29 February 2020 | New Scientist | 51JAMES WINSPEARUSUALLY we strive to avoid letting
microbes colonise our food. But
not all of them are bad for us,
and many cuisines preserve food
by encouraging the growth of
non-harmful microbes and
discouraging the bad ones. We call
this fermentation. Kimchi, a staple
food in Korea, is a prime example.
In this case, the microbes we
want to cultivate are principally
lactic acid bacteria (LAB). These are
hardy – able to tolerate acidic, salty
and low-oxygen conditions. They
make lactic acid as a result of their
metabolism and this, along with
the salt used to make kimchi, kills
most other bacteria and allows
LAB to dominate.
As fermentation progresses,
some bacteria flourish and then
are supplanted by others. Lactic
acid levels rise, so the most
acid-tolerant microbes come
to dominate. The most abundant
in a mature ferment is typically
Lactobacillus plantarum. Once this
species has become established,
the kimchi should also be acidic
enough to keep pathogens at bay.
LAB are found in very low
numbers on growing vegetables
under normal conditions and
make up less than 1 per cent
of bacteria in soil or on farms.
When cabbages are grown in
sterile conditions and inoculated
with LAB, the bacteria won’t
persist on them, according to
microbiologist Ben Wolfe at Tufts
University in Massachusetts.
So where do they come from?
Wolfe told Gastropod, a US food
science podcast, that his team are
testing the idea that insects bringthem to cabbages, and they may
reside mainly in insects’ guts.
In whatever way they get there,
the cast of microbial characters
will be different in every jar of
kimchi, depending on the
vegetables, where they were
grown and the kitchen. Along with
lactic acid, the bacteria produce
carbon dioxide, acetic acid and a
range of flavour compounds that
give kimchi its complex, tangy
character. You can make kimchi
with a variety of vegetables, but
cabbage kimchi is the best known.
To make your own, chop a
cabbage into small, evenly sized
pieces and put in a large bowl. For
every 400 grams of cabbage, add a
teaspoon of finely ground salt and
mix well, pressing and squeezingthe cabbage until water leaches
out. Chop a few spring onions,
some cloves of garlic and a piece of
ginger and add these, along with
some Korean chilli flakes, known
as gochugaru, or chilli powder.
Pack the mix tightly into a large
jar. The vegetables should be fully
submerged – if not, add more
water and press down on them.
Leave some space at the top for the
CO2 that will build up, and release
the pressure every couple of days.
Leave the jar somewhere cool.
After a week, have a taste. If you
like the taste, move it to the fridge,
or leave it for a few more days for
the flavour to develop further. It
will keep for a couple of months in
the fridge, but will become more
sour over time. ❚Encourage the growth of a special bacterial ecosystem and you
have a tasty way to preserve vegetables, says Sam WongPuzzles
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Follow him @samwong1Science of cooking Week 9
Make friends with microbes
Science of cooking online
All projects are posted at
newscientist.com/cooking Email: [email protected]What you need
Large jar
Cabbage (any variety will do)
Spring onion
Salt
Garlic
Ginger
Chilli
For next week
White and wholemeal flour
Water
Salt
Glass jar, casserole pot
Next in the series
1 Caramelising onions
2 Making cheese
3 Science of crispiness
4 Tofu and Sichuan pepper
5 Gravlax and curing
6 Tempering chocolate
7 Umami and flavour
8 Perfect pancakes
9 Kimchi and fermentation
10 Sourdough bread
Harness wild yeast
to make bread
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