Orange found over grapefruit, lemon and lime favorable for diet and teeth in protein concentration, sugar and pH tests



Group ABCD

Christina Cherian

Deanna Lanier

Bailey Schroeder

Austen Thelen




Abstract: Written by Austen Thelen

Edited by: Christina Cherian and Deanna Lanier                                        

The human diet consists of a variety of foods, and citrus fruits are considered an important component because they contain a large amount of vitamin C (Hay, 1998).  Humans lack an enzyme that manufactures this vitamin, so we must obtain it through diet (Hay, 1998).  Although citrus fruits are a good source of vitamin C for a balanced diet and the immune system, they contain organic acids (citric, malic, and lactic acids) that can contribute to the weakening of tooth enamel (Dental Websmith, 2001-2003). Oranges, grapefruit, limes, and lemons were tested in order to decide which fruit is least acidic, and also contained the greater amounts of natural sugars and protein concentration.  Tests such as Benedict’s, Barfoed’s, and the Iodine Test were preformed to determine different sugars present.  Fructose was used as a control for these tests because it is the most common sugar found in fruits (Hay, 1998).  Grapefruits, limes, and oranges yielded a positive result (contained reducing sugars), while lemons showed a negative result in the Benedict’s Test.  In Barfoed’s Test, the grapefruits and oranges tested positive for monosaccharides while the lemons and limes tested negative.  We found that all fruits tested negative (contained no starch) in the Iodine Test.  The absorption spectrum showed at what wavelengths the fruit’s pigments absorbed light.  All spectrums were similar.  The last set of tests was the Bradford Assay for the total protein concentration, which were found to be inconclusive.  An additional test preformed by our group was to compare the pH of each fruit’s extract using a pH meter.  The pH level determined which juice would be most harmful towards tooth enamel.  Oranges were found to have the highest pH; then grapefruit, limes, and lemons.  After a thorough analysis of our data, oranges were found to be least harmful to tooth enamel while still containing monosaccharides.


Discussion:  Written by Christina Cherian

Edited by: Bailey Schroeder and Austen Thelen


            A study from the Division of Restorative Dentistry showed that the erosion of tooth enamel is influenced by dietary acids, pH and acid concentration (Hughes, et al, 2000).  The lower pHs initiate more damage to tooth enamel (Dental Websmith, 2001-2003).  The purpose of our research was to find which of four citrus fruits – grapefruit, lemon, lime and orange – was the best for the human diet and least harmful to the teeth.  We decided that the citrus fruit with the greater variety of dietary (reducing) sugars, including monosaccharides, the highest concentration of proteins and the highest pH would be the most beneficial to eat and the least damaging to tooth enamel.  This was determined because monosaccharides are easier to digest because of their simplicity, and that higher concentrations of proteins are digested to provide higher quantities of amino acids, which are used by the body.  Our prediction was that the orange had the greatest variety of dietary (reducing) sugars, the highest level of proteins and was the least acidic.  The orange therefore would be the most beneficial to the human diet.

We thought this because human taste buds are able to distinguish between sweet and bitter tastes and the orange tasted the sweetest, therefore it would be the one with the highest pH. 

During the course of our investigation we performed three sets of experiments to determine which was the best fruit.  The first set were assays to determine the presence of various sugars in the fruits.  Benedict’s Test was performed to determine if reducing sugars (sugars that are able to reduce ions) were present in the fruits.  The results of this test revealed that grapefruit, lime and orange were positive for reducing sugars while lemon was negative.  Fructose, the control, also resulted in a positive outcome.  It is known that there are reducing sugars in lemons, but they can have as little as one percent within their contents where other citrus fruits such as oranges can have up to nine percent (Salunkhe, 1995).  Reducing sugars are most likely present in lemons in such a low amount that any precipitate was not recognizable during the experiment. 

Next, we performed Barfoed’s Test to distinguish the fruits that contained monosaccharides from the ones that did not.  The first time that Barfoed’s Test was completed it was invalid.  Fructose is a known monosaccharide and should have reacted positively to form a red participate, however fructose did not react in the first trial.  We believe that this trial did not react correctly because the water was not hot enough.  The second trial of Barfoed’s Test produced a positive result for fructose.  Grapefruits and oranges tested positive for monosaccharides and lemons and limes tested negative.  Though only grapefruits and oranges tested positive for monosaccharides, all of the fruits may contain di and polysaccharides.  This is because Barfoed’s Test can only determine if there are monosaccharides present, it cannot confirm or refute the presence of di- or polysaccharides.  Also, lemons and limes may contain monosaccharides, but in amounts too low to produce a positive result.  Monosaccharides are more beneficial to consume because they are less complex than polysaccharides and are therefore easier to digest.  Since oranges and grapefruits contain monosaccharides they are more favorable. 

