The Effects of Sugar, Acidity and Pectin on Gel … Effects of Sugar, Acidity and Pectin on Gel...

The Effects of Sugar, Acidity and Pectin on Gel Strength in a Naturally Low and

High Pectin Fruit Varieties

Jason A. Frey

Purdue University: Dietetics, Nutrition Fitness and Health Double Major

Jason Frey: 500 N River Rd. West Lafayette, IN 47906

765-743-8729

[email protected]

Running Head: Pectin and Gel Strength

Abstract

This experiment focused on the effects of sugar, pectin, and acidity on gel strength in low

and high pectin fruits. The purpose was to see how high and low pectin fruits reacted to

the different variables. The low pectin fruit, strawberries, demonstrated greatest gel

strength using the penetrometer when the 2 grams of pectin was added and the sugar

concentration doubled (6.3 g average for pectin, 7.1 g average for 2X sugar vs. an

average of 3.95 g for acidity and control). The high pectin fruits: apples and plums

revealed large increases in gel strength readings from the penetrometer when the sugar

concentration was doubled (762 g apples, 43.1 g plums vs. an average of 6.73g acidity

and 7.1 g for pectin, and 7 g for control). The linespread test indicated similar results.

The low pectin fruit had the firmest gel when the sugar and pectin was added (2.8”

average traveled for sugar and pectin compared to 6.5” average for the control and acid).

The high pectin fruits demonstrated the firmest gel when the sugar concentration was

doubled (1” distance for 2X sugar compared to an average of 2.6” for control, acid, and

pectin). The amount of natural pectin does appear to alter the affects of acidity, sugar and

added pectin. Doubling the sugar concentration had the greatest effect on gel strength for

fruits naturally high in pectin. Increasing sugar and pectin had the largest increases in gel

strength for the naturally low in pectin fruits.

Introduction

This experiment will focus on how acidity, pectin, and sugar affect gel strength in

low and high pectin fruits. The literature reviewed makes it clear that different fruits will

have different pectin concentrations. The literature has also divided different fruits in

groups based on their acid and pectin concentration. The pectin in some fruits is high

enough to form gels without adding additional pectin. As long as the fruit juice is heated,

appropriate pH and sugar concentrations maintained, the gel will set in 24 hours

(www.gialaska.edu/ScienceForum/ASF8/887.html 1988). In other fruits pectin must be

added for a satisfactory gel to form. The National Center for Home Preservation website

provided the listing of fruits divided into three groups based on pectin content. For this

experiment a high and low pectin fruit will be used. Plums and Apples will serve as the

high pectin fruit and Strawberries will serve as the low pectin fruit

http://www.uga.edu/nchfp/how/can_07/jellied_product_ingredients.html, 2005). The

purpose of this project is to understand how acidity, sugar and pectin concentrations

affect gel formation strength in naturally low and high pectin fruits. The independent

variables being studied are pH, sugar concentration, and pectin. The dependent variables

being studied are taste and texture, both being performed by a taste panel.

Dr. James R. Daniel explains in his lecture notes how pectin plays a role in gel

formation. Pectin is present in many varieties of fruit in varying concentrations. The

pectin can be found in the lamella of plants and it acts to hold cells together. The pectin

chemical structure is a 6 carbon ringed compound with methyl ester groups attached.

The six ringed compounds can link with each other by alpha 1-4 linked-D-Galacturonic

Acid Groups. This linkage forms a straight long chain of pectin molecules. This straight

chain can further react with other pectin chains by forming junction zones between their

hydrogen and oxygen molecules. These junction zones trap water and form a gel. For

these junction zones to form certian conditions must be in place. The pH of the solution

containing the pectin must be between 3.2 and 3.6 to form a gel. The decreased pH

eliminates the negative charge surrounding the pectin chain, which allows the chain to

bind with other pectin chains. Sugar is also needed to remove the water shell from

pectin. A concentration of 65% is optimal for pectin gelling to occur. If the

concentration is too low the gel won’t be able to from because the pectin molecule will be

surrounded by water and unable to make contact with each other. If the sugar

concentration is too high the solution will begin to crystallize and a hard candy will be

formed. To achieve the optimal concentration of sugar the solution is heated to 104.5ºC

allowing for evaporation of water until the correct temperature is reached. The pectin

concentration is also crucial in gel formation. A concentration of 1% is optimal for gels.

