Journal on Processing and Energy in Agriculture 20 (2016) 2 69 Biblid: 1821-4487 (2016) 20; 2; p 69-72 Review Paper UDK: 632.123 Pregledni rad WATER ACTIVITY VS. EQUILIBRIUM MOISTURE CONTENT VODENA AKTIVNOST NASUPROT RAVNOTEŽNE VLAŽNOSTI MATERIJALA Monika LUTOVSKA, Vangelce MITREVSKI, Tale GERAMITCIOSKI, Vladimir MIJAKOVSKI, Igor ANDREEVSKI Faculty of Technical Sciences, 7000 Bitola, Makedonska Falanga 33, Macedonia sahdooel@hotmail.com ABSTRACT Water activity is an important parameter that can be used to predict the stability and safety of food with respect to the most microbiological and biochemical reactions, conditioned by its value. In other words, it describes the degree of the ̎bound ̎ level of water in the dry material, as well as the indicator of the ability of water to act as a solvent, participant in various biochemical reactions, or growth of different microorganisms. The determination of the minimum value of water activity that allows the growth of certain microorganisms and mechanical and thermal equilibrium of the material with the surroundings, are the most important parameters that characterize the stability of the material. The equilibrium moisture content has fundamental role in food industry and technology from the aspect of design and optimization of drying processes and equipment, design of packing materials for dried products, prediction of quality and stability, as well as for calculation of changes in the moisture content of dried material during its storage. Considering the relationship between these two physical parameters, their interdependence has been shown in this paper. Key words: water activity, equilibrium moisture content, food materials. REZIME Vodena aktivnost je ključan parametar za predviđanje stabilnosti i roka trajanja prehranbenih materijala, iz razloga što je najveći broj mikrobioloških i biohemiskih reakcija uslovljen njenom vrednošću. Drugim rečima, ona je pokazatelj stepena veze vode sa suvim skeletom materijala, kao i pokazatelj sposobnosti vode da reaguje kao rastvarač, da učestvuje u različitim biohemiskim reakcijama ili u razvoju određenih mikroorganizama. Poznavanje minimalne vrednosti vodene aktivnosti koja omogućava razvoj određenih mikroorganizama, kao i odnos mehaničke i termičke ravnoteže materijala sa okolinom, su jedni od najvažnijih parametara koji karakterišu stabilnost materijala. Ravnotežni sadržaj vlage ima fundamentalnu ulogu u prehranbenoj industriji i tehnologiji u smislu dizajna i optimizacije procesa i opreme za sušenje, dizajniranje ambalaže za pakovanje sušenog proizvoda, predviđanje kvaliteta i stabilnosti materijala, kao i proračun promene sadržaja vlage u sušenom materijalu tokom skladištenja. Obzirom na relaciju ove dve fizičke veličine u radu je prikazna njihova međusobna zavisnost. Ključne reči: vodena aktivnost, ravnotežni sadržaj vlage, prehranbeni materijali. INTRODUCTION Food materials are multicomponent systems, including water as one of the most important constituents. The state of water in the food system, under different external conditions, temperature and pressure, has crucial role for the protection and preservation of nutritional and organoleptic characteristics of food material. The concept of water activity has fundamental role in the drying process and food materials stability. The first food stability map based on the concept of water activity, reflects the development of microorganisms and different types of biochemical reactions that occur in the material, such as: oxidation process, browning reactions, enzyme activities and vitamin degradation (Labuza et al., 1972). Later, the upgraded food stability diagram has been proposed (Rahman, 2010). In order to overcome the limitations of the water activity concept, in the scientific literature the glass transition concept has been proposed (Rahman, 2012). On the other hand, the equilibrium moisture content has fundamental role in food industry and technology from the aspect of design and optimization of drying processes and equipment, design of packing materials for dried products, prediction of quality and stability, as well as for calculation of changes in the moisture content of dried material during its storage. MATERIAL In this paper, the importance of water activity as one of the most important factors affecting the food quality and stability, as well as the interrelations between two the most important concepts for stability criteria determination: the water activity concept and the glass transition concept, has been highlighted. By analyzing the impact of the water activity value on the achievement of mechanical and thermal material equilibrium, the sorption isotherms as a unique opportunity to experimentally determine the equilibrium moisture content of food materials, have been defined. DISCUSSION Water activity and concepts of water activity Water activity is an important parameter that can be used to predict the stability and safety of food with respect to the most microbiological and biochemical reactions, conditioned by its value. In other words, it describes the degree of the ̎bound ̎ level of water in the dry material, as well as the indicator of the ability of water to act as a solvent, participant in various biochemical reactions, or growth of different microorganisms. The determination of the minimum value of water activity that allows the growth of certain microorganisms and mechanical and thermal equilibrium of the material with the surroundings, are the most important parameters that characterize the stability of the material. Water activity, a w , is defined as a ratio of the partial vapor pressure of water on the material surface, р s , and the partial vapor pressure of pure water, р 0 , at the same temperature (Rahman, 2009): 0 p s p a w = (1)