Any halfway decent scientist knows why. And after Monday's biology lab, all of the students in Dominican University's Biology class (including myself) do too.
I will explain. But to do so, we must zoom in to what's called the "particulate" world, the world we can't see with our own eyes ...
The failure of the gelatin to set has to do with a type of enzyme (protein) called "gelatinase" found in those tropical fruits. (In case you were wondering: Yes, these fruits do contain protein.) If active, gelatinase breaks down the gelatin protein found in your typical packet of JELL-O, preventing the JELL-O from setting.
How does this work? Understanding this phenomenon requires a bit of biology and a bit of chemistry. (But don't be scared! It's actually really cool how this works.) Proteins, such as the gelatinase found in the tropical fruit mentioned, are organic molecules that have very important roles in the function of living cells (including our own cells). One role that many proteins have is that of an enzyme, or biological catalyst. These catalysts basically facilitate chemical reactions in living cells by reducing the amount of energy required for those reactions to take place. Without these chemical reactions, life would cease. So proteins (as enzymes) are pretty important to us, and to other living things.
Enzymes function as a result of a very specific three-dimensional structure. This structure is determined by the DNA that "spells out" the instructions for making the enzyme. Each enzyme, then, has its own unique structure. Because of this individualized three-dimensional structure, each enzyme usually fits, in the same manner as a lock and key, with only one other molecule (called a "substrate") and catalyzes a reaction there. So each enzyme does one specific job. The enzyme will not function (act as a catalyst) if a different molecule is present. This property is called "enzyme specificity" or "substrate specificity."
However, proteins are very sensitive. If they get too warm, or the solution around them changes in pH (how acidic/basic the solution is), then the protein can lose its three-dimensional structure. And when that happens, the protein can no longer bind with its substrate -- the key doesn't fit into the lock anymore. This is called "denaturing" of a protein.
Now let's return to the "macroscopic" world, the world we can see unaided by microscopes and chemical models. The world of the lovely, green, sweet-and-sour kiwi ...
Like all good scientists, once we learned the principles of how enzymes work, we tested them out. Using (yes) kiwis and (yes) gelatin. Raspberry gelatin, actually. The experiment was simple, yet elegant, explanatory, and enlightening. Here is my data chart. I will explain it below.
|Assay of Gelatinase From Fresh Kiwi Fruit|
|Tube No.||Contents of Tube||Gelatin set?||Active Enzyme present?||Is the tube a control or test sample?|
|1||gelatin + water||Y||negative control|
|2||gelatin + water||Y||negative control|
|3||gelatin + known gelatinase||N||X||positive control|
|4||gelatin + known gelatinase||N||X||positive control|
|5||gelatin + fresh kiwi extract||N||X||test sample|
|6||gelatin + fresh kiwi extract||N||X||test sample|
|7||gelatin + boiled kiwi extract||Y||negative control|
|8||gelatin + boiled kiwi extract||Y||negative control|
|9||agar + fresh kiwi extract||Y||test sample|
|10||agar + fresh kiwi extract||Y||test sample|
We tested 10 tubes of solutions (five different mixtures, each mixture done twice to see if we got the same result). After mixing the solutions, we set them all in an ice bath to find out whether they would set into hard gelatin, or remain as liquids.
Tubes 1 and 2 had only gelatin + water (no gelatinase). These tubes were what is called a "negative control" -- they had a known negative test in an experiment (no enzyme activity). Tubes 3 and 4 had gelatin + a "known gelatinase" -- an enzyme from a pineapple prepared by our lab professor. So as predicted, they did NOT set because there was active enzyme present. These tubes were "positive controls"-- a known positive test (enzyme acting on the gelatin). Tubes 5 and 6 were test samples -- tubes we were interested in the results of. And they behaved as predicted -- the gelatinase from the kiwi extract did interact with the gelatin and prevent the gelatin from setting! Contrast this with tubes 7 and 8, which contained gelatin and boiled kiwi extract. Remember that heat will "denature," or unfold, an enzyme. That's exactly what happened -- boiling the kiwi rendered the gelatinase ineffective, so tubes 7 and 8 did set (more negative controls). Tubes 9 and 10 were filled with agar, another jello-y substance, and fresh kiwi extract. This time, we were testing the "enzyme specificity" concept -- would the kiwi's gelatinase also work on agar, as well as the gelatin? The answer, we found, was "no." The gelatinase is specific to the gelatin, and would not work on a different substrate (agar).
As I said, simple yet elegant, and very hands-on. My kind of learning.