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With the improvement of living standard, the requirements on the quality of milk become higher and higher, and then it is urgent to find a rapid and accurate technique to detect the milk quality. With the traditional detection method, only one component in the milk is detected, this always causes various incidents, such as leather milk incidents, reprocessed milk incident, recycled milk incident, et al. Since the liquid milk in the market basically preserves the nutrient ingredients of original milk, and fluorescence spectra have many advantages including rapidness and high sensitivity, fluorescence method is an ideal technique for the milk detection. This paper presents detailed experimental and theoretical studies on the fluorescence spectra of liquid milk.Using AvaSpec-2048 and FLS920 spectrometer, we get different brands milk of various concentrations and measure their absorption spectra, fluorescence spectra, excitation spectra, time-resolved spectra, polarizing fluorescence spectra, and synchronous fluorescence spectra as well. At the same time, the experimental data is analyzed with related software, and the results are explained theoretically. .The results shown as blow:1. Milk has a strong absorption in the range of 200nm-600nm, with the increasing concentration, the absorption peak red-shifts and the spectral range broadens. When the concentration is 0.8%, the saturated absorption phenomenon occurs. The proteins and a variety of vitamins contribute to the absorption spectra between 200nm-300nm, and VB2 mainly contributes to the spectral range longer than 300nm. As the concentration of milk increases, dimer may be formed between the different components, which gives rise to the generation of conjugated chromophore structure, auxochromous group, and absorption peak red-shift.2. Different brands of milk at various concentrations generate fluorescence when the excitation light is between 255nm and 310 nm. Emission spectra ranges from 285nm to 460nm. The intensity of emission spectra is very strong. Different brands of milk with the same concentration have a similar spectral pattern except for different intensity. This provides a rule for us to choose milk as the study object.3. With multi-peaks fitting method, fluorescence spectra of milk are analyzed under different excitation wavelengths. The results show that fluorescence spectra of milk are composed of contributions from many components. The proteins contribute most of the spectrum, and some vitamins in the milk also contribute a little. So, the main components of milk are able to be preliminarily recognized through multi-peaks fitting. This can be the experimental and theoretical foundation for components analysis.4. The wavelength range of synchronous flurescence spectra of milk is 200-450nm, which has high intensity. Especially, more spectral information can be obtained from the milk of concentration of 1%, and the spectral can be fit with 8 curves. The analyzing result show that 279nm and 332nm is generated by lactose, and the peaks of 295nm, 216nm, and 390nm are the result of protein, vitamin E, vitamin B1, respectively. While 349nm is the joint effect of protein and lactose.5. There are many kinds of short lifetime fluorophores in the milk. The lifetime of protein, lactose, vitamin E, vitamin B6, vitamin B1 are roughly 6ns, 1.2ns, 11.9ns, and 5ns, respectively.6. Many peaks appear in polarized fluorescence spectra of milk. We conclude that the peak of 315nm is the partial result of protein fluorescene affected by lactose, and the peak of 330nm is caused by vitamin E and unsaturated fatty acids, peak of 348nm directly is generated by the protein, and the peak of 356nm may also be generated by protein. Vitamin B1 leads to the peak of 415nm, and vitamin A results to peak of 490nm.Therefore, the polarized of fluorescence is able to reflect more milk components, which paves a new way for milk components testing.
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