Chemistry, Biology




SRDJAN JANKOVIC                                                            Institute for Occupational Health

                                                                                                Deligradska 29, Belgrade 11000,




     Enzymes are biochemical catalysts of reactions inside living beings. This simple and inexpensive experiment aims to demonstrate the enzyme activity in an attractive way, while proving two important points:

1.    "Natural" products do not mean "safe" products,

2.    In vivo and in vitro experiment results can differ significantly.

It also offers some ideas for further, similar activities.



        Three water glasses

        A small piece of beef meat

        One unopened bottle of carbonated cola drink

        One fresh pineapple (a small one, about kg will do)

        A food processor (blender)



Fig. 1. Materials.



This activity requires the materials to stand for 24 hours, so it can either be done in class in two consecutive days, or the students can be asked to perform the experiment at home, writing down their observations as instructed. These observations can then be discussed in class.

Take a piece of beef meat and cut three smaller pieces out of it, approximately of the same size. Get three identical glasses (approx. 200 mL each) and rinse with tap water, leaving the inner surface wet. Pour the tap water into one of the glasses, until about full, leave on the table and label it "A".

Take a fresh pineapple, peel it and chop into small pieces, as if it was to be eaten. Put the pieces into a food processor, add a glass of tap water (Fig. 2) and process for about 30 seconds, until the mix achieves a uniform consistency. Pour the juice into one of the glasses, until about full. The blender usually has a crude filter so the juice will be partially transparent (which is OK), but if you prefer a clear liquid, you can squeeze it through a piece of cotton cloth. Put the glass, containing the juice, onto a table and label it "C". Pour the same amount of cola drink into the third glass, and label it "B" (Fig. 3).


Fig. 2. A blender cup with  pineapple chunks and water.

Fig. 3. Glasses with liquids marked A, B and C and the prepared chunks of meat.


Note the meat consistency. Put the meat pieces, one into each of the glasses (Fig. 4), loosely cover the glasses (Fig. 5), record the beginning time of the experiment and let the glasses stand for 24 hours. Avoid direct sunlight and try not to use water that is too cold (about 18-22 C is adequate). The room should be warm as well.



Fig. 4. Meat pieces in glasses.

Fig. 5. Loosely covered glasses.


After 24 hours, note the changes in the liquids: their color, presence of foam or precipitate (Fig. 6). Remove the meat pieces with a spoon and examine their appearance and consistency (Fig.7). The meat chunk in water (A) did not suffer much change in appearance or consistency. The one in the cola drink (B) made the liquid transparent by separation of a colouring agent (Caramel) together with coagulated blood in the foam and precipitate. However, the chunk did not decompose nor disintegrate: its consistency remained the same. The meat chunk in the pineapple juice (C) decomposed into a slimy pulp, due to the action of proteolytic enzymes (bromelain). If left to stand for longer times, esp. with agitation, it would decompose even further. This dispels the popular "myth" about the cola drinks being able to decompose a meat chunk overnight, but also raises the question: is it then safer to drink artificial soft drinks than natural pineapple juice? Is it dangerous to drink pineapple juice?


Fig. 6. Glasses after 24 hours.

Fig. 7. Meat pieces after 24 hours .




      Compared to the plain tap water, serving as a "control" in the experiment, the cola drink does coagulate blood, but does not attack the meat protein structure significantly, because it does not contain enzymes. The proteolytic enzymes (bromelain) in the pineapple juice cause the decomposition of proteins in meat. Bromelain, along with some other natural enzymes, is used in commercial meat tenderizers.

      There is a widespread misconception that a product being derived from natural sources automatically means it is safe. "Natural" does not always mean "safe"; also, "artificial" doesn't always mean "bad." For example, there is a number of plant-derived (naturally occurring) poisons that were historically used around the world to poison arrow heads for the purposes of hunting (e.g. tubocurarine). Also, one of the strongest known toxins for humans is the Botulin toxin, produced in nature by the Clostridium botulinum bacteria.

      This experiment could lead to a false conclusion that it is unsafe to ingest fresh pineapple juice. It, actually, positively impacts one's health. What the results really demonstrate is that the in vivo and in vitro results can differ dramatically, so that in vitro models may not always be adequate to describe a living system. When ingested in normal quantities by a living human, pineapple enzymes are broken down and any superficial damage repaired, thus no ill effects should be expected.

      Ideas for further experiments: concerns have been raised about the possibility that certain soft drinks attack teeth [3] but little experimentation of this kind was done about fruits containing proteolytic enzymes. Part of a tooth is made of a protein-containing substance called dentin. A similar setup can be used to check if a dead tooth can be attacked by bromelain from pineapple juice, papain from papaya etc. Any experiment should be done against a control (a tooth from the same animal, in water) and, if possible, with recording the temperatures during the process. It is a good opportunity to relate simple, educational experimentation to the useful scientific results. Remember to use only fresh (non-cooked) teeth (ask the butcher to save You some) and don't use canned or processed pineapple, because the enzymes get inactivated when treated by heat during production.




2.      J. A. von Fraunhofer, M. M. Rogers. Dissolution of dental enamel in soft drinks. General Dentistry, pp. 308-312, July-August 2004.