| Milk is rich in protein,fat,vitamins and other nutrients,and their proportion is very balanced.It is the food whose nutritional value is closest to perfection recommended by the World Health Organization,and has become an indispensable diet in daily life.The demand for dairy products is also increasing,but raw milk is prone to be contaminated by harmful microorganisms,which will affect the quality of milk and subsequent products,and even threaten people's lives and health.In addition,due to regional differences and environmental hygiene of milk stations,microbial contamination in raw milk will also be different.Therefore,the control and prevention of microbial contamination is a crucial and indispensable process in the production and processing of raw milk.However,the traditional thermal sterilization technology suffers from incomplete killing of microorganisms at low temperature and great influence on the nutrition and flavor of milk at high temperature.In recent years,non-thermal physical sterilization technology has been widely concerned by researchers because of its low sterilization temperature,low damage to natural characteristics of products,no addition,green and many other advantages.In this thesis,raw milk from 4 milk stations around Hohhot,Inner Mongolia Autonomous Region was taken as the research object to analyze and compare the microbial contamination of raw milk.The Photon-Phonon coupling cold sterilization equipment was developed and designed to study the sterilization effect and sterilization mechanism of raw milk,the influence of different sterilization conditions on milk sensory quality and nutritional composition,and the influence on the structural properties of casein and whey protein in milk.The specific research contents are as follows:(1)Four milk stations around Hohhot were selected to collect fresh raw milk samples,and 3M Petrifilm test tablets were used to culture raw milk samples from four milk stations.It was demonstrated by drug sensitivity test that the main microorganisms in raw milk of different milk stations had different degrees of drug resistance,and then Fourier transform infrared spectroscopy(FTIR)coupled with the principal component analysis(PCA)were used to distinguish these microorganisms.It was found that there were differences in the microorganisms in different raw milk.(2)The sterilization effect of ultraviolet sterilization technology on microorganism in raw milk was studied and the influence of some related factors on the sterilization effect was analyzed.According to the single factor experiment of UV irradiation,the longer the UV treatment time,the better the sterilization effect;the thinner the raw milk liquid thickness,the better the sterilization effect;and the closer the UV irradiation distance,the better the sterilization effect.The germicidal parameters were optimized by response surface method,and the germicidal equation was obtained.It was found that the thickness of raw milk is most decisive,followed by irradiation time,and irradiation distance shows minimum impact.Due to the low penetration rate of ultraviolet light in raw milk,its bactericidal effect is poor.In this study,ultraviolet(UV)technology and ultrasonic(US)technology were combined to develop Photon-Phonon coupling cold sterilization equipment.Through mechanical action and reactive oxygen species caused by ultrasonic cavitation,bacterial cell membrane structure and intracellular substances are damaged.Ultrasonic resonance can make raw milk evenly mixed and microorganisms are fully irradiated by UV light.Its bactericidal effect is much better than that of low temperature and UV treatment alone,and the killing rate against total bacteria is up to 99.9%,realizing the effective control of microorganisms in raw milk.(3)The effects of different sterilization technologies on sensory quality,nutrient composition and structural properties of casein and whey protein in raw milk were also studied.The results showed that the color of raw milk still maintained within the acceptable range after being treated by Photon-Phonon coupling sterilization equipment.The results of electronic tongue test showed that the effect of Photon-Phonon coupling cold sterilization on the taste of raw milk was less than that of low temperature and long time thermal treatment.When the electronic nose test was conducted,the Photon-Phonon coupling cold sterilization treatment had less effect on volatile compounds of raw milk,such as aldehydes,ketones,nitrogen oxides,sulfides and benzene aromatic components.The results of milk composition analysis showed that Photon-Phonon coupling cold sterilization treatment had little effect on the nutritional composition of raw milk.The results of enzyme linked immunosorbent assay showed that the active proteins such as immunoglobulin and lactoferrin in raw milk treated with Photon-Phonon coupling cold sterilization were more comprehensive than those treated with low-temperature and long-term thermal treatment.After the particle size and scanning electron microscopy(SEM)analysis,the Photon-Phonon coupling cold sterilization technology can make protein particle size decreases and dispersion.When Infrared and fluorescence analysis were conducted on protein,the hydrophobicity of casein was reduced after the Photon-Phonon coupling cold sterilization treatment,while whey protein showed opposite trend,indicating that sterilization treatment impacted differently on the structure of casein and whey protein. |
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