“[Jesus] told them another parable: ‘The kingdom of heaven is like leaven that a woman took and hid in three measures [about 30 kg] of flour, till it was all leavened.’ ”
According to commentators, this woman was making sourdough bread, by adding some sourdough starter (which can be produced from water, flour, and perhaps a few grapes or sultanas):
Such starters contain a mixture of wild yeast, and the kind of bacteria (lactobacillus) also found in yoghurt:
Microscopic view of yeast and bacteria in a one-year-old starter (photo by “Karniecoops” at sourdough.com)
The growth of these microorganisms varies with temperature (and so will the taste of the bread), but at the peak rate, bacterial numbers double every hour.
Bacteria & yeast growth rates, from here (see also this paper). The bacteria causing sourness (red and green) grow best around 33°C (91°F), while the yeast (blue) grows best around 27°C (81°F). Growth rates for bacteria and yeast are the same around 23°C (73°F).
Such exponential growth recalls the story of the man who wanted grains of wheat on a chessboard: 1 on the first square, 2 on the second, then 4, 8, 16, 32, 64, 128, 256, 512, 1024, ... up to 9,223,372,036,854,775,808 on the last square (for a spectacular total of hundreds of billions of tonnes).
The point of the parable is outlined by C.S. Lewis in his wonderful little book Mere Christianity:
“All sorts of people are fond of repeating the Christian statement that ‘God is love.’ But they seem not to notice that the words ‘God is love’ have no real meaning unless God contains at least two Persons. Love is something that one person has for another person... And now, what does it all matter? It matters more than anything else in the world. The whole dance, or drama, or pattern of this three-Personal life is to be played out in each one of us: or (putting it the other way round) each one of us has got to enter that pattern, take his place in that dance. There is no other way to the happiness for which we were made. Good things as well as bad, you know, are caught by a kind of infection... [Christ] came to this world and became a man in order to spread to other men the kind of life He has – by what I call ‘good infection.’ Every Christian is to become a little Christ. The whole purpose of becoming a Christian is simply nothing else. ”
Given exponential growth and time, a little leaven can pass on the good infection to most of the human race – all it takes is regular doubling. And when the world has been leavened, it will come out of the oven as a perfect loaf of bread.