AOAC Methods in Codex STAN 234 (Preliminary Methods Review)

858

BARBANO ET AL.: J. ASSOC. OFF. ANAL CHEM. (VOL. 73. NO. 6. 1990) Table e. Linear regre11lon of Ir, RSD,. •R, RS~, r value, and R value v• percent protein (all regrealona comprlMd 9 telt valuH with 7 degrea1 of freedom) for eecond trlal In collaboratlve 11udy• Corr. Equipment Statistic Mean Slope lnte,cept eoeff.• % ,-value

2.670' 2.195 2.023

50.45 40.77 36.89 21.21 50.45 36.89

-0.0474 -1.0259 - 0.0338 -0.5261 -0.1341 -0.0958 -0.0675 -1.5849 -0.0270 -0.3130 -0.1909 -0.0780 -0.0225 -0.3920 -0.0484 -0.9430 -0.0620 -0.1350

0.0171 0.3924 0.0144 0.2841 0.0483 0.0407 0.0229 0.5519 0.0126 0.2119 0.0647 Q.0356 0,0092 0.1929 0.0184 0.4012 0.0258 0.0519

s,

0.011 0.307 0.015 0.438 0.030 0.042 3.396 0.010 0.289 0.015 0.440 0.028 0.042 3.394 0.009 0,263 0.014 0.420 0.025 0.043

All

RSD,. o/o

Sf!

1.373

RSo,,_, % r va.lue R value % protein

2.670" 2.023

1.896 1.589 0.846 0.435 1.896 0.846 1.286 0.894 2.222 1.674 1.286 2.222

33.93 26.52

Block

9,-

RSD,, %

9.26 3.57

SR

RS011, % rvalue A value % protein

33.93

9.26

19.10 10.24 41.36 28.56 19.10 41.36

s,

Tradltional

RSO,, %

!lfl

RSo,,_, %

rvalue R value

3.397

% protein

• Significant at ,x = 0.05. a Values for all and for tradltloMI exclude values for laboratory F materlal 5 because it severely distorted regression results for the traditional equipment. thus, statistical values are slightly different from those In Table S.

cent protein vs each AOAC statistical parameter for each type of equipment were run. There was no significant innu– ence of protein concentration on AOAC statistical parame· ters for use of traditional equipment in trial I or trial 2. Thus, there is no j ustification for an extra 0.5 h boiling time after clearing (J h was just as acceptable as 1.5 h boiling after clearing to obtain complete release of the bound nitrogen) for the traditional Kjeldahl equipment in the laboratories that participated in this study. Unlike trial 1, there was no signifi cant influence of protein concentration on any of the bloc~ digestor AOAC statistical parameters in trial 2. This indi– cates that some laboratories using block digestors may need to boil samples for up to 1.5 h after clearing to obtain L complete release of nitrogen from high protein samples However, not all laboratories using block digestors could successfully conduct a l.5 h boiling after clearing; when a sample crystallized during the cooling time after removal from the digestion heat (as in laboratory G), that sample had lower test results than its blind duplicate pair (iodieatin~ excessive acid and nitrogen loss during digestion). Each lab– oratory needs lo determine an appropriate 'boil time afkr clearing of l to l .5 hon tho basis of their test results. Summary and Conclwlons The preliminary research and the subsequent collabotafot study has resulted in the development of a detailed methro.t for the determination of the total nitrogen co:ntent of milk Iii the Kjeldahl method using a copper catalyst. The perfor mance of the traditional Kjeldahl equipment and block dige tor/steam distillation equipment was compared. The trau, tional equipment and the block digestor equipment g;ive similar performance with respect to repeatability and repr~– ducibility when the pruuedures wen: conducted as 11pecilied

loss of nitrogen). The 2 samples that crystallized were the Jower test values of the blind duplicate pairs as was expected from previous nitrogen loss studies. All data from laboratory K were invalid because this laboratory did not follow the procedure (a class A buret was not used for the study as mentioned in the discussion of trial 1 results) . Laboratory I could not participate in trial 2 because their equipment was not working during the week of the study. There was 1 statistical outlier. Laboratory D material 8 was an outlier by the Cochran test (very large difference between duplicates compared to performance of the other laboratories on that sample). The test value for laboratory F material 5 was deter.mined not to be a. statistical outlier (Cochran test was 0.696 and critical value for discard was 0.754). The- me.an test results are reported in Table 4 by material. Blank values from laboratory to laboratory ranged from 0.035 to 0.22 mL, which is similar to trial I. The AOAC statistical parameters for each sample materi– al and mean AOAC statistical values for the second trial are shown in Table 5. Method repeatability and reproducibility were very good, as in the first trial with the shorter boiling time after clearing. Mean statistical performance for the second trial was slightly better than for the first trial (Table 2). When the AOAC statistical parameters from Table 5 were evaluated as a function of protein concentration, there was a significant (P < 0.05) influence of protein concentra– tion on s, and r values (Table 6). No other statistical parame· tcr was significantly influenced. by protein conceutration. Next, statistical analysis was done separately for tradition– al equipment and block equipment to compare their pcrfor. mance in the second trial (Table 6). Performance of the 2 different types of equipment were similar when used with the longer boiling time after clearing. Linear regressions of per-

Codex Trial Method Review