Internet Version [Naruhodo no Mori] Vol.56

by Yumiko Mori

Vol.56 March 15th, 2001

Increasing Yogurt Bacteria

Research on Bacteria

Home science experiments for mothers of elementary school children

Satoshi: "I didn't know yogurt is made of milk."
Mom: "Look! This yogurt has a picture of cows on the top lid. "
Satoshi: "I saw that."
Mom: "Do you know when we add yogurt to milk, yogurt bacteria will grow?"
Satoshi: "They grow?"
Mom: "If we heat up the milk, yogurt bacteria should grow. I wonder it's really going to work."

Mom and Satoshi watch what happens. They notice the milk is becoming more viscous.

Mom: "Satoshi, look! Yogurt is ready."
Satoshi: "Is this ready?"
Mom: "Yes. But why are you asking?"
Satoshi: "Mom, you said 'yogurt will grow'. I thought it is going to fill this cup. This is not growing at all."

Materials

Milk	500 ml
Skim milk	1/4 cup
Sugar	50g
Yogurt	50g
A container with its lid (heat resistant one)
A Pot
A pair of chop sticks
If available, a thermometer

How to make yogurt

Required time: A half day or a full day

Put milk into a pot and heat up. Before the milk reaches boiling, stop heating.
Add skim milk and cool down the mixture to 40-43 degree Centigrade.
Add yogurt and sugar to the mixture. Put the mixture into a container.
Place the container in hot water (60-70 degree Centigrade) for 3-4 hours until the mixture gets solidified.
If water gets cooler, then heat up the water. Alternatively, place the container near the heater for a half day.
Also you can place in a warmed thermos for 3-4 hours. (Thermos has to washed with hot water for disinfection before use.)
In summer you may leave the mixture in a warm room.
Refrigerate the solidified yogurt in the refrigerator for several hours.
The yogurt get even more solidified.

* Yogurt bacteria are called lactobacillus. Various kinds of lactobacillus are known.
Lactobacillus bacteria breaks lactose into lactic acid.
This lactic acid makes milk proteins to solidify.

* If you add more skim milk, you can get harder yogurt.

* Eat yogurt with fruits or nata de coco, it is delicious!

Research on Bacteria

How much did the number of the bacteria increase in the yogurt? A common human bacteria, Escherichia coli, divides once every 20 minutes in favorable conditions. That means the number of the bacteria grows 2 times in 20 minutes, 4 times in 40 minutes, 8 times in 1 hour, and 4096 times in 4 hours. 1 gram of yogurt contains several billion bacteria cells. So, the number of bacteria will become huge. Microorganisms such as bacteria, fungus, and mold are used in various fields such as food industry, medicine, agriculture, energy industry, and environment improvement. I think microbiological research has bright future along with other biotechnologies.

Author	Yumiko Mori (Zushi, Kanagawa, Japan)
Illustration	Mitsue Ogata
Translator	Akira Ishihara
HTML format	Muneo Kume