EXPERIMENT 9: ENZYMATIC BROWNING OF FRUITS/VEGETABLES.
Introduction:
The purpose of this experiment is to monitor the level of enzymatic browning of apple slices. Apples contain an enzyme called polyphenol oxidase (phenolase). In the presence of oxygen from the air, this enzyme catalyses the formation of brown pigments called melanins. Treatment of the apple slices with ascorbic acid, citric acid, or acetic acid will reduce the level of browning. The reduction in browning is dependent on the type of substance and its concentration. oa!ing in water alone will temporarily reduce the level of browning by restricting the amount of oxygen in contact with the apple slice. "hen fruits or vegetables are peeled or cut, enzymes contained in the plant cells are released. In the presence of oxygen from the air, the enzyme phenolase catalyses one step in the biochemical conversion of plant phenolic compounds to form brown brown pigments !nown as melanins. This This reaction, called enzymatic browning, occurs readily at warm temperatures when the p# is between $.% and &.%. The presence of iron or copper can increase the rate of reaction. This can be easily observed when fruit is cut with a rusty !nife or mixed in a copper bowl. 'ruising or other inury to the plant tissue disrupts the arrangement of chemicals within the cells and allows these chemicals to ma!e contact with oxygen. This may lead to browning of uncoo!ed fruits. nzymatic browning can be a significant problem, limiting the shelf life of many fruits and vegetables which have had little heat applied during processing. #owever, enzymatic browning is not always a defect. The browning browning reaction contributes to the desirable colour and flavour of raisins, prunes, coffee, tea, and cocoa.
Objective:
The monitor the level of enzymatic browning of apple slices.
Materials/Apparatus:
*resh fruit+egetable, (Apple) - ascorbic acid - citric acid - acetic acid / acetic acid 'ea!er or cup 0aper towel Tong 1love 2nife topwatch or timer Procedure:
-. The fruit (apple) was slice into 3 pieces with same size. /. The unheated fruit slice was place on a paper towel. The towel was label as 4control5. 6. Another apple slice was dip into one of the test solution (- ascorbic acid) using tong for -% minutes. 7. The tong was rinse and repeat the same procedure for the other three solutions (- citric acid, - acetic acid, / acetic acid). $. The one apple slice was soa! in water for -% minutes, the apple slice was place it on a paper towel and label the towel 4water soa!5. 3. The time and level of browning in table was recorded. The slices every -% minutes for / hours and record your observations.
Results:
8evel of 'rowning
Time (min)
9ntreated (control)
Ascorbic acid (-)
:itric acid (-)
Acetic acid (-)
Acetic acid (/)
"ater soa!ed
%
-
-
-
-
-
-
-%
-
-
-
/
/
-
/%
-
-
/
/
/
-
6%
-
-
/
/
6
/
7%
/
-
/
6
7
/
%$/
-
/
6
7
/
3%
/
-
/
6
7
/
&%
/
-
/
6
7
/
;%
/
-
/
6
7
/
<%
/
-
/
6
7
/
-%%
/
-
/
6
7
/
--%
/
/
/
6
$
/
-/%
/
/
/
6
$
/
Key $ = :ompletely dar! brown 7 = *ully covered light brown 6 = #alf>covered light brown / = light or scant brown patches - = ?o browning present
Discussion:
In this experiment, we need to monitor the level of enzymatic browning of apple slices. nzymatic browning is a chemical process, involving polyphenol oxidase, catechol oxidase, and other enzymes that create melanins and benzo@uinone from natural phenols, resulting in a brown color. In general, enzymatic browning re@uires exposure to oxygen, for example the browning that occurs when an apple is cut. everal substances have been used in the food industry to prevent browning of fruits and vegetables. ulfites prevent browning by releasing sulfite ions, which prevent melanin formation. The Australia ?ew ealand *ood tandards :ode permits the presence of sodium or potassium sulfites in apples for manufacturing purposes to a level of /%%mg+!g (ppm). ulfites are not allowed in foods in Australia unless expressly permitted by the *oods tandards :ode. ?ext, ascorbic acid (vitamin :) acts as an antioxidant. Bxygen is more li!ely to react with the ascorbic acid, rather than with the phenolic compounds in the fruit or vegetable. 'rowning does not proceed until most of the ascorbic acid is used up in the reaction. :itric acid and acetic acid lower the p# of the fruit tissue to retard the action of the phenolase. If the p# is reduced below 6.%, the activity of the phenolase will be severely inhibited. 0lacing fresh fruit in a water bath will temporarily inhibit the browning reaction, since water restricts the amount of oxygen in contact with fruit tissues. #eating also prevents browning by inactivating the phenolase. ince heat also coo!s the fruit, heating cannot be used as a treatment for fruits which will be served fresh. There are some apples seem to brown faster than others. This is because all plant tissues contain polyphenol oxidase (00B), however, the level of 00B activity and concentration of substrate is different each other. The phenolic compounds can vary between varieties of fruits. In addition, a tissueCs 00B level can vary depending on growing conditions and fruit maturity. Bne approach the food industry employs to prevent enzymatic browning is to select fruit varieties that are less susceptible to discoloration either due to lower 00B activity or lower substrate concentration. In the home !itchen enzymatic browning can be prevented by either reducing 00B oxidation activity or lowering the amount of substrate to which the enzyme can bind. :oating freshly cut apples in sugar or syrup can reduce oxygen diffusion and thus slow the browning reaction.
