content, such as milk, meat, fruits, and vegetables, undergo rapid microbial deterioration. The concept of water activity (a-w) gives information about the availability to microbial growth and the stability of food. It is expressed in terms of vapor pressure generated by an aqueous system relative to that of pure water at the same temperature. Growth and survival of food spoilage organisms (bacteria, yeasts and molds) are a function of water activity and other environmental factors including temperature, pH, oxygen, and carbon dioxide concentration, and the presence of preservatives.

Fuer die Beschreibung von Trocknungsprozessen ist die Kenntnis einer Reihe von Stoffdaten, insbesondere des Wasserdampf-Sorptionsverhaltens der Trocknungsprodukte, erforderlich, da diese es gestatten, Verpackungs-, Lager-und Stabilitaetsprobleme richtig einzuschaetzen bzw. loesen zu koennen. Anhand einer umfangreichen Literaturrecherche wurde eine tabellarische Zusammenfassung der fuer den Temperaturbereich von 40 bis 80 Grad C gueltigen publizierten Sorptionsisothermen erstellt. Darueber hinaus werden zwei unterschiedliche Methoden beschrieben, die es gestatten, mit relativ einfachen Labormitteln die Wasserdampfisorptionsisothermen bei hoeherer Temperatur mit hinreichender Genauigkeit experimentell zu ermitteln.

This study has deduced a correlation between points of inflection of water activity and loss factor with respect to moisture content. A point of inflection in loss factor with respect to moisture content was found to coincide with the sorption isotherm point of inflection that defines the transition from multilayer to solution in every instance analysed, with an average difference of just 0.01 kg kg−1. Food can support microbial growth and chemical reactions in water activity levels above this critical transition. This correlation was discovered using published dielectric and sorption data for specific foods at similar temperatures. It was found that low sugar foods containing high levels of hydrocolloids generally exhibited different behaviour from fruits. This shows that microwave heating behaviour will be different in fruits compared to low sugar foods with high hydrocolloid content when drying to achieve a certain water activity and therefore shelf life.

Der Trocknungsverlauf wasserreicher, zylinderfoermiger Lebensmittel bzw. Modellsubstanzen (Konvektionstrocknung) wurde untersucht. Bedingt durch das Schrumpfen der Proben veraendern sich waehrend des Trocknungsprozesses die Gutsoberflaeche und der-geometrieabhaengige -Stoffuebergangskoeffizient. Es wird gezeigt, dass durch Verknuepfen der beiden Geometriefunktionen die flaechenbezogene Trocknungsgeschwindigkeit aus der Massenabnahme und den geometrischen Anfangswerten fuer Oberflaeche und Volumen bestimmt werden kann. Eine einfache Ueberpruefung der Richtigkeit der gefundenen Beziehung ergibt sich aus der Beurteilung des 1. Trocknungsabschnittes (freies Wasser an der Gutsoberflaeche): Die gemessenen Verdunstungsverluste muessen konstante Werte fuer die flaechenbezogene Trocknungsgeschwindigkeit ergeben. Die Berechnungsmethode kann auf mehrere geometrische Formen des Trocknungsgutes uebertragen werden.

A method for estimation of the upper temperature limit of water loss by food systems during preservation (drying, baking, extrusion, smoking, etc.) is proposed. These temperatures are related to the lower and higher critical solution temperatures, which were shown to depend on the chemical structure of system components. A determination method for the lower and higher critical solution temperatures in the plasticization curves obtained by calorimetry was developed.

