Watch a cloudy protein mixture change

Neutral protease in nature

Proteases cut peptide bonds. They are involved in digestion, protein turnover and many regulated cellular processes. Microbes can secrete proteases to obtain nutrients from proteins outside the cell. Neutral proteases are selected for useful activity around neutral pH.

What chemical reaction does it catalyse?

Enzymes speed up a reaction without being consumed in the overall process. The preparation, accessible substrate and reaction conditions determine what you can measure.

Industrial applications

Proteases are used in food protein processing, some cleaning formulations, brewing and other applications where controlled protein breakdown changes material properties.

Experiment overview

Use a dilute skimmed-milk suspension as a cloudy protein substrate. Compare light transmission in enzyme and control tubes at the same pH and temperature.

Equipment and materials per working group

• 0.5% w/v skimmed-milk powder suspension freshly prepared
• Neutral protease liquid and compatible buffer near pH 7.0
• Three clear tubes, pipettes and a printed black cross
• 40°C water bath; optional colorimeter
• Timer and eye protection

Plan a class of 30

Use 10 working groups of three. Multiply the per-group quantities in this guide by 10, then allow an appropriate preparation reserve. Keep concentrations unchanged when scaling. Assign a timer, a practical operator and a recorder, then rotate roles. Set up teacher-prepared comparison samples in advance where a reaction takes longer than the lesson.

Teacher and technician preparation

1. Pilot 0.5 mL stock plus 4.5 mL milk suspension. This is a high classroom screening dose, not an established specification. Trial lower doses if background colour obscures the comparison.
2. Check an enzyme-only blank, a milk-only control and a matched no-enzyme diluent. The preparation may affect colour or pH.
3. Use neutral conditions and avoid heating or acidifying milk until it precipitates.

Step-by-step class method

1. Label ENZYME, CONTROL and ENZYME BLANK.
2. Add 4.5 mL milk suspension to ENZYME and CONTROL and 4.5 mL buffer to the blank.
3. Equilibrate all tubes at 40°C for 3 minutes.
4. Add 0.5 mL pilot-approved protease dose to ENZYME and ENZYME BLANK; add 0.5 mL matching diluent to CONTROL.
5. Mix gently, start timing and place tubes against the same black-cross background.
6. Record cloudiness or transmission at 0, 2, 5 and 10 minutes.
7. If no clear difference appears, retain the tubes for the pilot-established longer observation period.
8. Compare to both controls and distinguish clearing from curdling, sedimentation or colour contributed by the stock.

What should students look for?

Protein hydrolysis can reduce milk opacity. A usable visual contrast must be demonstrated with this preparation before the class.

Interpretation and troubleshooting

This is a pilot practical, not a guaranteed 10-minute endpoint. If milk clots or remains opaque, use a validated protease assay suited to the product.

Questions for the class

• What changed in the enzyme-treated sample compared with the control?
• What bond or substrate does this enzyme act on?
• Which conditions did we keep the same, and why?
• Could colour, pH, warming, stirring or background sugars explain part of the result?
• What would you measure or change in a follow-up investigation?