No. 87/2 e Application Bulletin Of interest to: Foodstuffs, Dairy products A E 1, 7 Analysis of dairy products Summary This Bulletin describes potentiometric titration methods for the determination of the acidity in milk and yoghurt, as well as the chloride content in milk, butter and cheese. The determination of the oxidation stability of butter is also described. See Application Bulletin No. 86 for the determination of the pH value in dairy prod- ucts and Application Bulletin No. 235 for the determination of calcium and magne- sium. Apparatus and accessories • Titrino or Titrando with Dosino or Dosimat • Magnetic swing-out stirrer • Exchange unit(s) (ev. for NaOH with PP reagent bottle 6.1608.040) • Porotrode (comb. pH glass electrode) 6.0235.100 • Ag Titrode 6.0430.100 • Electrode cable 6.2104.020 • Rancimat (for the determination of the oxidation stability of butter) Reagents These are given for the individual determination methods. 1. Determination of acidity (milk, yoghurt) General Titrate milk and yoghurt to a pH value of 8.3 with NaOH. The results are given as acid number (mL NaOH, c = 0.1 mol/L for 1 L milk), respectively as acid number according to Soxhlet-Henkel - oSH (mL NaOH, c = 0.25 mol/L for 100 mL milk). It is important to titrate quickly. Application Bulletin No. 87/2 e Analysis of dairy products Page 2 Reagents • c(NaOH) = 0.1 mol/L • Demineralised or dist. H2O • Buffer solutions pH = 4.0 and pH = 7.0 (Metrohm 6.2307.100 and 6.2307.110) • Porolyte (electrolyte for Porotrode) 6.2318.000 Analysis First calibrate the Porotrode with the buffer solutions pH = 4.0 and pH = 7.0 on the titrator. Titrate 50 mL milk (or 10 g yoghurt with 40 mL dist. H2O) to pH = 8.3 with c(NaOH) = 0.1 mol/L against the Porotrode and using following parameters on the Titrator: Determination of the acidity of milk 'pa 'fr 736 GP Titrino 04268 736.0011 736 GP Titrino 04268 736.0011 date 99-11-02 time 09:52 7 date 99-11-02 time 09:52 7 SET pH Acidity pHc(init) 6.74 SET pH Acidity parameters smpl size 50 ml id#1 milk >SET1 EP1 7.712 ml 8.30 EP at pH 8.30 SZ 154 dynamics 1 'SH 6.2 max.rate 10 ml/min ============ min.rate 25 ul/min stop crit: drift stop drift 20 ul/min >SET2 EP at pH OFF >titration parameters titr.direction: auto pause 1 0 s start V: OFF pause 2 0 s extr.time 0 s dos.element: internal D0 meas.input: 1 temperature 25.0 °C >stop conditions stop V: abs. stop V 99.99 ml filling rate max. ml/min >statistics status: OFF >preselections conditioning: OFF req.ident: OFF req.smpl size: OFF activate pulse: OFF ============ Fig. 1 Parameters and results report on the Titrino. Determination of the acidity in milk. Application Bulletin No. 87/2 e Analysis of dairy products Page 3 Calculations RS1; Acid number (AN) = EP1 * C01 / C00 RS2; Acidity (oSH) = RS1 / C02 The acid number is reported without decimal place and the acidity to 1 decimal. C00 = sample size in mL C01 = 1000 (for 1 L) C02 = 25 (conversion factor) Remarks • The determined acid number in milk is given mainly through the absorption of hydroxyl ions via milk proteins and milk salts at a pH value of 8.3. The acid number increases with bacterial acidification and with enzymatic lipolysis. • The following values are characteristic in assessing milk samples: Acid number Acidity Normal, fresh milk 160 - 190 6.5 - 7.5 Milk from cows with diseased udders < 150 < 6.0 Sour or rancid milk > 200 > 8.0 Literature • Schweizerisches Lebensmittelbuch, Kapitel 1 „Milch“ (milk). 2. Determination of chloride / kitchen salt (milk, butter, cheese) General Since the presence of albuminoids interferes with this determination, they must first be removed using a precipitation procedure. Reagents • w(aluminium sulphate) = 20 % 200 g/L Al2(SO4)3 * 18 H2O • c(NaOH) = 2 mol/L • c(HNO3) = 2 mol/L • c(AgNO3) = 0.1 mol/L (standardise by titrating against pure NaCl) Sample preparation A) Milk Mix 20 mL milk with 150 mL dist. H2O and 7 mL NaOH in a 200 mL graduated flask. Now add 10 mL aluminium sulphate, fill up to the mark with dist. H2O and shake thoroughly. Allow to settle and filter off the clear blue solution through a chloride-free filter paper. (If the solution is not clear, the NaOH concentration is too high). Application Bulletin No. 87/2 e Analysis of dairy products Page 4 B) Butter Weigh exactly 1 ... 