To Clean Or Not To Clean

based on 6 ratings
Author: Maxine Levaren

Did you ever wonder if your ready-to-eat packaged lettuce is really free from bacteria? Does the brand of packaged lettuce really matter? Or if bacteria are present, how can you wash the lettuce to get ride of bacteria? Jane Alejandro really wanted to know those answers.

Check out Jane’s project display in the figure.

Figure: Project display for “To clean or not to clean.”


I believe that washing pre-washed lettuce with a vinegar and water solution will remove more bacteria than washing with water only.

Independent variables

Type of wash used for lettuce

Dependent variables

Amount of bacteria on samples after three-day test


  • Lettuce samples
  • Sterilization techniques

Experimental groups

  • Lettuce washed with plain tap water
  • Lettuce washed with mixture of vinegar and water

Control groups

Lettuce that wasn’t washed


  • 37 nutrient agar plates (which contain a culture to grow bacteria)
  • 40 sterile plates
  • Six packages prewashed lettuce
  • 70 percent ethanol
  • De-ionized water
  • Two sets of tweezers
  • Two 100-ml beakers
  • One 200-ml beaker
  • One Bunsen burner
  • 45 sterile cotton balls
  • One liter of tap water
  • One liter of vinegar (bottled)
  • Three stirring rods


To perform this experiment, I took a piece of lettuce from each package, swiped it with a cotton swab and streaked the swab on the nutrient agar plate. I then placed the agar plates in an incubator set at 37@dg C.

When bacterial growth was evident, I took the same pieces of lettuce and put them through either a tap water or vinegar/water wash. Note that on each day, the workstation, tweezers, and stirring rods are cleaned and sterilized with 70 percent ethanol.

The following is a closer look at the procedures.

Day 1: Incubate bacterial growth:

  1. Mark each lettuce sample as XI, XII, YI, YII, ZI and ZII.
  2. Label one nutrient agar plate the control plate and streak with de-ionized water.
  3. Divide the remaining 18 nutrient agar plates, into six groups of three. Label the first group XI, and assign a number to each plate (for example, XI 1, XI 2, and XI 3). Repeat with remaining groups (XII, YI, and so on).
  4. Dampen the cotton ball with the de-ionized water and streak it twice across the agar on the control plate.
  5. Place a piece of lettuce on each labeled sterile nutrient agar plate.
  6. Incubate nutrient agar plates overnight at 37@dg C.
  7. Refrigerate sterile plate with lettuce piece at 4@dg C.

Day 2: Wash lettuce with tap water or vinegar and water:

  1. Observe, note, illustrate, and photograph the agar plates.
  2. Divide the 18 nutrient agar plates into six groups of three. In each group label one as the control plate, the second PW (plain water) and the third group VW (vinegar/water mix).
  3. Group each sterile plate with its corresponding nutrient agar plate containing a lettuce sample.
  4. Bathe PW samples in tap water using stirring rod.
  5. Bathe VW samples in vinegar-water mix using stirring rod.
  6. Incubate the nutrient agar plates overnight at 37@dg C.

Day 3: Observe and record bacterial growth:

  1. Remove agar plates from incubator.
  2. Observe, note, illustrate, and photograph the agar plates.


All samples washed with tap water grew bacteria, while only one sample washed with vinegar mix grew bacteria.

On day 2, after incubating the bacteria overnight, each plate had bacterial growth except for the control plate. Many distinct colonies formed, with two massive colonies in one of every group. All brands had relatively equal bacterial growth.

On day 3, all the control plates had bacterial growth. Only one of the VW plates had bacterial colonies formed, but all the PW plates had evident bacterial growth.

None of the plates were clear of bacterial growth.


My hypothesis was that re-washing pre-washed lettuce with a vinegar and water solution removes more bacteria than re-washing with water. The results show that this hypothesis is correct.

The significance of my findings is that bacteria does exist in packaged, ready-to-eat lettuce despite its label, “no washing needed.” The best way to get rid of the bacteria is to wash the lettuce with a mixture of vinegar and water.

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