Conclusion:
In conclusion, recent researches have shown that the browning of apples can be prevented or inhibited using a combination of combinations, either natural products or their derivatives that are food additives. arious type combinations of enzymatic inhibitors, reducing agents and anti>microbial compounds containing :a/D were tested. They all slowed down microbial decay and changes in organic acids and sugars during storage. It was also found that irradiating gamma rays could reduce rate of browning of apple uice and increase anti>oxidant activity. This is found to have importance especially in improving the shelf>life of food. Eore evidently, fresh>cut fruits could now be preserved longer without the need of special pac!aging.
Questions:
-.
"hat causes browning when fresh fruits and some vegetables are peeled or cutF The browning is caused by the conversion of phenolic compounds to melanins catalysed by the enzyme polyphenol oxidase.
/.
"hat conditions promote the browning processF "hyF Bxygen is re@uired for the reaction to occur. 'rowning increases with increasing temperature until the polyphenol oxidase is inactivated. The optimal p# for the browning reaction is between $.% and &.%. If the p# is significantly below 6.%, the enzyme will be inactivated. Eetal ions such as iron and copper can increase the rate of the reaction.
6.
#ow do food additives or treatment processes in use today prevent or retard browning in fruits and vegetablesF ulfites will inhibit melanin formation. Ascorbic acid (vitamin :) will prevent oxygen from reacting with the polyphenol oxidase. :itric acid and acetic acid will lower the p#, which decreases the enzyme activity. :itric acid also ties up copper ions and prevents them from participating in the reaction. #eating process inactivates the polyphenol oxidase.
7.
"hy do citrus uices retard browning in fresh fruitsF 'ecause ascorbic acid and citric acid are present in the fruit uices and they lower the p# which may reduce the activity of the enzyme.
References:
-. httpG++www.sciencedaily.com+releases+-<<<+%-+<<%--6%&7;-/.htm.com /. httpG++www.nps.ars.usda.gov+publications+publications.htm.com.F H?B--$=-//<6; 6. 0olyphenol BxidaseJ. "orthington nzyme Eanual. Ketrieved -6 eptember /%--. 7. Eayer, AE (?ovember /%%3). J0olyphenol oxidases in plants and fungiG 1oing placesF A reviewJ. 0hytochemistry 6 (/-)G /6-;/66-. doiG-%.-%-3+.phytochem./%%3.%;.%%3.0EIL -3<&6-;;. $. Huestions and AnswersG B!anagan pecialty *ruits5 ?on>'rowning Apple (vents 1L&76 and 1&;7)J (0L*). A0#I. Ketrieved -; Earch /%-7. 3. Eil!ovich, E. J?on>browning apples cause controversyJ *ruit 1rowers ?ews. April /<, /%--.
References:
-. httpG++www.sciencedaily.com+releases+-<<<+%-+<<%--6%&7;-/.htm /. httpG++www.nps.ars.usda.gov+publications+publications.htmFH?B--$=-//<6;