If the concentration is too low a gel will not be able to form.

The literature also lists several factors that affect the strength of gels. The type of

fruit, amount of pectin, acid and sugar concentration all affect gel formation. A. Voragen

states that pectin solutions are most stable at a pH of 3-4. However, if the pH drops

below this range esters start to be removed and the neutral side chain sugars begin to be

hydrolyzed (Voragen, 1995). The role of the sugar is to pull the water molecules away

from the pectin so that it can begin to bind together

(www.gialaska.edu/ScienceForum/ASF8/887.html 1988). If the water molecules are not

pulled away from the pectin the pectin will not be able to bind and syrup will be formed.

The pectin is used to form the three dimensional linked structure that gives the gel its

firmness. M. Jarvis explains the important role that pectin plays in the development of

texture. He explains how each fruit and cooked vegetables texture is related to the

amount of pectin present in the fruit at that time (Jarvis, 1984). The gaps that remain are

what effects added pectin will have on a low and high quantity pectin fruit gel. I am

studying this problem to gain a better understanding of the gel forming capabilities of

various fruits. I also want to gain a better understanding of the role that pectin plays in

gel formation and how it’s affected by increased sugar, increased acid, and increased

pectin. This information could prove useful to those who make their own jelly and want

to know if a fruit will be a good gel former and how different concentrations of

ingredients will affect it.

The independent variables that will be examined in this experiment are sugar, acid

and pectin concentrations. The pectin will be measured to get baseline data for the

natural pectin concentration of the fruit by mixing the fruit with alcohol and weighing the

solid residue. The fruit will need to be of similar ripeness for these experiments to be

accurate, since pectin concentrations change with ripeness. The pectin can then be

increased from baseline using commercial pectin. The pH will also be measured of each

fruit to determine a baseline pH. The pH can then be decreased by adding lemon juice to

measure the effects of pH on gel formation. The sugar concentrations in each juice will

be increased to see if this will increase the strength of the gel. The independent variables

will all have an affect on texture, which will be measured using a penetrometer and

subjectively by a panel. The linespread will be used to measure the viscosity of the gel.

The pH and pectin concentration methods were selected to understand the two natural

variables of fruit and how they affect texture. A pH test will be performed on the juice to

get baseline data. An alcohol test will also be performed on 10 ml of the juice, added to

10 ml of alcohol to get baseline data for the pectin concentration of each fruit.

Comprehension of the baseline data for theses two areas will assist in understanding the

importance of these two variables on gel strength. By having baseline data of the higher

and lower fruit pectin concentration, it’s possible to form hypothesis as to why the lower

pectin variety of grape did not gel. The sugar variable was chosen to see if increasing the

sugar concentration in juices would result in increased gel strength. A Jelmeter is used to

get the initial sugar concentration to add to juice and a multiple of that will be used to

increase the amount of sugar for that variable. All the procedures performed in this lab

will follow standard procedures of pectin gel formation laboratories. The dependent

variables being studies are taste and texture. A taste panel will be arranged to perform

these tests. The panel will rate the samples on a scale of 1-10. A score of 10 will

indicate optimal texture and taste. A score of 1 will indicate poor texture and taste for a

jelly.

Methods

This experiment is designed to test low and high pectin fruits. The low pectin fruit used

will be strawberries. Plums and apples will serve as the high pectin fruits. Two fruits

were used since plums went out of season during the experiment trials. This project plans

to alter three dependent variables and perform six tests to measure the changes that these

variables produce. The variables that will be altered are sugar, pH and pectin. The tests

to be performed are pH test, pectin test using alcohol, Jelmeter test to determine the

amount of sugar needed for baseline, penetrometer test using a cone probe, viscosity

using linespread apparatus, and a subjective analysis of texture using a taste panel.

