Juicy Fruit: Why sugar is added to fruit.

Juicy Fruit: Why sugar is added to fruit.

Since our last two mini-experiments, for our Science of Food and Cooking course, require us to choose a recipe from our textbook, and to refer to the science concepts behind them, I began my experimental search by browsing over the table of contents. One of the concepts that caught my eye was concept 49, which was tilted Sugar and Time Make Fruit Juicer. My interest was caught by this concept, because fruit is often already very sweet, and I just could not understand why anyone would want to add more sugar to it and make it even sweeter. After skimming the material, I learned that adding sugar to fruit actually makes it juicer, something I will explain later on in this blog.

I then turned my attention to the test procedure the authors of our textbook had provided in demonstrating this interesting concept. In the textbook, the authors’ test consisted of cutting 4 ounces of strawberries, into ¼ pieces, tossed them in 1 tablespoon of sugar, plied them in the middle of a napkin, and measured how far the juices had spread over a period of 15 minutes, with data being recorded in 5 minute intervals. They also place 4 ounces of cut strawberries on a separate napkin to serve as a control.

I decided to replicate this test, with the addition of a group of strawberries that had been tossed in 1 tablespoon of salt, in addition to the group tossed in sugar, and the control group. I also decided to collect data over a 25 minute period instead of 15.

My hypothesis for this study was that adding sugar to cut strawberries would cause them to release more liquid, then strawberries that were plain, because the sugar would cause an osmoses reaction to occur and draw the liquid out of the fruit.

My null hypothesis was that added sugar, and salt, would not affect the amount of liquid that was released by the strawberries, and then absorbed by the napkins.

My independent variables for this experiment were strawberries tossed in sugar (sugar) and strawberries tossed in salt (salt).

My dependent variable for this experiment was the amount of liquid released by the strawberries, which translated into how far the liquid had traveled on the paper towels.

My standard variables were the number of strawberries place on each paper towel (16 ¼ cuts), the amount of time each measurement was taken (5 minute intervals), the total time of the test (25 minutes), the measuring tool used (a 12 inch ruler), the size of the paper towel (10.5 inches x 11 inches), the same brand paper towel, the amount of sugar (1 tablespoon), the amount of salt (1 tablespoon), and the stop watched used to keep track of time (www.online-stopwatch.com).

The materials for this test included:

·         12 strawberries of approximately equal size

·         1 tablespoon salt

·         1 tablespoon sugar

·         3 paper towels of the same size, from the same roll

·         2 separate bowls to toss the strawberries in either salt of sugar

·         A ruler

·         A stop watch

·         A knife

To begin my experiment, I first began by acquiring 12 strawberries of similar size. Following this, I then removed the leaves from the top of all the strawberries. Once this was completed, I than acquired 2 separate plastic bowls. In one bowl I measured out 1 table spoon of sugar, and in the other bowl I measured out 1 table spoon of salt. I than place 4 strawberries on 3 separate Styrofoam plates. At this point I laid out 3 separate paper towels on my kitchen table, and place note cards next to each indicating which would hold what test group.

Once everything was prepared, I than cut the strawberries in groups of four. The strawberries were first cut in half horizontally, and then those halves were then cut in half again horizontally. This meant that each strawberry yielded 4 pieces of roughly equal size, resulting in 16 ¼th pieces per test group. I then proceeded to repeat this process for the other two sets of strawberries. Once all strawberries were cut, I then tossed one group in the bowl of sugar, until the strawberries had soaked up all the sugar in the bowl. Following this, I did the same thing with the remaining group of strawberries, in the second bowl containing salt. At this point, I put on 2 sets of food service gloves in order to avoid contaminating any of the batches with residual salt or sugar from one another.

At this point, I place each group of strawberries on their corresponding paper towel, shedding gloves in between each group, until the final group was placed with my bare hands. Once all groups had been placed on their paper towels, I started the stop watch. At every 5 minute interval, I measured the diameter of the released juice on the paper towel with my ruler. Data was gathered for 25 minutes.

Group of Strawberries without anything added, on paper towel, at the 0 minute mark.

Group of sugar tossed strawberries, on a paper towel, at the 0 minute mark.

3 groups of strawberries, after they were cut into 1/4 sized pieces, before being treated in their respective conditioning agents (sugar and salt).

Group of salt tossed strawberries, on a paper towel, at the 0 minute mark.

Groups of strawberries, before being cut.

The paper towels that were used to demonstrate the juice released from the strawberries.

