Yogurt is a semi-solid fermented milk product that is produced by milk used alone or in combination with bacterial culture. This bacterial culture contains bacterial that produces lactic acid during fermentation. The fermentation process is the most important stage in yogurt manufacture. The production of lactic acid decrease the pH value, which causes the milk to coagulate to form yogurt. During this stage, the yogurt curd is formed, and it is textural characteristics and distinct flavor are developed. The key factor of the fermentation process is the starter culture that acts through biochemical reactions and inductively causes the formation of the curd and the development of flavor components. Conventionally, the yogurt acidity is measured by a pH meter to determine the progress of fermentation. Nevertheless, pH meter requires calibration, and therefore, this method is inconvenient and time-consuming. Therefore, this method is not practical, Intensification of milk fermentation without negative influence on product quality is a priority research direction in dairy industry. One of the perspective tools for solving this problem is usage of ultrasound.
Ultrasound is a cyclic sound pressure that is higher than the upper limit of human hearing (20kHz) and is fast, non-invasive and inexpensive technique. Ultrasonic sensors are proved to be effective in many industrial and medical applications. Most known are nondestructive testing (NDT) and ultrasonic imaging. Ultrasonic sensors are used as flow and level meters in the process industry. Recently, new field of ultrasonic sensor technology have emerged and partly improved transducer development and signal processing. Careful selection of ultrasonic treatment regimens allows to activate lactic acid bacterial metabolic activity and to improve the efficiency of dairy production. Ultrasonic causes cavitation that brings about various chemical and physical changes.
Cavitation may cause the destruction of bacterial cells and at the same time, release of β-galactosidase. β -galactosidase is an enzyme that hydrolyzes the glycosidic bond between a galactose and another sugar or alcohol molecule. The measurement of ultrasonic parameters such as amplitude and time of flight of the echoes, backscattered by samples of yogurt, were able to provide information on the phases of the fermentation and on the state of yogurt, were able to provide information on the phases of the fermentation and on the state of yogurt over time. Sound velocity, sound absorption and acoustic impedance can be applied as acoustic properties together with advanced signal processing to measure liquid mixtures properties and to continuously control and monitor processes. Ultrasonic measurement is based on the changed in the properties of the transmitted acoustic wave, which are influenced by the medium. The literatures review show that the correlation between fermentation time and acoustic attenuation as well as acoustic velocity.