Decomposers | Activity Useful Decomposers

Activity: Useful Decomposers (40 min over 2 days)

Students compare aerobic and anaerobic decomposition and identify products in their homes and daily lives that are direct or indirect products of each kind of decomposition.

Resources Provided

Materials needed

  • Large container for boiling water
  • Tongs
  • 1 pint glass jar with lid per student or pair of students
  • Milk – 1 cup per student or pair of students
  • Saucepan for heating milk
  • Food-safe thermometer
  • 1 quart container of unpasteurized yogurt
  • Incubator set at 37o C or warm, not drafty corner of room


Print one copy per student of Useful Decomposers Worksheet.


1. Students read and discuss text in Section 1 of Useful Decomposers Worksheet.

Have students read and discuss Section 1 of the worksheet. Possible discussion questions include: what have students experienced? How can these microbe-laden products be safe? Which kind of decomposition, aerobic or anaerobic, is more likely to be smelly?


Assessment for this activity is meant to happen through discussion.


Important Safety Note: If students are interested in tasting their yogurt, inspect their results first to make sure it is safe. Stirring the yogurt before trying it is recommended as there will be some liquid that separates from the solids. Students should taste less than a teaspoonful. If the milk is still mostly liquid on the following day, it is not safe to consume and should be completely discarded. 

While successful attempts will produce something that looks like yogurt, it will not taste like the yogurt that most students are accustomed to. Most store-bought yogurts are high in added sugars, and so the yogurt produced in this lab will taste much more sour and not sweet at all as a result. Students should be warned of this in advance or they may reflexively spit out the yogurt when they try it.

The lab can be made to be more inquiry-based by offering students a couple of options for store-bought yogurt to use as their starting culture or store-bought dry yogurt cultures (available in health food stores). Dairy food manufacturers invest a large amount of money, time, and effort into finding bacterial cultures that produce consistent results, and different yogurt brands may be more or less successful. Students may make predictions about what brands or types might be more effective (e.g. name-brand vs. store-brand; regular vs. Greek, etc.).

(On a side note, CRISPR was actually discovered by Danisco yogurt microbiologists, which is a testament to the amount of research that still occurs in dairy foods).

Extending the Learning - Digging Deeper

Other useful microbial decomposers

Students can research other useful microbial decomposers. Some possibilities are:

  • Antibiotic producing molds and bacteria
  • Decomposers that can break down crude oil or plastics to help with oil spills or plastic in landfills
  • Disease causing vs helpful intestinal bacteria
  • Health benefits of probiotics (or lack thereof)