The juice for the tests will be made the first week of testing and stored in the

freezer to ensure freshness for the next two tests. We will follow the jelly recipes from

University of Georgia Cooperative Extension Service

(http://www.fcs.uga.edu/pubs/PDF/FDNS-E-43-8.pdf, 2000). The recipe will produce 3

quarts of jelly allowing the control and three independent variables to have 200 milliliters

of jelly for each variable, for the three weeks that tests are performed. The recipe

requires 1,359 grams of fruit, and 177 ml of water.

The first step in this experiment is to wash the fruit and remove stems. Crush the

fruit and then add the 177 ml of water cover and bring to a boil on high heat. After it

reaches boiling temperatures reduce heat and simmer for ten minutes. Extract the juice

by placing prepared fruit in a damp jelly bag. Hang the jelly bag over a large pot to

collect the juice. Then refrigerate jelly overnight and strain through two cheese cloths to

prevent tartrate crystal formation. After the jelly is strained collect one quart of juice for

the first experiment. The remaining two quarts can be frozen for next week’s

experiments. This procedure will be used for both varieties of fruit resulting in 6 quarts

of juice. To prepare jars for jelly, wash containers with soap and water. Boil the jars for

ten minutes and leave them in hot water until use. Wash the lids with soap and water to

prepare for canning. To prepare the jelly, first remove the juice from the fridge and allow

it to warm to room temperature. Then place finger on bottom of Jelmeter and fill it to the

top. Allow the juice to flow for one minute, then replace finger and note the amount of

sugar to add. Then take 15 ml of juice and place in a test tube. Add 15 ml of a 70%

alcohol solution to the test tube and shake gently. Remove the amount of solid jelly like

mass formed and weigh to get a baseline for pectin concentration of juice. Then take

200ml of juice and the appropriate amount of sugar based on the Jelmeter test. Heat both

in a small saucepan stirring constantly until sugar is dissolved. Calibrate a thermometer

by boiling deionized water and observing the 100 degrees Celsius point. Bring the

mixture to a boil rapidly until the temperature reaches 104.5 degrees Celcius. If the jelly

will gel it will form a sheet when dripped from a metal spoon. Then pour the jelly into a

jelly glass and let cool. After the jelly has cooled place on the lids and store in the fridge

undisturbed to allow the gel to set. Repeat the procedures with another 200 ml of juice,

but twice the recommended amount of sugar. Then reduce the pH reduced by 0.60 using

lemon juice and keep the recommended amount of sugar. Finally, add 2 grams of pectin

to the original juice and keep all other variables the same. Each of these tests will be

poured into a separate jelly jar, bringing the total amount of jelly jars to 4 for each variety

of fruit. Each variable will have pH and alcohol tests performed after adding the changed

variable, but before forming the gel. The procedures listed earlier will be followed for

each test.

After all variables have been given a day in the fridge the texture and viscosity

can be analyzed. The viscosity will be analyzed using a linespread apparatus. The

reading for viscosity will be taken by filling the middle ring with jelly and then removing

it quickly. The jelly is allowed to flow for 2 minutes, then the distance flowed is

recorded in four spots around the circular pie pan. The distance of each line on the

linespread sheet is 1/8”. To calculate distance: [(Add 4 readings/4)]/8=total distance

traveled. After this test is performed the penetrometer and cone attachment will be used

to measure the texture of the product. The settings for the penetrometer will be: mode

Force/Comp and the pre and post speed will be 1.0mm/s, post speed force 1.0 mm/s,

distance 4 mm, Count NA, Trigger 0.39 N, and PPS 200.00. Stable Micro Systems-

Texture Expert was the program used.

The final test that will be performed is the sensory analysis of texture. The four

variables: Strawberry control, lowered pH by 0.6, twice recommended sugar, and 2 added

grams of pectin will be analyzed subjectively for taste and texture. The same four

variables will be observed for the apples/plums. Each variable will be assigned a random

four digit number and placed in line for subjective analysis by a panel. Small paper

plates will be utilized to hold the jellies and they will be served plain. The panel will rate

texture and taste on a scale of 1-10 for each of the eight variables. The highest texture

scores will be rewarded to the samples most similar to jelly. The highest taste scores will

be given to samples that have the most satisfactory taste. The lowest scores will be given

to the samples least similar to jelly and with the poorest taste. The panel will consist of

24 students.

