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Precise Pasteurisation

Scientific discovery and discussion
stewamax
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Precise Pasteurisation

#147648

Postby stewamax » June 24th, 2018, 10:22 am

For the past two years or so I have found that milk - usually Tesco skimmed - will 'go off' on its use-by date almost exactly, give or take only one day.
It smells and tastes OK, but my test is simply whether it curdles when added to my morning cup of tea. I am guessing that the milk has become slightly acidic, and that the hot tea (loose-leaf Darjeeling/Assam mix*) is also slightly acidic and the combination is enough to create curds and whey.

My surprise is simply that, unlike most other grocery products, the use-by date for milk is so precise!

* - I originally selected this particular mix because, unlike cheap tea-bag teas, it was tangy and refreshing, so perhaps it is more acidic than most.

XFool
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Re: Precise Pasteurisation

#147686

Postby XFool » June 24th, 2018, 1:11 pm

stewamax wrote:For the past two years or so I have found that milk - usually Tesco skimmed - will 'go off' on its use-by date almost exactly, give or take only one day.
It smells and tastes OK, but my test is simply whether it curdles when added to my morning cup of tea. I am guessing that the milk has become slightly acidic, and that the hot tea (loose-leaf Darjeeling/Assam mix*) is also slightly acidic and the combination is enough to create curds and whey.

My surprise is simply that, unlike most other grocery products, the use-by date for milk is so precise!

This may be nothing to do with Tesco's milk, and I don't use skimmed milk. It seems to tie in with my experience, even milk kept in the fridge. Though I haven't kept an eye on precise dates.

tjh290633
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Re: Precise Pasteurisation

#147711

Postby tjh290633 » June 24th, 2018, 3:44 pm

I suspect that is more down to your tea drinking habit. It doesn't often happen that we have Tesco skimmed milk kept longer than the use by date, but for putting on cereals or putting in tea or coffee, a few more days seems to be fine.

Do you pre-lactate? A favourite expression of a friend of mine, sadlyno longer with us.

TJH

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Re: Precise Pasteurisation

#147741

Postby XFool » June 24th, 2018, 7:04 pm

tjh290633 wrote:Do you pre-lactate? A favourite expression of a friend of mine, sadlyno longer with us.

Could be a lesson there? ;)

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Re: Precise Pasteurisation

#147755

Postby XFool » June 24th, 2018, 7:59 pm

...Further thoughts.

First my own: Milk seems to go off quite suddenly. Milk is a liquid, so whatever makes it go off is likely evenly distributed throughout it already and if it goes off in one place it likely goes off everywhere else at the same time. This is unlike a solid food where a bad patch might occur and then have to spread over time to somewhere else. This is all the more likely if microbes that effect milk are already inherent in the milk.

From the Internet, these snippets:

"Milk spoils when bacteria converts the lactose into glucose and galactose, producing lactic acid."

"But lactic acid bacteria occur naturally on plants like grasses and can easily end up in milk. They ferment lactose at room temperature and turn it into lactic acid."

These suggest to me that a bacteria inherent in milk converts the lactose to produce lactic acid. Rather like fermentation this chemical reaction will proceed at a set speed, presumably affected by temperature. So yes, one can see how any quantity of milk could all go off after a set period.

tjh290633
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Re: Precise Pasteurisation

#147830

Postby tjh290633 » June 25th, 2018, 9:24 am

My observations suggest that the sign, of the milk having gone off, is when it curdles as tea is poured in. Then you have small particles suspended in the liquid. Up to this point, no obvious change in either smell or taste.

Back in the days before refrigeration, and our milk came unpasteurised straight from a churn, my mother would "scald" the milk, by putting the jug in a pan of boiling water, if there was a risk of it going off in hot and sultry weather. Jim Burborough delivered it on his bike and ladled it from the churn into the jug, always adding "a drop for the cat", as was the tradition.

TJH

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Re: Precise Pasteurisation

#147847

Postby Watis » June 25th, 2018, 10:32 am

I regularly buy Tesco skimmed milk and find it often lasts several days beyond an already generous 'life' - the expiry date can sometimes be a week beyond the date of purchase.

I've noticed that other stuff in the fridge also stays fresh for days after it might be expected to have gone off. Although I wouldn't take a chance on meat based products or those where getting it wrong might lead to a bout of food poisoning.

The only variable under my control is the temperature setting, so my suggestion is that you turn the thermostat in your fridge up (or down) - whichever lowers the temperature - and see if that improves things.

HTH,

Watis

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Re: Precise Pasteurisation

#147853

Postby XFool » June 25th, 2018, 11:15 am

tjh290633 wrote:Back in the days before refrigeration, and our milk came unpasteurised straight from a churn, my mother would "scald" the milk, by putting the jug in a pan of boiling water, if there was a risk of it going off in hot and sultry weather.

In a word: pasturisation

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Re: Precise Pasteurisation

#149706

Postby Hardgrafter » July 3rd, 2018, 2:17 pm

You could try filtered milk. Its often sold at a small premium to ordinary, unfiltered, pasteurised.

Filtered milk goes through an extra, fine filtration process to remove souring bacteria, and is packed in opaque bottles, and can extend shelf life up to 45 days. According to The Dairy Council, no nutrients are lost in the process.

dspp
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Re: Precise Pasteurisation

#150557

Postby dspp » July 6th, 2018, 3:41 pm

stewamax wrote:For the past two years or so I have found that milk - usually Tesco skimmed - will 'go off' on its use-by date almost exactly, give or take only one day.
It smells and tastes OK, but my test is simply whether it curdles when added to my morning cup of tea. I am guessing that the milk has become slightly acidic, and that the hot tea (loose-leaf Darjeeling/Assam mix*) is also slightly acidic and the combination is enough to create curds and whey.

My surprise is simply that, unlike most other grocery products, the use-by date for milk is so precise!

* - I originally selected this particular mix because, unlike cheap tea-bag teas, it was tangy and refreshing, so perhaps it is more acidic than most.


For me the curdle in the tea moment, is also my end of use moment.

I have just checked my Tesco blue top full fat, and it is now 10 days beyond its use by date, and still not curdled. This is not unusual for me.

My observation is that there is a very strong correlation between the number of times the container is opened, and how long it is left open, and in what environment, and the curdle date.

In my household where only I drink blue top, and I am travelling for many days of most weeks, the milk sits quite happily in the fridge and is very seldom opened. When opened it is for tea, and goes straight back in the fridge. Result = long useful life.

In my GF's household where many people all dip in and out of the blue top, and most of them leave it with lid off and sitting around for 30 or 40 minutes whilst they eat cornflakes or whatever, then maybe clear up, result = it curdles much faster.

So I don't think it is as precise as you suggest, just that there are different use-cases out there and you of course are only one use case.

regards, dspp


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