Several days after choosing peroxidase inactivation, it looks like I naively picked the wrong fight. From what I have read so far, peroxidase may be the most heat resistant enzyme [5] in fruits and vegetables but I now think it isn't the worst enemy.
In preserving the quality of fruits and vegetables, blanching (currently thermal treatment) is done to inactivate enzymes. By inactivating enzymes, biochemical reactions in plant tissues can be stopped thereby halting aging and other quality degradation.
How tough is peroxidase? Peroxidase in coconut water requires at least 90 0C microwave-induced thermal treatment to effect inactivation [4]. Other enzymes can be denatured at a lesser amount of heat. At 10-20% (w/w) sucrose solution, peroxidase has greater thermal stability [2].
Being the most heat stable, peroxidase (enzyme classification, EC 1.11.1.7) is the strategic target of blanching. Once inactivated, it is assumed that all other enzymes are considered denatured already. Hence, no further tests for other specific enzymes are required [5].
In the past several days, I have been thinking why should I hit peroxidase? What specific damage does it bring to plant tissues?
So far, I haven't found any serious charge that can be lodged against it. On the contrary, I found some benefits from the presence of peroxidase in plant tissues. It has been reported that respiration produces active oxygen species (harmful compounds) such as O2 - and peroxide (H2O2). Together, these form hydroxyl radical (OH-) and singlet oxygen, which is considered the most reactive species in chemistry, according to Trujillo and others in their 2003 paper published in the Journal of Agricultural Food and Chemistry. These compounds can cause lipid peroxidation, protein denaturation and DNA mutation [4]. Enzymes like peroxidase can prevent this excessive damage by breaking down the active oxygen species before they even become harmful radicals. For instance, peroxidase can break down peroxide into harmless water and oxygen molecules.
I remember in our microbiology class, peroxide can inhibit growth of some microorganisms. To survive, some bacteria produce their own peroxidase to neutralize peroxide. So, it is alright to let peroxide untouched by peroxidase in plant tissues to inhibit growth of peroxidase-negative microbes. In this case, inactivating peroxidase in plant tissues looks wise but I doubt if there is a significant microbial threat inside the plant tissues. Healthy, fresh fruits and vegetables should have minimal microbial load at harvest and these germs are likely on the surface, not inside the tissues where peroxide exists.
I also recall that peroxide (agua oxenada) is used to treat wounds. The peroxide works by oxidizing the microbes in the wound making the injured area more sterile thereby protecting us against infection. The reactivity of peroxide is generally lethal to microorganisms (and tolerable for humans), but I now digress.
In fruits and vegetables, I think it is better to ignore peroxidase and instead focus on other more damaging enzymes. This brings me to polyphenoloxidase (EC 1.14.18.1), an enzyme associated with browning in fruits and vegetables [1]. Notice that some fruits and vegetables become brown when tissues are cut and exposed to oxygen. Examples are potato, sweet potato, apple, mushroom and banana.
Browning happens when phenols in fruits and vegetables are oxidized into quinones by polyphenoloxidase in the presence of oxygen. Upon condensation, these quinones further react with phenolic compounds and amino acids to form complex brown polymers [1].
Treating fruits and vegetables with sulfites is one common way of preventing browning but sulfites have been associated also with allergic reactions, especially among sensitive asthmatics [1]. Some acids also inhibit to some extent the activity of polyphenoloxidase.
In my case, my interest is inhibiting polyphenoloxidase activity or inactivating polyphenoloxidase altogether using physical treatment (electric field), not chemical intervention.
My next concern was, what product should I choose? After some contemplation, I think banana is an excellent experimental sample because of two things. First, browning in banana tissues can significantly turn off consumers. In potato and mushroom, I think browning may not be as noticeable and customers are less likely to take issue with the discoloration. Browning of apple is also a serious concern but Philippines does not produce apple. This is my second reason for my pick. Banana is an economically important export commodity of the Philippines. By working on the polyphenoloxidase in bananas, I might stumble into a precious discovery.
If I can inactivate polyphenoloxidase in bananas, will it prevent browning even if a banana slice is left on the table, exposed in the air? Will there be no more discoloration?
If you are an executive or scientist from a multinational corporation (like Dole) involved in banana export, will you be interested in my work?
By getting affirmative answers to the questions above, I'll certainly go bananas!
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[1] Chaisakdanugull, C., Theerakukait, C. and Wrolstad, R. 2007. Pineapple juice and its fractions in enzymatic browning inhibition of banana [Musa (AAA group) Gros Michel]. J. Agric. Food Chem. 55 (2007) 4252-4257.
[2] Chang, B., Park, K. and Lund, D. 1988. Thermal inactivation kinetics of horseradish peroxidase. Journal of Food Science, May 1988, Vol 53, Issue 3, pp. 920-923.
[3] Fernandez-Trujillo, J., Nock, J., Kupferman, E., Brown, S. and Watkins, C. 2003. Peroxidase activity and superficial scald development in apple fruit. Journal of Agricultural and Food Chemistry, 51 (2003) 7182-7186.
[4] Matsui, K., Granado, L., de Oliviera, P. and Tadini, C. 2007. Peroxidase and polyphenol oxidase thermal inactivation by microwaves in green coconut water simulated solutions. J. of Food Science and Technology, June 2007, Vol. 40, Issue 5, pp 852-859.
[5] USDA. 1975. Enzyme inactivation tests for frozen vegetables. http://www.ams.usda.gov/fv/ppbweb/PPBfilecodes/135a12.pdf. Last access: April 4, 2008.
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Some interesting pieces about bananas here:
1] here, there and everywhere
2] vegparadise
Saturday, April 05, 2008
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