The Scorecard will be as follows:

Product # ________________________ Texture 1□ 2□ 3□ 4□ 5□ 6□ 7□ 8□ 9□ 10□ Least like Jelly Most like Jelly Taste 1□ 2□ 3□ 4□ 5□ 6□ 7□ 8□ 9□ 10□ Dislike Neutral Like Very Much Product # ________________________ Texture 1□ 2□ 3□ 4□ 5□ 6□ 7□ 8□ 9□ 10□ Least like Jelly Most like Jelly Taste 1□ 2□ 3□ 4□ 5□ 6□ 7□ 8□ 9□ 10□ Dislike Neutral Like Very Much Product # ________________________ Texture 1□ 2□ 3□ 4□ 5□ 6□ 7□ 8□ 9□ 10□ Least like Jelly Most like Jelly Taste 1□ 2□ 3□ 4□ 5□ 6□ 7□ 8□ 9□ 10□ Dislike Neutral Like Very Much Product # ________________________ Texture 1□ 2□ 3□ 4□ 5□ 6□ 7□ 8□ 9□ 10□ Least like Jelly Most like Jelly Taste 1□ 2□ 3□ 4□ 5□ 6□ 7□ 8□ 9□ 10□ Dislike Neutral Like Very Much

Discussion

Calculations:

Linespread Apparatus-- [14+16+19+12/4]/8=1.9” traveled

Discussion

The results of this study indicate significant differences between the variables. The low

pectin fruit, strawberries, had significant increases in gel strength measured by the

penetrometer. This difference is evident in Figure 1. The second trial of strawberries

resulted in a less exaggerated difference, but it was still evident in Figure 4. The average

increase in grams of force required to break the gel by the cone probe was 6.33 grams for

the 2 grams of added pectin and doubled sugar variables, compared to 3.95 grams for the

control and lemon juice variable. This data is clearly indicated in Table 1. The reason

for the increase in gel strength is likely a result of increased pectin present to form

linkages and hold water. Jarvis states in the Structure and properties of pectin gels in

plant cell walls that low pectin concentrations will yield weak jellies (Jarvis, 1984). This

information is also stated in Dr. Daniel’s notes. By increasing the pectin concentration

the solution is able to gel. To understand why the increased sugar results in a stronger gel

it’s necessary to view the linespread data. Figure 5 and 7 make it clear that the increased

pectin and sugar concentration jellies were very firm. Table 4 reveals that the distance

traveled by the high sugar gel was an average of 1.89” for both trials. The 2g of added

pectin gel traveled 2.3” average for both trials. The control and lowered pH variables

traveled 6.15” and 6.8” respectively. This large difference indicates how firm the sugar

gel and pectin was. The increased sugar concentration resulted in decreased water

available to keep pectin from interacting. The result was increased gelling and possibly

some crystal formation due to the lowered water concentration. These results are in line

with research presented by Jarvis M.C. The high pectin fruits indicated slightly different

results. Figure 2 and 3 show a large increase in gel strength for the double sugar

concentration. It’s important to note that the plums went out of season during the middle

of this experiment. As a result the plums were replaced with apples. The apples are also

high in pectin and should have similar jelling characteristics as the plums. Tables 2 and 3

show that the doubled sugar variable resulted in gel strengths of 761 and 47.1

respectively compared to single digit reading for all other variables. This large increase

is likely a result of crystallization. The research completed by Thakur BR indicates

similar results to high amounts of solutes in the juice (Thakur, 1997). Thakur completed

an article review of the chemistry and uses of pectin, one article discussed how a large

increase in sugar results in crystallization. The greater the amounts of sugar present in

the jelly the less water to interfere with bonding and crystal formation. The result is a

very firm gel. The linespread apparatus also indicates this showing no movement for the

doubled sugar concentration in Table 4 and 6. This crystal formation led to a gel that was

similar to hard candy in texture. The sensory panel also made similar comments on

texture. Table 8 indicates a lower rating for the apple variable with twice the sugar.

