The proteases in cow’s milk have a great influence on the flavor and quality, especially on the shelf life of dairy products, but there is no uniform reference standard for the detection of protease activity in dairy products. This paper elaborates the sources, types and characteristics of protease in cow’s milk, and summarizes recent progress in the detection of the activity of different types of protease in cow’s milk, pointing out that the precise detection of protease activity in milk has great significance for dairy production and quality assurance.

A method for the determination of stevioside, rebaudioside A, rebaudioside B, rebaudioside C, rebaudioside F, dulcoside A, rubusoside and steviolbioside in fermented milk by high performance liquid chromatography (HPLC) was developed and validated. The chromatographic conditions and sample pretreatment conditions were optimized. Samples were ultrasonically extracted with water for 20 min and for another 10 min after addition of 30% acetonitrile in water bath at 45 ℃. Potassium ferrocyanide and zinc acetate were added to precipitate proteins. The chromatographic separation was achieved on a C18 with gradient elution using a mobile phase consisting of sodium phosphate buffer and acetonitrile, and the analytes were detected by an ultraviolet (UV) detector at 210 nm. The results showed that the method was linear in the concentration range of 1–50 μg/mL, the limit of detection (LOD, RS/N = 3) was 3.0 mg/kg, and the limit of quantitation (LOQ, RS/N = 10) was 10.0 mg/kg. The average recoveries of all analytes spiked at 15–200 mg/kg were between 92.0% and 103.1%, with relative standard deviation (RSDs) (n = 6) less than 3.0%. This method was rapid, accurate, stable, sensitive, and suitable for the analysis of the content of steviol glycosides in fermented milk.

Inductively coupled plasma-mass spectrometry (ICP-MS) was used to measure the aluminum content in the quality control sample of milk powder, and an X-control chart was established for runaway judgement and handling during the operation process. The precision and median line of the control chart for new data points obtained one year later were evaluated using Minitab software with test for equal variance and analysis of variance. The standard deviation for the previous 25 data points was 0.186 mg/kg compared to 0.126 mg/kg for the 35 new ones, while the median values of the two groups did not differ from each other. More reliable control limit was calculated based on all the data points, and the new median value was 1.334 mg/kg with a standard deviation of 0.154 mg/kg. The decrease in standard deviation suggested that the precision was improved with smaller data fluctuations. In conclusion, calculation of new and more reliable control limit from more data is useful to find obvious systematic errors, abnormal data and slow changing trend in a timely and effective way and can provide evidence to decide whether data should be retained or removed so as to directly reflect the stability and tendency of the detection.

A spectrophotometric method for the determination of tea polyphenols in milk tea powder was developed by using Folin-phenol reagent. Crucial operating parameters such as extraction reagent, number of extraction cycles, temperature and derivatization reagent were investigated. Extraction using 70% methanol solution as the extraction reagent in a hot water bath at 70 ℃ performed twice and oxidization of the resulting extract with Folin-phenol reagent proved optimal. The detection wavelength was selected as 765 nm, and gallic acid was used as the calibration standard for the quantitation of tea polyphenols. The recoveries for whole milk powder spiked at concentration levels of 850–4 500 mg/kg were in the range of 98.9%–104.9%. Good stability and accuracy were achieved. When milk tea powder samples with different contents of tea polyphenols were detected in six replicates, relative standard deviations (RSDs) of 0.29%–0.84% were obtained, indicating good precision. This method proved to be accurate, easy to operate and repeatable with simple and easily available instruments, and thus suitable for the analysis of tea polyphenols in milk tea powder.

Inductively coupled plasma emission spectrometry (ICP-OES) was used to determine the contents of 12 trace elements (K, Na, Ca, Mg, Fe, Zn, Cu, Mn, P, Sr, Al, and Cr) in premixed feed and crude feed for dairy cows. Samples were pretreated with microwave digestion before being analyzed. The calibration curves of all elements were linear with correlation coefficients above 0.999 8. The spiked recoveries for this method were 90.0%–106.9%, and the coefficients of variance (CV) for six replicate determinations were 0.69%–8.70%. With the advantages of low limits of detection and high sensitivity and accuracy, this method could be useful for the simultaneous determination of multiple trace elements in dairy feeds.