Gelatin Catalysis - Jell-O + Pineapple

1 Gelatin Catalysis - Jell-O + Pineapple

The topic for today is a classroom demonstration of catalysis using an enzyme. In particular this involves a gelatin dessert (such as Jell-O^tm) and cysteine protease enzymes (such as the bromelain found in fresh pineapples, the papain found in papayas, or the actinidin found in kiwifruits). For more about these enzymes, see reference 1 and references therein.

On the retail box of gelatin dessert there are instructions to avoid adding certain kinds of fresh fruit. It says if you add such fruit before the gelatin has set, it will never set. For some experimental data on this, see reference 2.

That’s all nice and true, but today we wish to investigate what happens if enzymes are introduced after the gelatin has set.

Here’s the procedure:

Acquire a box of your favorite flavor of powdered gelatin dessert and an assortment of fruits. In my experience pineapple and kiwi work fine. The unripe fruits have a higher concentration of the enzyme of interest.

No later than the day before the demo, mix the gelatin with boiling hot water in accordance with the instructions. Pour it into molds. A cupcake pan will serve nicely if you don’t have anything fancier. A sample size of 50 grams per pit is convenient. More is harmless.

Let it set overnight in the refrigerator.

Pop the samples out of the mold into somewhat larger dishes, so that if/when they melt and run down they have some room to spread out. Dipping the bottom of the mold-pan in a basin of hot water helps the samples release from the mold. Label the dishes.

If the samples happen to have a shallow depression on top, that’s great. If not, you may wish to make a shallow depression using a hot spoon. Try not to break or crack the sample.

Removing a shallow core using a straw also works and serves the same purpose, i.e. creating a pit to retain some fruit juice.

Keep the samples cold or at least cool until it is time for the demo.

In the case of pineapple, take it to the kitchen, peel it and core it, and reserve the nice parts for eating. Take the peelings and cores to the lab for the experiment.

At the start of the demo, early in the class period, take a slice of fruit and crush it to release the juice. A couple tenths of a milliliter is enough. An entire mL is overkill but harmless.

Apply one type of juice to one of the samples. Note the label and the time. Similarly for the other types of juice.

Keep at least one sample unmolested as a control.

After an hour, each enzyme should have digested its sample of gelatin to a noticeable degree (but not completely). In contrast, the control should be more-or-less unchanged.

Variations abound.

For one thing, you could slice or crush some of the gelatin samples, to show that the rate depends on surface area, in particular the amount of area contacted by the juice.

Similarly you could carefully deposit the juice in isolated droplets here and there on the surface of the gelatin, to show that the attack is local. A wider, thinner slab of gelatin might be convenient for this.

You could investigate the dose/response relationship to prove that we are seeing a catalyst not a reactant.

You could investigate how much heating is required to denature the enzyme.

2 Remarks

A – There are two main options:

Set the gelatin in advance, and then digest it during the class period, as recommended here.

The traditional (but not recommended) procedure is to add the catalyst to the liquid gelatin and observe that it doesn’t set (whereas the control does).

This is advantageous in terms of practicality, clarity, convenience, timing, and showmanship.

It’s hard to tell a story about what didn’t happen. Also, it takes a long time for the control to gel and it isn’t very interesting to watch during the process.

B – This demo can be carried out even in situations where “dangerous chemicals” are not allowed. You can explain that it’s just food from the grocery store.

On the other hand, I would not let students eat the fruits. Partly this is because I don’t want to make any exceptions to the rule about not eating anything in or from the lab ... but also because eating unripe fruit is not good for you. Fruits with high levels of cysteine protease in them can cause unpleasant swelling of the tongue and throat.

C – Other examples of catalysis that can be demonstrated using only over-the-counter supplies include:

Decomposition of hydrogen peroxide catalyzed by a slice of potato. Or even by saliva. You get to decide whether to use 3% H₂O₂ (from the grocery store) or 30% H₂O₂ (from the beauty-supply store).
Last but not least: Diet Coke + Mentos, or Diet Coke + rock salt. See reference 3 and references therein.

3 References

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