Table 9 contradicts with a higher rating for texture for the 2 x sugar gel; however this

result is likely due to the soupy texture of the other apple gels. Figures 10 and 12 also

show these results. Table 7 and figure 9 and Figure11 indicate that the control was the

least favorable texture for the jellies. The low pectin fruit control did not gel and as a

result the control was a soupy texture, which would explain the low texture score. The

other variables did well on the taste panel texture scores as indicated in figure 11. The

taste panel gave high scores for all the variables in the taste category except the

strawberry variables in trial 2, indicated by Figure 11. This lower score may have been a

result of the order that the taste samples were presented. The second strawberry trial

followed the apple jelly taste samples. The apple jelly samples were well favored which,

may have caused lower scores for the strawberries presented afterwards. Greater

randomization between samples should be presented in future taste panels.

The research performed indicates that the amount of pectin naturally present in

fruit does alter how acid, sugar concentration and added pectin affect it. This is evident

by the data presented and the wide variations in results between the low pectin and high

pectin fruits. Further research should examine the exact amount of pectin that is initially

present in fruits and how this affects the results. This experiment was not successful at

isolating pectin by adding alcohol to the juice. A solid precipitate was not formed, so the

exact amount of pectin could not be measured. If this variable could be recorded it would

increase the understanding of the effects of other variables on texture, since the initial

pectin concentration could be isolated.

Results

Table 1. Strawberry Penetrometer Gel Strength Trial 1 Trial 2 Variable force (g) force (g) Strawberry Control 5.2 3.2 2 gram pectin 5.5 6.3 2 x sugar 6.4 7.1 Strawberry w/ Lemon 4.7 2.7

0

1

2

3

4

5

6

7

8

Strawberry Control 2 gram pectin 2 x sugar Strawberry w/ Lime

Variable

Grams of fo

rce

Figure 1. Strawberry Penetrometer Trial 1

Table 2. Apple Penetrometer Gel Strength Trial 1

Variable force (g) Apple Control 7.8 2g pectin 7.3 2x sugar 761.8 Apple w/ Lemon 5.1

0

100

200

300

400

500

600

700

800

Apple Control 2g pectin 2x sugar Apple w/ Lime

Variable

Gra

ms

of F

orce

Figure 2. Apple Penetrometer Trial 2

Table 3. Plum Penetrometer Gel Strength

0

5

10

15

20

25

30

35

40

45

50

Plum Control 2g pectin 2x sugar Plum w/ Lime

Variable

Gra

ms

of F

orce

Figure 3. Plum Penetrometer Trial 1

Trial 1 Variable force (g) Plum Control 3.2 2g pectin 6.3 2x sugar 47.1 Plum w/ Lemon 2.7

Table 1. Strawberry Penetrometer Gel Strength

Trial 1 Trial 2 Variable force (g) force (g) Strawberry Control 5.2 3.2 2 gram pectin 5.5 6.3 2 x sugar 6.4 7.1 Strawberry w/ Lemon 4.7 2.7

0

1

2

3

4

5

6

7


Variable

Gra

ms of

For

ce

Figure 4. Strawberry Jelly Penetrometer Trial 2

Table 4. Strawberry Linespread Apparatus

0.00

1.00

2.00

3.00

4.00

5.00

6.00

7.00

8.00


Variable

Dis

tanc

e Tr

avel

led

(inch

es)

Variable Distance (inches) Trial 1 Trail 2 Strawberry Control 6.50 5.80 2 gram pectin 2.25 2.30 2 x sugar 1.88 1.89 Strawberry w/ Lemon 6.88 6.72

Figure 5. Linespread Strawberry Trial 1

Table 5. Apple Linespread Apparatus Variable Distance

(inches) Apple Control 1.43 2g pectin 1.60 2x sugar 1.00 Apple w/ Lemon 2.31

0.00

0.50

1.00

1.50

2.00

2.50


Variables

Dis

tanc

e Tr

avelle

d (in

ches

)

