How is yogurt made?

This exercise is a part of Educator Guide: New Rules for Finding Antibiotics / View Guide

Class time: 30 to 50 minutes during two class periods. (Plus two to three days between class periods for the milk to solidify.)

Purpose: Students can grow safe yogurt bacteria in sterilized milk, observe that the bacteria solidify the milk into new yogurt and test how antibiotics affect the bacteria.


•     Activity Guide for Students: How is Yogurt Made?

•     Small boxes of Parmalat or Lil’ Milk ultra-high-temperature (UHT) pasteurized milk (typically sold in the non-refrigerated baking aisle of the grocery store)

•     Cup(s) of yogurt (plain yogurt is simplest)

•     Tube(s) of triple antibiotic ointment

•     15 ml graduated sterile plastic test tubes

•     Lab markers (preferably alcohol-resistant)

•     Paper towels

•     70% ethyl or isopropyl alcohol in squirt bottles (rubbing alcohol)

•     Non-latex gloves

•     Safety goggles

•     Lab coats or lab aprons

•     Alcohol burners or Bunsen burners

•     Flame-sterilizable inoculation needles or loops (straightened paper clips taped to pencils can also be used as long as the paper clips have no plastic coating)

•     Water bath (optional): Styrofoam cooler, aquarium heater, thermometer, plastic test tube racks, paper clips, nearby sink

Notes to the teacher: Yogurt contains harmless live bacteria (typically Lactobacillus and non-pathogen Streptococcus species). When a drop of these bacteria is added to fresh milk, they can acidify and solidify the milk to make new yogurt.

Triple antibiotic ointment contains three antibiotics (bacitracin, neomycin and polymyxin B). When a small amount of antibiotic ointment is added to fresh milk along with the yogurt bacteria, the antibiotics will kill the bacteria and prevent the milk from solidifying into new yogurt. This lab activity is a good demonstration to show that antibiotics kill bacteria.

This activity is also a good opportunity to discuss and demonstrate the importance of using sterile techniques in science experiments. In this experiment, we want only yogurt bacteria to grow in the milk, not other bacteria from the classroom, students’ hands or elsewhere. To minimize or avoid introducing other bacteria, students should:

  • Wear gloves, sterilize gloves with alcohol by pouring some alcohol on the gloves and rubbing it around the gloves. After sterilizing the gloves with alcohol, students should avoid touching anything with their gloves that is not absolutely essential for the experiment.
  • Use UHT milk, in which all of the bacteria have already been killed by high heat (unlike in regular milk). Because of this, UHT milk can be stored at room temperature (until it is opened and is exposed to new bacteria from the environment).
  • Use sterile test tubes, keep them tightly capped except when adding something to them and be very careful handling the test tube caps.
  • Sterilize an inoculation needle by passing its end through a flame for a few seconds, then let it cool in the air for a few seconds before using it to transfer a drop of yogurt or antibiotic ointment.
  • Use separate inoculation needles for yogurt and antibiotic, and wipe and flame them between uses to avoid cross-contamination.

The yogurt bacteria will solidify a tube of milk overnight at 37° C, or in two to three days at room temperature. If you have a heated water bath, incubator or a warm room (like a boiler room), you can put the sealed tubes there to incubate. If you would like to create a water bath, you can easily make one by filling a large Styrofoam cooler with water, dropping an aquarium heater and a thermometer into it and adjusting the thermostat on the aquarium heater until it heats and maintains the water at around 37° C. Using paper clips like hooks, you can attach plastic test tube racks to the inner walls of the cooler, so that the racks will keep test tubes mostly immersed in the water with just their tops sticking out of the water.

Please make sure students follow all applicable safety rules in working with flames and using inoculation needles. Students should not consume any of the yogurt or milk before, during or after the experiment.

If you have enough time, your students can dilute a drop of yogurt in water, streak it on a microscope slide, Gram’s stain the slide and observe the yogurt bacteria at 400x or 1000x under a microscope. Lactobacillus bacteria look like chains of tiny sausages linked end-to-end, and Streptococcus bacteria look like chains of tiny spheres. Both types of bacteria are gram-positive and should appear purple when Gram’s stained. Students could test for bacteria from a cup of fresh yogurt and/or from their milk tubes after the samples have been incubated.

Materials for observing bacteria under a microscope:

•     Activity Guide for Students: Gram Staining

•     Gram’s stain (such as this Gram’s stain kit from Home Science Tools)

•     Pre-cleaned glass microscope slides

•     Pre-cleaned thin glass cover slips

•     Optional: Canada balsam or microscope slide mounting cement

•     Microscopes with 400x (or 1000x and immersion oil)

•     Squirt bottles with water