At 5 minutes in, the strawberries with no salt or sugar did nothing; the only visible marks on the paper towel were where the strawberries had initially come into contact with the paper towel. The sugar coated strawberries on the other hand, had a significant visual ring of water on the paper towel, while the strawberries tossed with salt had a visible ring around them, but smaller compared to the sugar. By 10 minutes, the rings had grown about an inch for the salt and sugar strawberries, while the control group still had not produce any changes. The surprise was that, at 15 minutes, the strawberries covered in salt had begun to release more liquid than the strawberries covered in sugar. This continued until the end of the experiment. The diameter of the water marks for each group were as follows:

Time	Control- Nothing added	Strawberries tossed in sugar	Strawberries tossed in salt
5 minutes	N/A -  only initial contact marks	4 inch	3 inch
10 minutes	N/A- only initial contact marks	5 inch	4.5 inch
15 minutes	N/A- only initial contact marks	5.5 inch	6 inch
20 minutes	N/A- only initial contact marks	6 inch	7 inch
25 minutes	N/A- only initial contact marks	7 inch	8 inch

As you can see by the data, sugar did in fact cause the strawberries to become juicer than strawberries with nothing added. However, the data indicates that salt was had a greater effect on the strawberries than the sugar. After I some more of the science behind this phenomena, I learned that sugar has 1/10 the power of sugar in terms of causing this effect (Editors of Cook's Illustrated & Crosby, 2012). This explains why the strawberries tossed in salt started releasing more liquid than the ones tossed in sugar. A point of interest, was that while the strawberries tossed in salt released more overall liquid than the strawberries tossed in sugar, the liquid on the paper towel of the strawberries tossed in sugar seemed to be more concentrated juice, as it was much redder, and thicker than the water on the paper towel with the strawberries tossed in salt.

The graph above demonstrates the diameter of the water marks made by the juice released from each of the test groups of strawberries (salt, sugar, and control). The diameter of the water mark was measured in inches over a 25 minute period, with measurements taken in 5 minute intervals. The control group does not have a visible bar, because the juice collected on the paper towel, was only that of the initial contact made with the paper towel upon placing them there, and was measurable.  In turn, since the control data was not applicable, it was recorded in the data collection as 0 for each of the intervals. The red bars represent the juice released by the sugar tossed strawberries, and the blue bars represent the juice released by the salt tossed strawberries. 

A picture of the final water mark left on the paper towel, made by the control group of strawberries at the 30 minute mark. 

A picture of the final water mark left on the paper towel, made by the control group of strawberries at the 30 minute mark. 

An up close picture of the surface of the control group of strawberries at the 0 minute mark. 

An up close picture of the surface of the sugar group of strawberries at the 0 minute mark. 

An up close picture of the surface of the salt group of strawberries at the 0 minute mark. 

A picture of the final water mark left on the paper towel, made by the sugar group of strawberries at the 30 minute mark. 

Now that we have discussed the experiment, let now examine the scientific aspect behind why these results occurred. According to concept 49, when sugar is added to fruit, it produces osmotic pressure, which in turn pulls the water out of the fruit’s cells (Editors of Cook's Illustrated & Crosby, 2012). The reason sugar creates this osmotic pressure is, because sugar is hygroscopic, which means that it has a high affinity for water molecules (Editors of Cook's Illustrated & Crosby, 2012). In fact, sugar is so hygroscopic, it can even draw moisture out of the surrounding air (Editors of Cook's Illustrated & Crosby, 2012). The process by which sugar draws the water molecules out of fruit is call maceration, which not only draws the moisture molecules out of the fruit, but also changes the texture of the fruit by making them softer and less waterlogged (Editors of Cook's Illustrated & Crosby, 2012). This textural change occurs, because cells that are filled with ware are firm and more ridged, while cells that contain less water become flaccid, and in turn softer, which is similar to that of a wilted plant when it begins to dry out (Editors of Cook's Illustrated & Crosby, 2012). Once maceration occurs, the resulting liquid can be used to moisten dishes like fruit salads or fruit cakes since it is a very flavor rich liquid, or it can simply be discarded to prevent keep baked goods such as crumbles or pies from becoming too soggy (Editors of Cook's Illustrated & Crosby, 2012).

If I were to do this experiment again, one of the things I would do differently would be to cut the strawberries into smaller pieces. The ¼ sized pieces were simply too large, making them hard to pile in the center of the paper towel, which also made it kind of hard to measure how far the liquid had spread.

In addition, it was not until after I had completed the experiment did I realize that we were supposed to cook a dish using a recipe from our textbook, instead of conducting a test to demonstrate a scientific concept. So for the final experiment, I will not make this same mistake.

Source Citation:

Editors of Cook. , & Crosby, G. (2012). The science of good cooking. Brookline: Cook's Illustrated