2 g butter in an Erlenmeyer flask, add 7 mL NaOH and 20 mL hot dist. H2O. Transfer quantitatively the melted mass with approx. 100 mL hot dist. H2O in a 200 mL graduated flask. Allow to cool. Add 10 mL aluminium sulphate, fill up to the mark with dist. H2O and shake thoroughly. Allow to settle for 30 min, then filter through a chloride-free paper filter. C) Cheese Weigh exactly 1 ... 2 g cheese (broken into pieces) in a 200 mL graduated flask, add 7 mL NaOH and 120 mL hot dist. H2O and shake thoroughly to dissolve. Allow to cool. Add 20 mL aluminium sulphate, fill up to the mark with dist. H2O and shake thoroughly. Allow to settle, then filter the upper liquid through a chloride free paper filter. Analysis Place 100 mL filtrate into a beaker and add 2.5 mL HNO3. Titrate with c(AgNO3) = 0.1 mol/L against the Ag Titrode. Determination of chloride in milk 'fm 'pa 736 GP Titrino 04268 736.0011 736 GP Titrino 04268 736.0011 date 99-11-02 time 14:39 11 date 99-11-02 time 14:39 11 DET U Cl- Milk DET U Cl- Milk >calculations parameters Chloride=EP1*C01*C02*C03/C00;0;mg/dl >titration parameters C00= 20 meas.pt.density 4 C01= 3.5453 min.incr. 10.0 ul C02= 100 titr.rate max. ml/min C03= 2 signal drift 30 mV/min ============ equilibr.time 32 s 'fr start V: OFF 736 GP Titrino 04268 736.0011 pause 0 s date 99-11-02 time 14:39 11 dos.element: internal D0 U(init) 131 mV DET U Cl- Milk meas.input: 1 smpl size 20 ml temperature 25.0 °C EP1 2.784 ml 11 mV >stop conditions Chloride 99 mg/dl stop V: abs. stop V reached stop V 6 ml ============ stop U OFF mV 'cu stop EP 9 736 GP Titrino 04268 736.0011 filling rate max. ml/min date 99-11-02 time 14:39 11 >statistics start V 0.000 ml DET U Cl- Milk status: OFF 2.0 ml/div dU=50.0 mV/div >evaluation EPC 5 EP recognition: greatest fix EP1 at U OFF mV pK/HNP: OFF >preselections req.ident: OFF req.smpl size: OFF activate pulse: OFF ============ ============ Fig. 2 Parameters and results report on the Titrino. Determination of the chloride content in whole milk. Application Bulletin No. 87/2 e Analysis of dairy products Page 5 Calculations 1 mL c(AgNO3) = 0.1 mol/L = 3.5453 mg Cl- resp. 5.8443 mg NaCl The results are given in mg chloride / 100 mL without decimal place in milk and in % NaCl in butter and cheese. Milk mg/100 mL chloride = EP1 * C01 * C02 * C03 / C00 C00 = sample size in mL C01 = 3.5453 C02 = 100 (for 100 mL) C03 = 2 (half of the initial sample is titrated) Butter/cheese % NaCl = EP1 * C01 * C02 * C03 / C00 C00 = sample size in g C01 = 5.8443 C02 = 0.1 (for %) C03 = 2 (half of the initial sample is titrated) Remarks • Contrary to the original specification, AgNO3 is used instead of the toxic Hg(NO3)2. • The chloride content in milk lies between 90 and 110 mg / 100 mL. • The NaCl content in unsalted butter lies under 0.1 %, in salted butter between 0.5 % and 2 %. • The NaCl content in cheese can vary within wide limits. 3. Oxidation stability (butter) General Susceptibility to rancidness can be tested with the oxidation stability. Certain in- formation concerning storage stability or deterioration during storage is also possi- ble. The analysis regulation corresponds with the following generally acknowl- edged methods: AOCS Cd 12b-92 (1992) Sampling and analysis of commercial fats and oils. Oil Stability Index (OSI) Analysis First test the temperature with the GLP Test Set according to the instructions for use (Rancimat) and enter the temperature difference on the instrument. Weigh out 3 g sample into the reaction vessel (melting beforehand if necessary) taking care that none of the sample touches the sides of the vessel. Application Bulletin No. 87/2 e Analysis of dairy products Page 6 Fill each absorption vessel with 60 mL dist. H2O. The induction times are usually determined by 120 oC and 20 L/h air throughput. (See instructions for use of the Rancimat). 25 11.30 20 15 µS/cm 10 5 0 0 1 2 3 4 5 6 7 8 9 10 11 12 h Determination Tempera- Induction time Block Channel butter 04.01.2000 120°C 11.30 h B 4 Fig. 3 Report Rancimat 743 for cooking butter Remarks • The induction times are reduced approx. to the half through a temperature in- crease of 10 oC,. • Pay heed that the reaction vessels are meticulously clean! Dirt particles (also e.g. residue from a washing agent) catalyse the reaction.