Milk

[pleasehelpme]

Does stay fresh for longer in a tetra pack than in the plastic translucent/transparent ones? i heard that because the tetra packs are opaque they let no light in (obviously) and are so slightly cooler. as a result of the slightly cooler temps and no light micro-organisms and whatnot do not develop as quickly. is this true or just an urban legend?

[LazyGun]

If the cartons are staying for the majority of the time in the fridge, I find it difficult to believe that the miniscule temperature difference as a direct consequence of opacity would make any difference at all.

Again, assuming that the packs are spending the majority of the time in the fridge, the transmission of light would also be a non issue, I would have thought. (Unless the light stays on all the time in the fridge :) )

[theprof]

Can I just clarify what you have in mind here?

When you mention plastic translucent/transparent bottles are you talking about the ones containing conventional chilled pasteurised milk (i.e. fresh milk) or the type of unchilled UHT (i.e long-life) ambient milk sold in bottles with the tetra packs in supermarkets?

What about 500ml and one litre fresh milk cartons?

[pleasehelpme]

thanks for your answers, i feel quite stupid now because i didnt think of the light not being on in the fridge when the fridge door is closed!?! the plastic translucent/transparent ones to which i refer are these ones:
http://www.chevening.kent.sch.uk/images/Milk_C artons.jpg

[Gnisy]

if the packaging lets no light in, it's most probably because the milk is fortified with 'light-sensitive' vitamins like Vitamin B-something ( i forgot, didnt pay attention in class during this lecture ) - sensitive as in it's not stable in light - and hence the packaging that is not translucent. cool or not, means nothing if u keep it in the fridge ( which is what you should do to keep milk consumable). Hope that helped.

[theprof]

The storage temperature and the treatment the milk has undergone are the deciding factors here. There are differences in shelf -life that arise if the milk has been pasteurised rather than being UHT treated.

Tranlucency or opaqueness of the storage containers is of little consequence except when an algal bloom would develop in pasteurised milk that had been left for prolonged periods in sunlight. Aseptically packaged UHT milk would not encounter this problem. Bacterial and fungal growth at room temperature or above is another matter. On occasions, internal temperatures of tetra-packs can be higher than translucent containers when not refigerated.

cont

[theprof]

Because milk provides nutrients, near neutral pH and the high water activity prefered for reproduction of microorganisms, it can serve as a growth medium for a wide varity of microorganisms. These microorganisms are known as psychotrophs.

Psychotrophs are microorganisms that can grow at refrigerated temperatures even though their optimal growth rate is above 20 degrees Celsius. These psychotrophs can grow in pasteurised milk resulting in spoilage due to degradation of carbohydrates, proteins and lipids. Psychotrophs carry out many biochemical reactions at higher temperatures, but reaction rates are slowed at low temperatures. Nevertheless, several weeks at refrigerated temperatures may be necessary for extreme changes to occur.

The presence of psychotrophs in all milk leads to decreased shelf life, development of rancid flavours and odours and gel formation.

cont

[theprof]

Conventional pasteurisation kills most psychotrophs. However, some species such as Arthrobacter, Bacillus, Clostridium, Lactobacillus, Corynebacterium and Microbacterium are resistant to pasteurisation. Nevertheless, refrigeration retards the growth of these endemic bacteria

Incidentally, there are about nine yeast species and eight mould species that are not destroyed by pasteurisation. Once again their growth is retarded by refrigeration.

Cont

[theprof]

UHT milk treatment has never been completely foolproof. Some bacterial spores and bacterial or native enzymes, particularly proteases and others survive UHT treatment

These proteases that have not been deactivated by UHT processing can cause problems in the milk during extended storage leading to similar problems to that encountered with pasteurized milk. Nevertheless, the theoretical extended storage life of UHT milk far outweighs the risks associated with its selection for commercial use. Modified UHT treatments have been tried with some success but there are commercial barriers to overcome.

[Kingaroo]

Pleasehelpme, don't feel bad about the light-in-the-fridge thing. While the milk is in the store, it is in the light the whole time. I've heard that light does degrade some vitamin or other (as Gnisy mentioned).

Light also affects beer, which is why it's usually put in colored bottles.