Figure 6. Linespread Apple Trial 2

Table 4. Strawberry Linespread Apparatus

Variable Distance (inches) Trial 1 Trail 2 Strawberry Control 6.50 5.80 2 gram pectin 2.25 2.30 2 x sugar 1.88 1.89 Strawberry w/ Lemon 6.88 6.72

0.00

1.00

2.00

3.00

4.00

5.00

6.00

7.00

8.00


Variable

Dis

tanc

e Tr

avel

led

(inch

es)

Figrue 7. Linespread Strawberry Trial 2

Table 6. Plum Linespread Apparatus Variable Distance (inches) Trial 1 Plum Control 1.20 2g pectin 1.53 2x sugar 1.00 Plum w/ Lemon 2.18

0.00

0.50

1.00

1.50

2.00

2.50


Variable

Dis

tanc

e Tr

avel

led

(inch

es)

Figure 8. Linespread Plum Trial 1

Table 7. Strawberry Sensory Panel Sensory Panel Trial 1 Trial 2 Texture Taste Texture Taste Strawberry Control 4.80 7.3 3.50 3.7 2 gram pectin 6.80 6.7 6.50 5.2 2 x sugar 8.20 5.8 6.20 5 Strawberry w/ Lemon 7.20 7 6.5 3.7

0.00

1.00

2.00

3.00

4.00

5.00

6.00

7.00

8.00

9.00


Variable

Ave

rage

Rat

ing

(1-1

0)

TextureTaste

Figure 9. Strawberry Trial 1 Taste Panel

Table 8. Apple Sensory Panel

Variable Texture Taste Apple Control 8.30 8.3 2g pectin 3.30 8.2 2x sugar 6.70 5.3 Apple w/ Lemon 3.30 6.5

0.00

1.00

2.00

3.00

4.00

5.00

6.00

7.00

8.00

9.00


Variable

Ave

rage

(1-1

0)

TextureTaste

Figure 10. Apple Trial 1 Taste Panel

Table 7. Strawberry Sensory Panel Sensory Panel Trial 1 Trial 2 Texture Taste Texture Taste Strawberry Control 4.80 7.3 3.50 3.7 2 gram pectin 6.80 6.7 6.50 5.2 2 x sugar 8.20 5.8 6.20 5 Strawberry w/ Lemon 7.20 7 6.5 3.7

0.00

1.00

2.00

3.00

4.00

5.00

6.00

7.00


Variable

Rat

ing

(1-1

0)

TextureTaste

Figure 11. Strawberry Trial 2 Taste Panel

Table 9. Plum Sensory Panel

Variables Texture Taste Plum Control 4.00 7.2 2g pectin 8.50 6.5 2x sugar 6.80 7.7 Plum w/ Lemon 8.70 8.7

0.00

1.00

2.00

3.00

4.00

5.00

6.00

7.00

8.00

9.00

10.00


Variable

Rat

ing

(1-1

0)

TextureTaste

Figure 12. Plum Trial 2 Taste Panel

References

Jellied Product Ingredients. May 2005. The National Center for Home Food Preservation.

September2005,<http://www.uga.edu/nchfp/how/can07/jellied_productingredients.html>.

On the Jelling of Jelly Article #887 August 1988. Alaska Science Forum. September

2005, <http://www.gialaska.edu/ScienceForum/ASF8/887.html 1988>.

Preserving Food: Jams and Jellies. November, 2000. University of Georgia Cooperative

Extension Service. September 2005, < http://www.fcs.uga.edu/pubs/PDF/FDNS- E-43-

8.pdf>.

Jarvis M.C.(1984) Structure and properties of pectin gels in plant cell walls. Plant Cell

Environ.,7(1984), 153-156.

Voragen(1995), Pilinik W., Thibault J.F.,Axelos M.A.V. and Renard C.M.C.G.

Pectins in Food Polysaccharides and Their Applications (Marcel Dekker, New York,

(1995) pp. 287-339.

Thakur & Singha (1997),Thakur BR, SinghRK, Handa AK, Chemistry and uses of

pectin—a review. Crit Rev Food Sci Nutr. 1997Feb;37(1):47-73.

The Effects of Sugar, Acidity and Pectin on Gel … Effects of Sugar, Acidity and Pectin on Gel...

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