The Iodine Test was performed to determine if any of the fruits contained starch.  After completion of the Iodine Test the results suggested that none of the four citrus fruits contained starch.  The Iodine Test concluded that none of the fruits contained starch.  Since starch is long chained polysaccharide it is harder to digest within the body, its absence in fruits makes the fruit easier to digest.

During the second set of experiments we performed our pH test and the absorption spectrum experiment.  The pH levels showed that the lemon was the most acidic with an average pH of 2.75, followed by the lime: 2.88, grapefruit: 3.65, and then the orange at 4.35.   Since studies show that the lower the pH the greater the erosion of tooth enamel, this supports our hypothesis that the orange is the least damaging to teeth (Dental Websmith, 2001-2003).  The orange is followed closely by grapefruit and then by lime and lemon, which cause higher levels of tooth enamel erosion. 

The absorption spectrum experiment showed which wavelengths of light the pigments in the fruit juice absorbed.  Grapefruit had an absorption peak at 400 nm with its lowest point at 700 nm.  Orange had an absorption peak at 400 nm with its lowest point at 700 nm.  Lime had an absorption peak at 400 nm with its lowest point at 685 nm.  Lemon had an absorption peak at 400 nm with its lowest point at 700 nm.  Because all of the fruit are in the same family, being citrus fruits, it is not unexpected that their absorption spectrums were very similar.  The fruits may contain similar pigments that provide them with their similar spectrums.  These results for the absorption spectrums, being so similar, are inclusive relative to our hypothesis.

The Bradford Assay tested the total protein concentration of each fruit.  To find the protein values for each fruit to compare to one another we chose to use 20 ml of the diluted fruit juices.  For the first trial the protein values found for all the fruits was 20 mg at an absorbance of 0.075 for grapefruits and lemons, 0.05 for limes and 0.025 for oranges.  In the second trial grapefruit’s absorbency was 0.81, lime’s 0.025, lemon’s 0.075 and orange’s was 0, again for 20 ml.  Because there is no visible pattern between the fruit spectrums and the absorbance values are not consistent between the two trials, no solid conclusion can be reached to determine which one has the greater amount of protein.  The results of the Bradford Assay were inconclusive.

            After thoroughly conducting all of the experiments and a very detailed investigation analyzing all of the results, our hypothesis was supported by the sugar and pH tests.  Oranges were found to be the least acidic and contained monosaccharides.  Thus, having the least damaging affect on tooth enamel decay while being more beneficial to the diet (monosaccharides are easier for the body to digest).  Monosaccharides also provide for a greater variety of reducing sugars in the orange.  The Bradford Assay results pointed to no clear conclusion and the absorbance spectrums neither supported nor refuted our hypothesis; therefore all aspects of our hypothesis were not supported. 

There are several sources of error that may have occurred with in our experiments.  One of them may have been that the citrus fruits used may have varied in age.  As fruit matures, the sugar content is known to rise, while acidity level decreases (Hay, 1998).  Though we tried to pick out fruits with approximately the same size, shape, color and firmness for each set of experiments, we did use new fruits for each set so the maturity level may have differed.  We also found research showing that fruit composition is affected by factors such as growing conditions, variety, and climate (Hay, 1998).  When Meijers was out of a particular fruit we were forced to go to Farmer Jack’s to purchase it.  Since stores buy their fruits from companies, such as Del Monte, Outspan and Sunkist that grow the fruits it is possible that these stores did not purchase their fruits from the same company.  Also, because companies grow their fruit in multiple locations it is most likely that the fruits we used were grown in different locations.  The use of fruits grown in different locations for each set of experiments could have caused a discrepancy in the fruit juice composition, therefore providing us with varying results.  Another source of error may have been human error that occurred while conducting the numerous trials of our experiments.  The experiments we performed required close detail to the amount of solutions, such as our juices, put into them and to time restrictions within the experiments.  Many of the experiments performed required small quantities of solutions are were very sensitive to the amount added, such as the Bradford Assay.  It is likely the somewhere in the course of our experiments the proper amount a substance was not added or that the timed research was not timed correctly.








I like cows. 



Figure 1: Benedict’s Test for Fructose, Grapefruit, Lemon, Orange, and Lime All four citrus fruits were compared to the positive control, fructose. A red precipitate indicated the presence of a reducing sugar. (a) Grapefruit turned orange on top and red on the bottom for all three trials signifying a positive result. Trial one for lemon turned olive green while trials two and three turned green on top and blue on the bottom (n=3). (b) Orange was orange on top, a darker orange in the middle, and red on the bottom, showing a positive result (n=3). Lime turned orange on top, red in the middle, and dark red on the bottom, showing a positive result (n=3).