AOAC Official Method 972
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DA1D9318FB2E4F80AA3D4CA49C058B00 |
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文件大小(MB)： |
0.03 |
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页数： |
4 |
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日期： |
2016-2-28 |
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文本摘录（文本识别可能有误，但文件阅览显示及打印正常，pdf文件可进行文字搜索定位）：
33.2.31,AOAC Official Method 972.16,Fat, Lactose, Protein,and Solids in Milk,Mid-Infrared Spectroscopic Method,First Action 1972,A. Principle,Analysis of milk by IR is based on absorption of IR energy at specific,wave numbers by CH groups in fatty acid chains of fat molecules,(3.48 mm), by carbonyl groups in ester linkages of fat,molecules (5.723 mm), by peptide linkages between amino acids of,protein molecules (6.465 mm), and by OH groups in lactose molecules,(9.610 mm). Total solids (TS) or solids-not-fat (SNF) are computed,by assigning experimentally determined factor to percentage,of all other solid milk components, and by adding this amount to appropriate%,fat, protein, and lactose, or by direct mulitple regression,calculations using instrument signals at combinations of,above-mentioned wavelengths. Latter method has been shown to be,more accurate method of determining milk solids. Analysis by IR is,dependent on calibration against suitable standard method. See,“Definitions of Terms and Explanatory Notes,” for calculation of regression,lines.,The above wave numbers can be generated by means of (1) series of,filters, each absorbing all wave numbers of electromagnetic spectrum,but one, or (2) a Michelson interferometer, each wave number of interest,selected from the full spectrum by mathematical means.,B. Performance Specifications,Number of firms manufacture various model instruments based,upon principle,A. It is imperative that individual instrument utilized,meet following performance specifications, based upon analysis of,8 test samples:,Standard deviation of difference between duplicate instrument,estimates:,Fat, protein, and lactose . . . . . . . . . . . . . . . . . . ￡0.02%,Total solids . . . . . . . . . . . . . . . . . . . . . . . . ￡0.04%,Mean difference between duplicate instrument estimates:,Fat, protein, and lactose . . . . . . . . . . . . . . ￡0.02%,Total solids . . . . . . . . . . . . . . . . . . . . . . . . ￡0.03%,Standard deviation of difference between instrument estimates,and values by reference methods:,Fat [989.05 (see 33.2.26)], protein [991.20 (see 33.2.11)],and lactose [896.01B (see 33.2.21),or 930.28 (see 33.2.22)] . . . . . . . . . . . . . . . . . . ￡0.06%,Total solids [925.23A (see 33.2.09)]. . . . . . . . . . . . ￡0.12%,Mean difference between instrument estimates and values by reference,methods:,Fat, protein, and lactose . . . . . . . . . . . . . . . . . . ￡0.05%,Total solids . . . . . . . . . . . . . . . . . . . . . . . . ￡0.09%,Calculate standard deviation of difference as in 969.16D (see,33.2.29), where SD = algebraic difference either between duplicate,instrument estimates or between instrument estimates and values by,reference methods.,C. Precautions,Differences in fat readings for homogenized and unhomogenized,test portions of same milk should be <0.05% to assure accurate,results at high fat levels. If larger differences occur and,servicing homogenizer does not correct fault, consult manufacturer.,Changes in moisture vapor content within instrument console,will cause changes in optical 0 and shift in calibration level.,Replace desiccant frequently, preferably at end of each day, as,3–4 h are required to restore equilibrium conditions. For best accuracy,calibrate with type of milk to be analyzed (herd, individual,cow, homogenized, unhomogenized, market, etc.). Do not use,mixtures of cream and milk for calibration. Avoid abnormal (low,lactose) milks for calibration. Single pumping of milk through instrument,sample cell should purge 399% of previous test portion.,To test purging efficiency, perform fat determinations on H2O,and single pasteurized, homogenized whole milk in sequence:,H2O, H2O, milk, milk, H2O, etc., until total of 20 determinations,have been obtained. Calculate,Purging efficiency = (S S ),S S,M W,M W 1 2,2 2,100 - ′,-,whereM1 andM2 are first and second values for milk andW2 second,value for H2O.,Instrument must be well maintained and functioning correctly.,Malfunctions that influence calibration can cause large errors.,I. Fat,D. Calibration,Before first calibration, check linearity of output signals. Mix accurately,measured volumes of H2O and homogenized cream to prepare,ca 8 mixtures of known relative fat contents covering required,range. Prepare test portions as in 925.21 (see 33.2.02), and pump,each mixture into instrument twice, using second readings to prepare,plot against relative concentrations. If plot is not linear, adjust as indicated,in operating manual. Repeat measurements and adjustments,until plot is linear.,If analyzing unhomogenized milks, check linearity again with ca,4……
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