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The last word–
PART-TIME CURDLINGOn 3 July last year I opened a 2-litre
plastic bottle of semi-skimmed,
homogenised milk, with a use-by
date of 5 July. I poured out a small
cup of what turned out to be curds:
a sniff confirmed the milk was off.
I guessed that it had been
inadequately refrigerated but
because I was making pancakes I
continued to pour it out, suspecting
the sour taste would not be noticed.
To my surprise, the remainder was
fine and still was a day later: the
curdling was restricted to the top
4 centimetres of the milk, where
the bottle was fairly narrow. How can
this have happened?
● Pasteurisation, and, to a degree, homogenisation which evens out the populations of bacteria, reduce the number of bacteria in milk sufficiently to prevent souring for impressive periods, if it is kept cool. Later, it takes very few bacteria to start spoilage, which makes it critically important to keep the equipment sterile between treatment and bottling. That is where most slip-ups occur, not in the pasteurisation itself.
In your case the souring could have resulted from a microscopic amount of, say, Lactobacillus or Streptococcus inside the cap or neck of the bottle.
Possibly the bottle had been allowed to warm up a little in the shop or when it was being
transported. If the bottle was not severely shaken or tipped at any point, it would take a long time for the culture to spread through the bottle, because the species mentioned above do not swim and curdling solidifies the liquid, inhibiting convection currents in the milk.
Also, bacteria consume some of the dissolved solids, such as milk sugar, so the whey would be less dense than the milk and float above it, helping to keep the culture up in the neck of the bottle.Jon Richfield
Somerset West, South Africa
SHAPING THE MOULDBelow is a photograph of a pear that
started to go bad in my fruit basket.
I discovered it one evening with a
perfect bullseye pattern of mould.
Sixty hours later it had grown more
(partial) rings of mould. Another 48
hours later it had grown still more
partial rings, always separated by the
same gap and all still roughly
concentric. At that point it was
getting pretty rotten, so I threw it
away. What causes the mould to
grow in rings like this?
● The pear shown in the photograph is suffering from brown rot disease which is caused by the pathogenic fungus Monilinia fructigena. This is a very common and widespread disease of apples, pears and stone fruits and spreads through the air as spores. The spores germinate on areas of damaged fruit, attacking it where the fungus has easy access to the unprotected, nutrient-rich fleshy parts inside.
The fungal threads, or hyphae, grow and branch within the tissue and degrade the flesh. At first, the disease is invisible to the naked eye, but as it spreads, the pear responds with the typical “browning” reaction seen in the photograph which gives the disease its name.
As it grows, daylight prompts the fungus to produce more spores
on specialised hyphae which grow back out of the skin, forming the grey-brown pustules you can see in the photograph.
A new crop of fungal spores is therefore produced with each period of daylight, and the fungus continues to grow through the flesh forming successively larger rings each day, giving this typical appearance.
A parallel situation can be seen in the “fairy rings” of dense, green grass growth and toadstools that appear in lawns – again it is a visible manifestation of a microscopic fungus growing beneath the surface.
In this case, however, it is the fungus breaking down organic matter in the soil which causes the release of nutrients to stimulate grass growth and provide the essential energy to form the spore structures of the fairy ring toadstool.Peter Jeffries
Faculty of Science, Technology and
Medical Studies
University of Kent, Canterbury, UK
THIS WEEK’S QUESTIONSSpirit in the sky
How do manufacturers calibrate spirit levels? I have two spirit levels and one is way out. Can I recalibrate it?Dave Gellard
London, UK
Eye liner
Make a pinhole in a piece of cardboard. Bring your eye close to it and look through the pinhole as you rotate the card. You will see the network of your retinal capillaries against the background of a cloudy sky. How does this happen? Doohan Cho
Seoul, South Korea
No swimming
Everyone I know was told as a small child not to swim within an hour of eating. Why is this?Louis Counter
Croydon, Surrey, UK
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“It takes a long time for spoilage
to pass through the milk because
the bacteria don’t swim”
“New spores are produced with
each period of daylight leading
to the appearance of rings”
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