Journal on Processing and Energy in Agriculture 15 (2011) 1 17 Biblid: 1821-4487 (2011) 15; 1; p.17-22 Original Scientific Paper UDK: 591.146:621.3.017 Originalni naučni rad EFFECT OF TEMPERATURE ON MILK RHEOLOGIC AND THERMOPHYSICAL PROPERTIES UTICAJ TEMPЕRTURE MLEKA NA REOLOŠKE I TERMOFIZIČKE OSOBINE Peter HLAVÁČ, Monika BOŽIKOVÁ Faculty of Engineering, Department of Physics, Slovak University of Agriculture in Nitra, Tr. A. Hlinku 2, SK - 949 76 Nitra, Slovak Republic, e-mail: Peter.Hlavac.MF@uniag.sk ABSTRACT This article deals with rheologic and thermal properties of different types of milk. If we want to protect high standard of quality during the processing and storage of milk we need to know physical parameters which can evaluate the quality of food material. The most important properties are mainly mechanical and thermophysical parameters. The article presents the results from rheologic and thermophysical parameter measurements of milk with different fat content. For rheologic property measurement, the digital rota- tional viscometer Anton Paar DV-3P was used. Thermophysical properties were measured by Hot wire (HW) method. We measured the effects of temperature on rheologic parameters such as: dynamic and kinematic viscosity, fluidity and thermophysical parameters as: thermal conductivity and diffusivity. All the samples were measured in the temperature range (5 – 25)ºC in the laboratory set- tings. The results of the measurement showed that temperature and different fat content of milk have significant influence on milk rheologic and thermophysical parameters. Key words: milk, milk temperature, milk fat content, dynamic viscosity, kinematic viscosity, thermal conductivity, thermal diffusiv- ity. REZIME Ovaj rad se bavi reološkim i termičkim svojstvima različitih vrsta mleka. Ako se želi zaštititi visoki standard kvaliteta u toku pre- rade i skladištenja mleka treba da se poznaju fizički parametari na osnovu kojih može da se proceni kvalitet prehrambenog materi- jala. Najvažnije su, uglavnom, mehaničke i termofizičk osobine. U ovom radu prikazani su rezultati merenja reoloških i termofizičkih parametara mleka sa različitim sadržajem masti. Za merenje reoloških osobina korišćen je digitalni rotacioni viskozimetar Anton Paar DV-3P. Termofizičke osobine merene su metodom zagrejane žice (HV metod). Određivani su uticaji temperature na reološke parametare kao što su dinamička viskoznost, kinematska viskoznost i fluidnost (recipročna vrednost od dinamičke viskoznosti) i ter- mofizičke parametre kao što su koeficijent toplotne provodljivosti i koeficijent temperaturne provodnosti. Svi uzorci mereni su u temperaturnom opsegu (5 - 25)ºC u laboratorijskim uslovima. Rezultati merenja su pokazali da različite temperature i različiti sa- držaji masti u mleku imaju značajan uticaj na reološke i termofizičke osobine. Ključne reči: mleko, temeperatura mleka, sadržaj masti u mleku, dinamička viskoznost, kinematska viskoznost, koeficijent pro- vođenja toplote, koeficijent temperaturne provodnosti. INTRODUCTION For the realization of many technological processes, espe- cially for the monitoring during production and storage it is nec- essary to have knowledge about physical properties of materials. HACCP (Hazard Analysis and Critical Control Point System) is internationally accepted system guarantying the foodstuff safety aimed at the identification, evaluation and risks control in the whole technological procedure of foodstuff production. The quality assessment and guarantee of the safety of foodstuff make the main priorities in food industry. For successful application of HACCP, it is necessary to know selected physical properties of food. In general, all food material during processing and storage goes through the thermal or mechanical manipulation, so it is convenient to know thermophysical and mechanical (especially rheologic for liquid materials) parameters. This article shows that the physical research applied on food materials is very im- portant, because of information about physical parameters which can determine quality of agricultural and food materials (Vozárová and Hlaváčová, 2008). MATERIAL AND METHOD Description of milk – milk is an emulsion or colloid of butterfat globules within a water-based fluid. Each fat globule is surrounded by a membrane consisting of phospholipids and pro- teins; these emulsifiers keep the individual globules from joining together into noticeable grains of butterfat and also protect the globules from the fat-digesting activity of enzymes found in the fluid portion of the milk. In unhomogenized cow's milk, the fat globules average about four micrometers across. The fat-soluble vitamins A, D, E, and K are found within the milk fat portion of the milk (McGee, 1984). When raw milk is left standing for a while, it turns "sour". This is the result of fermentation, where lactic acid bacteria ferment the lactose inside the milk into lactic acid. Prolonged fermentation may render the milk unpleasant to consume. This fermentation process is exploited by the introduc- tion of bacterial cultures (e.g. Lactobacilli sp., Streptococcus sp., Leuconostoc sp., etc) to produce a variety of fermented milk products. The reduced pH from lactic acid accumulation dena- tures proteins and causes the milk to undergo a variety of differ- ent transformations in appearance and texture, ranging from an aggregate to smooth consistency. Some of these products include sour cream, yoghurt, cheese, buttermilk etc. Pasteurization of cow's milk initially destroys any potential pathogens and in- creases the shelf-life (FDA, 2010), (Janzen et al., 1982), but eventually results in spoilage that makes it unsuitable for con- sumption. This causes it to assume an unpleasant odour and the milk is deemed non-consumable due to unpleasant taste and an increased risk of food poisoning. In raw milk, the presence of lactic acid-producing bacteria, under suitable conditions, fer- ments the lactose present to lactic acid. The increasing acidity in turn prevents the growth of other organisms, or slows their