Effect Of Pulsed Magnetic Field On The Microorganisms And Enzymes In Milk

Milk is recognized as an ideal natural food, because the nutrients in milk are complete, and easy to be digested and absorbed. Fresh milk contains various microorganisms, including bacteria, mold, and yeast, and also different kinds of enzymes, such as lactoperoxidase, catalase, lipase, protease and etc. Microbes and enzymes are the main reasons resulting in food corruption. Traditional thermal sterilization methods can kill microorganisms, and inactivate the enzymes in milk, but it is likely to cause loss of nutrients and decline in sensory quality of milk. As a new non-thermal sterilization method, pulsed magnetic field treatment has incomparable advantages than traditional thermal treatment. The effect of pulsed magnetic field on the microbes and enzymes in milk was studied in this paer. Then, the mechanisms of sterilization for microorganisms and inactive to enzymes were also investigated.The main results of this thesis are as follows:1. After treated by pulsed magnetic field with over 8T, the survival rate of total plate count, coliform, mold and yeast decreased significantly with the magnetic field intensity, and pulse number increasing, but existed fluctuations. Different types of microorganisms showed different sensitivity to pulsed magnetic field. The E. coli was the most sensitive to pulsed magnetic field, while the yeast and mold had strong resistance.2. Pulsed magnetic field was remarkablely capable of inactivation to lactoperoxidase, lipase and catalase in milk, and the inactivation rised with increasese of the magnetic field intensity and the pulse number. Pulsed magnetic field had different impacts on various enzymes. Catalase was the most easy to be inactivated, while lactoperoxidase was the most difficult to be inactivated.3. The sensory quality, nutrition, physical and chemical properties of milk before and after pulsed magnetic field treatment were investigated. These results showed that the pulsed magnetic field treatment hardly affected the sensory quality of milk, did not cause the loss of nutrients, and had no adverse effects on physical and chemical properties of milk.4. The microorganisms treated by pulsed magnetic field were observed by scanning electron microscopy and fluorescence microscopy. It was indicated that the impacting location of pulsed magnetic field on the microorganisms was the cell membrane. The death of microbial cell lead by pulsed magnetic field was mainly caused by the destruction of the cell membrane and the loss of intracellular protoplasm.5. With analyzing the Fluorescence spectra and the circular dichroism spectra, it can be concluded that the inactivation of lactoperoxidase was mainly caused by the destruction of secondary structure, while changes of secondary and tertiary structure about other enzymes in milk needs to be further investigated.