The Continuous Observation of Milk Coagulation with Light Scattering by Means of Optical Fiber

Abstract

The tendency of light scattering at milk coagulation was investigated by means of optical fiber for process control of cheese or yoghurt processing. Ingredient or measuring direction of light scattering or wave length was varied and also an optical fiber with modified tip was employed. The tips of both optical fibers from the light source and the sensor were immersed in mixed ingredient solution and light scattering were measured continuously. Raw output signal and first derivative of output signal from the amplifier for the sensor were recorded. In all processes tested under different conditions of cheese processing, the beginning of coagulation was observed at the location immediately after the first obvious peak on the first derivative curve. Similar results were also obtained in the case of yoghurt. The tendencies of light scattering were correlated to those of curd-tension measured simultaneously in cheese processing. Some experiments, in which concentration of rennet or concentration of calcium chloride or heat treatment of raw milk were varied, were carried out. Two obvious peaks were observed on most of first derivative curves. To make heavier the heat treatment of raw milk caused the height of the first peak lower and the second peak later. When the amount of rennet and/or calcium chloride were increased, the appearances of both of the first and the second peaks became faster and the height of the first peak was increased. The relations between the curd-tension values obtained after 60minutes and the height of first peak or the appearance time of second peak were such that the higher the first peak became or the shorter the appearance time of the second peak became, the higher the curd-tension value tended to. The tendencies of light scattering depended on measured angle, but the locations of the peak on the first derivative curve coincided with each other. In the measurement of light scattering at the direction of 180°, when the tip of the double track optical fiber from the sensor and the light source was covered with plane glass plate, polarity of second peak reversed, and the whole shape of the light scattering curve showed reverse shape of the measuring curve at O° with respect to abscissa. These observations are supposed to mean rapid shrinking of the clot at the second peak. No fundamental difference was observed in the results of light scattering pattern at the direction of 180°, with red light (660nm) and green light (565nm) in the cheese and also yoghurt processing. These results suggest that the size of casein micelle does not increase at coagulation. Light scattering measurement seemed to propose the possibility for monitering of milk coagulation and for the prediction of the properties of finished curd.