Blanching - what is the most effective method?

This experiment definitely counts as one of your five fruit and vegetables a day. 

This experiment should take one to two hours.

Equipment

• Brussels sprouts, 1 kg
• Hydrogen peroxide solution (No significant hazard at this concentration), 0.5% v/v 
• Guaiacol solution (Solution at this concentration is a low hazard), 1% 
• Access to deionised water
• Teat pipettes
• Large beakers
• Bunsen burner
• Sieve
• Knife
• Access to balance
• Items from junk list to make grading device
• Safety glasses

Health, safety and technical notes

• Read our standard health and safety guidance here. 
• Wear eye protection.
• This is an open-ended problem-solving activity, so the guidance given here is necessarily incomplete.
• Remind students of the risk associated with food in a lab. 
• Chemicals used, at these strengths, pose no significant hazard.

Commentary 

This problem mirrors the process that is used in the food industry. Students should realise that the first task is to sort the sprouts into grades, three are sufficient. During trialling, two square holes were used for the grading: 18 mm and 22 mm.

Sprouts passing through the small hole were graded as small, while those that did not pass through the 22 mm hole were large; the remainder were medium. The sprouts should be blanched in boiling water, and it should be kept boiling during the blanching. Trialling showed that approximately 5 minutes is required, although the time depends on size, and the same water can be used repeatedly for the blanching.

However, it is important that a fresh water cooling solution is used each time. Cooling should take around 5 minutes. Adding 3 drops of each of the hydrogen peroxide and guaiacol solution to cut sprouts should give an immediate result. If a brown colour appears within 10 s then this is a positive test for peroxidase activity. If colour appears only after 10 s or does not appear, then this is a negative result.

During trialling, it was noted that a rapid colour change indicated severe under blanching, a slower colour change may indicate that blanching is about right, while a lack of any colour change after several minutes may indicate over blanching. Students who have difficulty with the test should try it out on un-blanched sprouts first.

Extension

This problem has been set as a competition. Students were asked to estimate the relative cost of blanching each fraction size; to plot blanching time against size; and to suggest what other vegetables could be used.

Broccoli and carrots can be used as alternative vegetables, and these do not need grading. They must, however, be cut up into uniform sized pieces. Mange-tout (or sugar-snap peas) can also be used, but need to be ground together with calcium carbonate and test for an alternative enzyme, catalase. 

Notes

This resource is part of a collection of problem-solving activities, designed to engage learners in small group work. Find out how to use these resources, and obtain a list of suggested ‘junk items’ here.