9 replies to this topic

[Brenkk]

The legal minimum pasteurisation temperature for Milk is 71.8C with a holding time of 15 seconds.
Can anyone help by suggesting what variables should be considered when determining the Set Point temperature to guarantee the legal minimum. eg. We use PT100 controllers that have a tolerence of +/- 0.5C. But the instrument used to calibrate the controllers also have an accuracy tolerence.

I need to develop a policy of how we arrive at our processing Set Point to guarantee legal pasteurisation.

[cazyncymru]

The legal minimum pasteurisation temperature for Milk is 71.8C with a holding time of 15 seconds.
Can anyone help by suggesting what variables should be considered when determining the Set Point temperature to guarantee the legal minimum. eg. We use PT100 controllers that have a tolerence of +/- 0.5C. But the instrument used to calibrate the controllers also have an accuracy tolerence.

I need to develop a policy of how we arrive at our processing Set Point to guarantee legal pasteurisation.

In the past, we have had a divert set at 73C.

The rational behind this is that if you have a tolerance on your thermometer of 0.5, and the calibrating thermometer is also 0.5 then you could potentially be 1 degree out. if you have your divert set at 72.5, 1 degree out from that would be 71.5 and therefore you won't be pasteurising.


When i audit, i always check divert temps and accuracy of thermometers.

caz x

[Tony-C]

In the past, we have had a divert set at 73C.

The rational behind this is that if you have a tolerance on your thermometer of 0.5, and the calibrating thermometer is also 0.5 then you could potentially be 1 degree out. if you have your divert set at 72.5, 1 degree out from that would be 71.5 and therefore you won't be pasteurising.


When i audit, i always check divert temps and accuracy of thermometers.

caz x

Agreed. Temperature is one of two factors, I am sure that you would check holding time verification as well.

Incidentally although the legal minimum is 15 seconds I believe most people in the UK hold for 25 seconds following research into the survival of Mycobacterium avium subspecies paratuberculosis on a 15 second hold.

Regards,

Tony

Edited by Tony-C, 18 March 2010 - 02:40 AM.

[cazyncymru]

Agreed. Temperature is one of two factors, I am sure that you would check holding time verification as well.

Incidentally although the legal minimum is 15 seconds I believe most people in the UK hold for 25 seconds following research into the survival of Mycobacterium avium subspecies paratuberculosis on a 15 second hold.

Regards,

Tony

Your right Tony, i would ask to see holding times done at least annually.

and again, most pasteurisaton is 25 seconds because of MAP, and is driven by our friends the retailers!

[Tony-C]

Your right Tony, i would ask to see holding times done at least annually.

and again, most pasteurisaton is 25 seconds because of MAP, and is driven by our friends the retailers!

Yes Retailers but love them or loathe them most people can't live without them.

Some of the more modern plants I have commissioned have also had flow meters that monitor the flow rate and divert if it exceeds a predetermined limit in a similar way to temperature monitoring.

Regards,

Tony

[SaRaRa]

Hello there!

I found some info online:

Modelling and predictive control of milk pasteurisation in a plate heat exchanger

Linear and nonlinear model predictive control design for a milk pasteurisation plant

Notes for guidance on HTST milk pasteurisation plant

Estimating the probability of undetected failure of pasteurization process control using fault tree analysis

Maybe they can be useful!


Cheers!
Filip

[Tony-C]

Hello there!

I found some info online:

Modelling and predictive control of milk pasteurisation in a plate heat exchanger

Linear and nonlinear model predictive control design for a milk pasteurisation plant

Notes for guidance on HTST milk pasteurisation plant

Estimating the probability of undetected failure of pasteurization process control using fault tree analysis

Maybe they can be useful!


Cheers!
Filip

There may be some useful stuff in these but I had to laugh at this from The Scottish Office Agriculture, Environment and Fisheries Department:

Thermometers can be:-
a. Mercury-in-glass;
b. Alcohol-in-glass; or
c. Electronic digital.
Some premises prefer not to use mercury in glass thermometers and for safety reasons are replacing them with other types of thermometer.

It's dated 1998 so you would expect it to be reasonably up to date with food safety standards.

Regards,

Tony

Edited by Tony-C, 12 April 2010 - 09:23 AM.

[SaRaRa]

LOL! you are damn right! I just saw that! :D
Well it can be used as an... archive! You know... just for the record! Ugh! hehehe :P

[Brenkk]

Thank you to all for your replies.

I was really trying to get at the "degrees of uncertainty" introduced by the instrumentation used for measurement and for calibration.

Cazyncymru's reply is something along the lines of what I'm trying to establish.

If the measuring instrument has a tolerencer of +/- 0.5C should the calibrating instrument have a higher accuracy?
Are there any other issues to be considered in relation to the calibration of the instruments?
Is the "Degree of uncertanty" simply the sum of the tolerences of the measuring and calibration instrumentation?

I am aware of the Holding Time as the second important criteria but my question is only in relation to calibration of the temperature measurement.

[Charles.C]

Dear Brenkk,

I know nothing about milk (although I read the last 2 Sarara documents, seems now that 71.7 > 71.8degC ) so I won’t pre-empt details from the experts who hv already posted in this thread but assuming the milk setup is similar to other haccp type / CCP heat treatment scenarios, a scientifically rigorous answer to yr question probably involves at least –

1. Understanding the basis of the derivation of the required minimum heating temperature / time, ie the validation. I anticipate that there are safety factors already built into the quoted figures although I suppose this is incidental if a regulatory CCP is involved . It is not unusual to find an “operational limit” further utilised (in addition to the correction factors in previous posts) so as to (hopefully) ensure non- failing the CCP. This approach is, I think, illustrated in the 1998 document system.The possibility of such an adjustment also relates to any incurred negative quality factors of course.

2. An appreciation of the statistical meaning of primary and secondary (eg using a master thermometer) calibration techniques. This involves dealing with unpleasant items like 95% confidence intervals which are typically (but usually only in short form) evident on a professional calibration certificate for a thermometer. The use of (+/-) factors is (or perhaps should be) an abbreviated interpretation of such statistical parameters although I think simple averages are quite popular in practice (preferably of >1 datum ) for the second of 2-stage calculations.( It can sometimes be quite depressing to examine conf.intervals too closely, especially at >95% levels.)

3. It is not unusual IMEX to make any master thermometer a “good” one as per your comment (this should hopefully be reflected in the calibration conf.interval) however it is also quite an achievement to get any calibrated thermocouple maintaining an accuracy of <= +/- 0.5degC IMEX, especially at a temperature of ca.71degC. ( I wonder if some people shortcut and do a primary calibration in-situ in steam at 100degC ?)

Otherwise I thought the practical information already given was fairly impeccable.

Rgds / Charles.C