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BOŽIKOVA MONIKA, HLAVAC PETER: Temperature and storing time infl uence on selected physical properties of milk and acidophilus milk. Acta Universitatis Agriculturae et Silviculturae Mendelianae Brunensis, 2013, LXI, No. 6, pp. 1589–1595 This article deals with thermophysical parameters as: temperature, thermal conductivity, diff usivity and rheologic parameters as: dynamic, kinematic viscosity and fl uidity of milk and acidophilus milk. For thermophysical parameters measurements was used Hot Wire method and for rheologic parameters measurements was used single – spindle viscometer. In the fi rst series of measurements we measured relations between thermophysical and rheologic parameters in temperature range (5– 25) oC for milk and acidophilus milk. Relations of all physical parameters of milk to the temperature showed infl uence of relative fat content. Eff ect of storage on milk and acidophilus milk is shown in the text. All measured relations for milk and acidophilus milk during temperature stabilisation had linear increasing progress with high coeffi cients of determination in the range (0.991–0.998). It was shown that increasing relative fat content has decreasing infl uence on milk thermal conductivity. Relations of rheologic parameters as dynamic and kinematic viscosity to the temperature had decreasing exponential progress, while relation of fl uidity to the temperature had increasing exponential shape with high coeffi cients of determination in the range (0.985–0.994).. Mathematical description of the dependencies is summarised by regression equations and all coeffi cients are in presented tables. milk, acidophilus milk, thermal conductivity and diff usivity, dynamic and kinematic viscosity, fl uidity, temperature and storing time, fat content At quality evaluation of food material is important to know their physical properties particularly mechanical, rheologic and thermophysical (Božikova, Hlavac, 2010). Automatically controlled processes at manufacturing, at handling and holding require exact knowledge about physical quantities of materials. Knowledge of physical properties of food materials has a decisive importance for the realization of many technological processes, especially for monitoring of their quality (Figura, Teixeira, 2007), (Barbano et al., 2006). Very interesting is monitoring of material quality in food industry, especially it is very convenient for food materials with short expiration time as dairy products (Forsbacke et al., 2011). So, the presented research was oriented on selected dairy products – milk with diff erent fat content and acidophilus milk. Still are detected new methods that are utilizing new modern apparatuses (Hlavacova, 2003) Because of necessity to measure many series of measurements in a short time, scientists preferred non stationary methods for thermophysical and rheologic parameters measurements to stationary methods which take a long time. On the base of presented facts there were created experimental apparatuses for detection of basic thermophysical and rheologic parameters. For thermophysical parameters measurements was used one of the basic dynamic measurement method – Hot wire method. The rheologic parameters were measured by rotational viscometer. Details of experimental apparatus were selected according to the character of the sample and according to the character of measured parameters. 1590 Monika Božikova, Peter Hlavac MATERIALS AND METHODS Transient methods represent a large group of techniques where measuring probes, i.e. the heat source and the thermometer, are placed inside the sample. This experimental arrangement suppresses the sample surface infl uence on the measuring process which can be described as follows. The temperature of the sample is stabilized and made uniform. Then the dynamic heat fl ow in the form of a pulse or step – wise function is generated inside the sample. From the temperature response to this small disturbance, the thermophysical parameters of the sample can be calculated (Wechsler, 1992). For measurements of thermophysical parameters was used one of the basic standard transient method – Hot wire method, which is applied in instrument Isomet 2104. Heat Transfer Analyser – ISOMET 2104 is a portable measuring instrument for direct measurement of thermophysical properties of a wide range of materials. Measurement is based on analysis of the temperature response of the analyzed material to heat fl ow impulses. Heat fl ow is excited by electrical heating of resistor heater inserted into the probe which is in direct heat contact with the tested sample. Evaluation of thermal conductivity and thermal diff usivity is based on periodically sampled temperature records as a function of time, provided that heat propagation occurs in unlimited medium (Davis, 1984). The broad measurement range of the Isomet allows to measure variety of materials with extremely diff erent mechanical properties. Isomet 2104 could be used for measuring of liquids, suspensions etc. It is equipped with two various types of probe, for liquid and suspensoid materials are convenient needle probes. The probes are selected according to expected range of sample thermal conductivity. Viscosity measurement can be done by various devices. O en are used these types of viscometers (Sahin, Samnu, 2006): capillary fl ow viscometers, orifi ce type viscometers, falling ball viscometers, rotational viscometers, vibration viscometers,... In rotational viscometers the sample is sheared between two parts of the measuring device by means of rotation. The shear rate is proportional to rotational speed so it is possible to measure the shear stress as the shear rate is changed. Sample can be sheared for as long is desired, therefore rotational viscometers are the best for non – Newtonian fl uids and fl uids with time dependent behaviour. We can divide rotational viscometers into four possibilities: concentric cylinder (coaxial rotational) viscometers, cone and plate viscometers, parallel plate viscometers and single – spindle viscometers (Steff e, 1996). For our research was selected rotational viscometer Anton Paar DV-3P, which works on principle of single – spindle viscometer so the measuring is based on torsion forces measurement required to overcome resistance of material at rotating spindle embedded in measured material. Spinning spindle is interconnected through spring to engine sha , which is rotating with defi ned velocity. Angle of angular rotation sha is measured electronically. From measured values is on the base of internal calculations directly displayed values of dynamic viscosity in mPa.s. This instrument works with several types of spindles and uses wide area of velocity, what allows measurement of viscosity in wide area. For liquids with constant viscosity resistance towards motion is growing proportionately with velocity and dimension of spindle. Combination of various spindles and velocities provides optimal selection extent for viscosity measurement. Measuring range for determination of rheologic properties of material can be changed by using other velocity at the same spindle. Milk – is an emulsion or colloid of butterfat globules within a water-based fl uid. Each fat globule is surrounded by a membrane consisting of phospholipids and proteins; these emulsifi ers 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 fl uid portion of the milk (McGee, 1984). In unhomogenized cow’s milk, the fat globules average is about four micrometers across. The fatsoluble vitamins A, D, E, and K are found within the milk fat portion of the milk (Janzen, Bishop, Bodine, Caldwell, 1982). Acidophilus milk – is milk which has been fermented with Lactobacillus acidophilus bacteria, creating a very distinctive tangy fl avour and slightly thickened texture. In addition to tasting a bit diff erent from regular milk, acidophilus milk is also believed to be benefi ciary, because the bacteria can help restore the healthy balance of bacteria. All measured samples of milk and acidophilus milk were provided in storage boxes at the temperature from 4 °C to 5 °C and 90 % of the air moisture content during 24 hours before measurement and relations of thermophysical and rheologic parameters to the temperature were measured during temperature stabilization of samples. All measurements were made in laboratory settings. The measurements were performed for milk with relative fat content 0.5 %, 1.5 % and 3.5 % and acidophilus milk in temperature range (5– 25) oC. From theory is evident that viscosity is infl uenced by temperature. This dependency can be described by Arrhenius equation

Temperature and storing time influence on selected physical properties of milk and acidophilus milk