Why Is an Apple Not Always Sweet?
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The following article was originally published in the journal for educators Biologia w Szkole (eng. Biology in School) (6/2016):
The Transformation of Sugars in Fruit of the Apple Tree
A botanist would define a fruit as an organ that develops from the ovary of the pistil in angiosperms. It contains seeds, serves a protective function for them, and facilitates their dispersal [1]. These fruits, called true fruits, are not the only type of fruit because in many plants, similar structures arise from other parts of the plant than just the pistil. We then call them false, pseudo, or accessory fruits, and they are common in the rose family Rosaceae, such as in strawberries Fragaria, raspberries Rubus, and also in apples Malus, pears Pyrus, and others [2].
The development of the fruit usually begins after the pollination of the flower and fertilization, although it is also possible to induce fruit formation in plants by applying appropriate hormones, even in the absence of fertilization [3].
The development of these plant organs is a complex process. Let's focus on one aspect of this phenomenon. We know that mature fruits, such as apples, often taste sweet, while immature ones are sour and tart. Starch is the most important carbohydrate in plants (as a reserve substance), and it is deposited, among other places, in fruits. However, starch has no sweet taste. Therefore, there must be a mechanism responsible for converting the starch contained in fruits into products with a sweet taste. In this article, I will present a simple method that allows for the examination of carbohydrate transformations in the ripening fruit, using the example of the domestic apple tree Malus domestica.
Experiment
To confirm the presence of starch in the apple fruit, we will perform an iodine test.
It is known that iodine, when it comes into contact with starch, forms a complex that, depending on the concentration, varies from blue to navy to almost black. You can easily verify this by adding some store-bought iodine tincture to almost pure potato starch powder (Fig. 1).
When iodine tincture comes into contact with potato starch, a dark blue color appears immediately (Fig. 2).
The same iodine tincture can be used for experiments with fruits. The iodine solution can be either of the following:
- an alcoholic iodine solution
- an aqueous iodine solution with potassium iodide KI
Since iodine has very low solubility in water, alcohol-based solutions or aqueous solutions of potassium iodide KI are used as solubilizers [4]. For this experiment, an aqueous solution is more suitable as it is less likely to damage plant tissues. The iodine solution can be made at home by dissolving 0.8g of potassium iodide KI in 200 cm3 (6.76 fl oz) of distilled water, then adding 0.2g of iodine I2 [5]. The resulting solution has a characteristic red-brown color (Fig. 3).
Warning: Free iodine is toxic and can irritate the skin and respiratory tract, as can its solutions. It leaves difficult-to-remove stains when it contacts the skin or organic substances.
For the experiment, select undamaged apples of any variety from the domestic apple tree Malus domestica (Fig. 4).
The most interesting results will be obtained if we use fruits at different stages of development. So, look for young fruits, several intermediate stages, and mature ones.
Each apple must then be cut in half across the seed cavity (Fig. 5).
Next, the fruit halves are immersed in the iodine solution as shown in Fig. 6.
After 1-2 minutes, remove the fruit from the solution (use the stem to handle the fruit more easily), rinse it under running water, and note your observations. An example result is shown in Fig. 7.
The picture that emerges from analyzing the degree and type of tissue coloration in apple fruit in iodine solution is interesting. In the case of the youngest fruits (Fig. 7A), we can observe that they contain large amounts of starch, as evidenced by the dark blue color in much of the cross-section. Only the area around the seed cavity remains lighter in color. As the fruit ripens, the starch content decreases (Fig. 7B). After some time, starch is only present in small areas located under the skin (Fig. 7C). A mature fruit no longer contains any starch, which is reflected by the complete lack of coloration (Fig. 7D).
Fruits are considered edible when they contain little to no starch.
Explanation
Starch, as a polysaccharide, consists entirely of glucose units linked by α-glycosidic bonds. It is the main storage substance in plants. It consists of amylose, which has a linear form (with only α-1,4-glycosidic bonds), and amylopectin, which has a branched structure due to additional α-1,6-glycosidic bonds [6]. It is stored in plant cells in the form of granules whose shape and size depend on the plant species. Their diameter typically ranges from 0.5 to 100 µm [3].
In the experiment, we have shown that starch is indeed stored in fruits. However, as the fruit matures, the amount of this polysaccharide decreases significantly. Eventually, in a ripe fruit, starch is no longer present.
This occurs because of the amylolytic enzymes, or amylases, produced in the tissues of the fruit. These enzymes catalyze the breakdown of starch into glucose. Through further chemical transformations, some glucose molecules are converted into fructose and sucrose. Both glucose, fructose, and sucrose induce sweetness, which, together with other compounds (such as organic acids), results in the characteristic taste of the fruit.
It should be noted that similar enzymes are also produced in animal organisms. The amylase present in saliva (also called ptyalin) efficiently breaks down starch into maltose and dextrins [7]. This is demonstrated in the situation shown in Fig. 8. A starch solution (Fig. 8A) turns dark blue when iodine is added (Fig. 8B). However, the addition of salivary amylase causes the starch to break down, and as a result, no blue coloration appears (Fig. 8C) [8].
Of course, there are many other processes occurring in fruits, and they are highly varied. In addition to the carbohydrate transformations presented here, one can also easily observe changes in the color of fruits during their ripening, as clearly seen in the case of raspberries Rubus (Fig. 9). I should note that the moments of taking both photographs were only 8 hours apart.
The color of the fruit's skin and flesh is influenced by the distribution and concentration of various pigments, including chlorophylls, anthocyanins, and carotenoids [9].
Literature:
- [1] Przywara L., Fruit, in: Otałęga Z. (ed.), Encyklopedia Biologii, Vol. VIII, Agencja Publicystyczno-Wydawnicza Opres, Kraków, 1999, pp. 30-33 back
- [2] Podbielkowski., Botanika, Wydawnictwo Naukowe PWN, Warsaw, 1994 back
- [3] Ozga J. A., Reinecke D. M., Hormonal Interactions in Fruit Development, Journal of Plant Growth Regulation, 2003, 22 (1), pp. 73–81 back
- [4] Janicki S., Fiebig A., Sznitowska M., Achmatowicz T., Farmacja stosowana: podręcznik dla studentów farmacji, Wydawnictwa Lekarskie PZWL, Warsaw, 2003 back
- [5] Sękowski S., Pierwiastki w moim laboratorium, Wydawnictwa Szkolne i Pedagogiczne, Warsaw, 1989, pp. 151-153 back
- [6] Leszczyński W., Skrobia – surowiec przemysłowy, budowa i właściwości, Zeszyty Problemowe Postępów Nauk Rolniczych, 2004, 500 (500), pp. 69–98 back
- [7] Konturek S., Fizjologia układu trawiennego, Wydawnictwa Lekarskie PZWL, Warsaw, 1985 back
- [8] Ples M., Enzymy - biologiczne katalizatory (eng. Enzymes: Catalysts of Life), Chemia w Szkole (Chemistry in School), 3 (2016), Agencja AS Józef Szewczyk, pp. 6-11 back
- [9] Rejman A., Pomologia, Państwowe Wydawnictwa Rolnicze i Leśne, Warsaw, 1976 back
All photographs and illustrations were created by the author.
Marek Ples