BS ISO 13082 pdf download Milk and milk products — Determination of the lipase activity of pregastric lipase preparation
4.1 Tributyrin (glycerintributyrate or glyceryl tributyrate), e.g.Merck No.1.01958.01001) or similar.4.2Sodium caseinate, e.g. Sigma C86541) or similar.
4.3Lecithin,from soya bean, e.g.BDH Prod.298631) or similar.
4.4Liquid paraffin. Use paraffin which is highly liquid (or similar light mineral oil), e.g.MerckNo.7174.100o1), or similar.
4.5Soda lime granules [Carbosorb1)], e.g, BDH no 3311041) or similar.
4.6Sodium hydroxide solution, c(NaOH) 0,025 mol, which can either be purchased or be prepared asfollows.
Using a pipette (5.1), add 25,00 ml of 1 moll sodium hydroxide with an accurately known titre into a container.Dilute with water to 1 0o0 ml.
The 0,025 moll NaOH solution can be kept in a closed container, protected against carbon dioxide in the airby use of a CO2 trap with soda lime (4.5) at room temperature for at least 1 month. lf necessary, seek advicefrom the supplier of the equipment or reagent. Change the soda lime at least once a year.
When changing the sodium hydroxide batch, check the actual stability of the titre by comparing the old andnew titrant,e.g.using a control sample.
For samples with low activity and manual titrations, use a 0,010 mo NaOH instead of a 0,025 mol/l NaOHsolution.As such，the 0,010 moll NaOH solution gives a higher and more useful consumption of titrant.Prepare the 0,010 moll NaOH solution freshly before use (unless the titre has been checked) as it is unstable.lf using the 0,010 mol/l NaOH solution, correct the calculation according to the formulae in 8.1.
4.7 Lecithin solution, with a mass per volume fraction of 10 %. Weigh 10,0 g of lecithin in a suitable bottle.Use magnetic strring to dissolve it in approx.95 ml of liquid paraffin，which may take between 1 day and2 days of mixing. When the lecithin is completely dissolved, make it up to a total volume of 100 ml with theliquid paraffin.
When stored in a refrigerator, the lecithin solution is stable for 1 year.
4.8 Control sample. Include a control sample of known activity in each series of test for lipase samples.Collect the results and use them for the evaluation of the variation of the test.
The control sample can be the last sample analysed or another well-known sample.
When carrying out the method for the very first time, use a control sample obtained from another laboratory orthe first sample analysed being kept as control sample for the next series of analyses. If needed, store thecontrol sample(s) in a freezer.
NOTE lt can be difficult to get a suitable control sample for rennet paste.
Usual laboratory equipment and, in particular, the following.
The laboratory equipment can be substituted by other equipment verified as giving similar results.
5.1 Micropipette or any other pipette, of capacities 1 ml and 10 ml with a repeatability of 0,5 % or higher.
5.2 One-mark volumetric flasks, of required capabilities, ISO 1042  class A.
5.3 Water bath, capable of circulating the water externally and of maintaining a constant temperature in the reaction vessel of 42 °C 0,5 °C.
5.4 Blender, Warren 1) , Ultraturax 1) or any equivalent apparatus.
5.5 pH stated equipment, including the following components:
a) a thermostated reaction vessel capable of stirring effectively, e.g. mechanical or magnetic stirring;
b) a burette for titration;
c) a recorder, printer or computer.
A Metrohm 718 Stat Titrino 1) is suitable for the purpose. A manual titration set-up may also be used but that can reduce the precision of the method.
For control purposes, therefore, mention the equipment used in the test report.
5.6 Stomacher and stomacher bags, for dissolving rennet paste, e.g. standard bags BA 6041 from Seward 1) or equivalent.
A representative sample should have been sent to the laboratory. It should not have been damaged or changed during transport or storage.
Sampling is not part of the method specified in this International Standard. A recommended sampling method is given in ISO 707 IDF 50 .
Test samples may be stored at a temperature of 5 °C or lower for 2 months. In case of a long storage period, store the test samples frozen, e.g. at 18 °C, as that will significantly improve the stability of the lipase powder.