Study On Instruments And Methods For Detecting Proteins In Dairy Products

Dairy products are natural nutritional foods of humans, especially for infant. Thequality of dairy products is related to types and concentrations of proteins in dairyproducts and can affect on health of consumers. In recent years, the dairy productswere adulterated with illegal materials by some manufactures in China for economicbenefit. In this thesis,4kinds of methods and3kinds of instruments for rapiddetermination of proteins in dairy products were developed. The determination andidentification of total protein, true protein, hydrolyzed animal protein and soybeanprotein in dairy products was investigated systematically. The instruments developedin this thesis, including near infrared total protein analyzer for milk, GDYN-200S trueprotein analyzer and GDYN-300S hydrolyzed animal protein analyzer for qualitycontrol of dairy products, the last two instruments have been produced by ChangchunJilin University Little Swan Instruments Co., Ltd and widely used in dairy plants andadministration departments of Chinese government.In the introduction, the composition and characteristics of typical proteins indairy products were summarized. The routine methods and instruments for thedetermination of different type proteins in dairy products were reviewed. The qualityof dairy products in Chinese dairy plants was analyzed. The instruments and methodsfor rapid detection of typical proteins in dairy products were proposed.In chapter2, a new type of portable short-wave near-infrared analyzer for rapiddetermination of total protein in raw milk was developed. The carefully numericalsimulation computation and design of optical and electrical systems were carried out, followed by the research on chemometric methods for outlier sample eliminating, datapretreatment, and construction of multivariable calibration model. When the Kjeldahlmethod was used as reference method, the correlation coefficient androot-mean-square errors of prediction (RMSEP) of the present method were0.9606and0.114, respectively. The halogen lamp, fixed grating and charge-coupled device(CCD), were used in the developed analyzer. The built-in32-bits ARMmicroprocessor and firmware can guarantee the computing capability of the analyzer.The analyzer and chemometric methods developed in this work are rapid, reagent-free,non-destructive, relatively inexpensive, and suitable for the filed or on-linedetermination of total protein in raw milk.In chapter3, a novel determination method for true protein in dairy productsusing organic dyes as molecular probes was investigated to eliminate the interferencefrom non-protein nitrogen in samples. The miniature colorimeter was developed basedon light-emitting diode (LED) and narrow-band interference filters. With the carefulchoosing of devices such as LED light source, glass cell and photoelectric sensor, andthe design optimization of controlling system, the miniature colorimeter developed inthis work was simple and has some advantages such as low production cost, highstability and short operation time(10min). The method and instrument were applied tothe determination of true protein in500samples, including fresh milk, drinkscontaining milk, milk powder, soybean powder, soybean milk and eggs. The relativeerror of protein concentration were less than±3.0%compared with proteinconcentration in powdered milk standard reference substance. The relative deviationof results of true protein in fresh milk and milk powder were less than±1.5%compared with these obtained with similar foreign instruments. The relative deviationof results of protein concentration in samples not containing non-protein nitrogenwere less than±5.0%compared with these obtained by Kjeldahl method.In chapter4, a method for detecting hydrolyzed animal proteins in dairy productsbased on the determination of the amino acid4-hydroxyproline (4-Hyp) wasestablished. An analyzer for detecting hydrolyzed animal proteins was developed. The analyzer was coupled with a special equipment for high-temperature hydrolysis ofdairy samples with25channels. The hydrolytic conditions of hydrolysis includingvolume and concentration of sulfuric acid, amount of catalyst, temperature and timeof hydrolysis were optimized. The results showed that samples can be hydrolyzedcompletely in5mL sulfuric acid (6mol/L) and300mg catalyst for35min (150℃).Compared with the GB standard hydrolysis method, the hydrolytic time wasshortened from16h to35min.Compared with the commonly used spectrophotometerthe analyzer is simpler, lighter and more suitable for the detection in the field.In chapter5, the preliminary study on the method for rapid separation anddetermination of milk proteins in dairy products using the micellar electrokineticcapillary chromatography was carried out. The separation of the milk proteins andsoybean proteins in dairy products was investigated. The effects of several separationconditions, such as concentrations of surfactant and viscosity-controlling agent,voltage, column temperature and time of separation were examined. The optimalconditions are as follows:120mg/L tween-20was used as surfactant,4.8mol/L ureawas used as viscosity-controlling agent, voltage column temperature and time ofseparation were25kV,30℃and30min, respectively. Limits of detection of5milkproteins in dairy product were in the range of2.50-5.61μg/L. The adulteratingsamples with concentration of soybean protein more than3.13mg/g or0.15mg/mLcould be distinguish properly. The recoveries of5milk proteins in3spiked samplesincluding skim milk powder, full-cream milk powder and pure milk are from89.16%to109.99%and the relative standard deviations are less than4%.