Paper described the method of measuring water content in food products, based on microwave heating and drying of the foodstuff samples. Power supplied to material is controlled as a function of sample mass and features, thus the sample is dried in a short time without overheating. Water content is automatically calculated from the mass losses during drying. The experiments conducted with yoghurt confirmed agreement of the results with a reference method

Heterogeneous and hygroscopic characteristics of plant-based food material make it complex in structure, and therefore water distribution in its different cellular environments is very complex. There are three different cellular environments, namely the intercellular environment, the intracellular environment, and the cell wall environment inside the food structure. According to the bonding strength, intracellular water is defined as loosely bound water, cell wall water is categorized as strongly bound water, and intercellular water is known as free water (FW). During food drying, optimization of the heat and mass transfer process is crucial for the energy efficiency of the process and the quality of the product. For optimizing heat and mass transfer during food processing, understanding these three types of waters (strongly bound, loosely bound, and free water) in plant-based food material is essential. However, there are few studies that investigate cellular level water distribution and transport. As there is no direct method for determining the cellular level water distributions, various indirect methods have been applied to investigate the cellular level water distribution, and there is, as yet, no consensus on the appropriate method for measuring cellular level water in plant-based food material. Therefore, the main aim of this paper is to present a comprehensive review on the available methods to investigate the cellular level water, the characteristics of water at different cellular levels and its transport mechanism during drying. The effect of bound water transport on quality of food product is also discussed. This review article presents a comparative study of different methods that can be applied to investigate cellular water such as nuclear magnetic resonance (NMR), bioelectric impedance analysis (BIA), differential scanning calorimetry (DSC), and dilatometry. The article closes with a discussion of current challenges to investigating cellular water.

Plant-based food materials are porous and hygroscopic in nature; therefore, it contains three water environments, namely, intercellular, intracellular water and cell wall water. The intercellular water is known as capillary water or free water which is less constrained than intracellular water, considered as loosely bound water (LBW), and cell wall water, which is recognised as strongly bound (SBW). During food processing such as drying, frying, heating and cooking, optimisation of heat and mass transfer is crucial. The existing heat and mass transfer models for food processing are developed based on the concept that all of the water inside the food material is bulk water, which can act as free water that can be easily transported. This simplistic assumption has been made due to a lack of sufficient data to enable consideration of the proportion of free and bound water in plant-based food materials. Therefore, the aim of the present study is to investigate the proportion of different types of water such as free, LBW and SBW in 11 different plant-based food materials. The water proportion was investigated using 1H NMR T2 relaxometry. The experimental results uncovers that plant-based food materials contain about 80 to 92% LBW, 6 to 16% free water and only about 1 to 6% SBW. This investigation also confirms that among the five different fruits, kiwi contains the lowest percentage of LBW while Apple contains the highest percentage of LBW. Among the vegetables, eggplant comprises the largest amount of LBW while cucumber contains least amount of SBW. An attempt was made to establish a relationship between physical properties of fruits and vegetables and the proportion of the different types of water. Interestingly, it was found that SBW strongly depends on the proportion of solid in the sample tissue whereas FW depends on the porosity of the material.Food preservation is a major concern in today's world as about one-third of the global food production is lost annually due to lack of proper processing and preservation. Food processing is very energy intensive process and it consumes about 15–20% of energy used in industrial processes. Quality of processed food is also a big concern in the industries. Therefore energy efficiency and food quality are two major concerns in the food processing industry.The current food processing techniques such as drying are unable to ensure best quality and energy efficiency as many microlevel fundamentals of hygroscopic food material are unknown. One of the major unknown is the proportions and characteristics of different types of water inside the food materials and because of this an optimised food processing cannot be designed in order to ensure high quality and energy efficiency. The existing heat and mass transfer models are based on some simplistic assumptions, for instance all of the water inside the food material is considered bulk water; which means that it acts as free water that can be transported easily. This simplistic assumption has long been used due to lack of sufficient data to enable consideration of the proportion of free and bound water. Therefore, the aim of the present study is to determine the proportion of different types of water such as free water, loosely bound water (LBW) and strongly bound water (SBW) and establish relationship between physical properties and water characteristics in hygroscopic food materials.The findings of this study will enhance the understanding of plant-based food tissue that will contribute to a better understanding of potential changes occurring during food processing and will contribute to the development of accurate heat and mass transfer models and prediction of deformation. These findings will ultimately be significant for the equipment design engineers in food processing industry.