How does blanching affect the enzyme polyphenol oxidase to stop the browning process?

 

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Polyphenol oxidase is a chemical enzyme found in fruits and vegetables that catalyzes the oxidation of phenols to quinones when exposed to air.  These quinones can then form polymers that are dark in color and that is why fruit will brown over time, especially if the skin is pierced thus exposing the inside to oxygen.  Blanching is a process that deactiviates the enzyme to slow down the browning process so the fruit or vegetable will remain fresher for longer.  Basically is it heating the food to 75-100 degrees Celsius for a short period of time to deactivate the polyphenol oxidase.  Enzymes are very temperature dependent to work properly and heating them even for a short period of time will cause then to distort their shape and therefore deactivate them from their chemical process.

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Explain how blanching affects the enzyme, polyphenol oxidase, to stop the browning process.While watching a cooking show, you learn that the browning of fruits such as apples is a chemical reaction. It is catalyzed by an enzyme called polyphenol oxidase. To prevent browning, the cooking show suggests that you blanch the fruit by placing it on boiling water or steam for a short time. Question Explain how blanching affects the enzyme, polyphenol oxidase, to stop the browning process.

Polyphenol oxidase, or PPO, is common plant enzyme which is believed to play a role in the plant's ability to resist pathogens; in some plants it may also have a role in defense against damage by herbivores. PPO has been intensively studied by the food industry. When cell damage occurs, PPO is released and, as its name indicates, it oxidizes phenol compounds. The addition of oxygen causes monophenols to become linked into larger biphenols and polyphenols, many of which serve as pigments and cause the appearance of browning or bruising.

There are several ways cooks avoid browning of cut fruits and vegetables which, while harmless, is generally considered unattractive. One option is to keep oxygen away from the cut surfaces. Submerging the food in water or wrapping it tightly will accomplish this over the short term.

There are also ways to disable PPO. One is to adjust the pH; PPO works best at a pH of 6.0-7.0, and is completely disabled at pH levels below 3.0; hence many cooks will add lemon juice to cut fruits to reduce browning. The other option is blanching. To blanch a food, you must apply heat to the food for a very short time, and then cool it rapidly; the objective is to apply enough heat, in the form of steam or immersion in boiling water, to disable the PPO, but not enough to cook the food.  Proteins like PPO have complex folded structures, and the energy of heat will disrupt the folding and make the molecule nonfunctional; we call this process denaturation.

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You would like to add some decorative slices of apple to the top of a baking desert. Whenever you try this, unfortunately, the apples brown before you have the chance to serve them to your dinner guests. While watching a cooking show, you learn that the browning of fruits such as apples is a chemical reaction. It is catalyzed by an enzyme called polyphenol oxidase. To prevent browning, the cooking show suggests that you blanch the fruit by placing it in boiling water or steam for a short time. Explain how blanching affects the enzyme, polyphenol oxidase, to stop the browning process.

Polyphenol oxidase is an enzyme contained in many fruits.  It catalyzes the oxidation of phenols into catechols by using oxygen from the air.  These catechols themselves then oxidize to quinones, which polymerize to brown colored pigments.  This is the reason for the browning of fruits, especially when the skin is pierced, thus letting oxygen contact the inside of the fruit.  If you heat the fruit in boiling water or steam it denatures the enzyme.  This means that the enzyme loses its shape and therefore its activity.  The amino acids are still connected to each other, its just that the tertiary folding structure of the overall enzyme is disrupted.  When the polyphenol oxidase loses its structure, it cannot perform its catalytic role in oxidation and the fruit will stay naturally colored for longer.  There are other ways to denature an enzyme including the use of surfactants (soaps), but heating is effective for cooking and won't spoil the taste.

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How does blanching affect the enzyme, polyphenol oxidase, to stop the browning process in fruit?

The browning of fruit over time is a chemical process.  Polyphenols are chemicals naturally found inside the fleshy bodies of all fruits (and other foods too).  Enzymes called polyphenol oxidases can oxidize these polyphenols into quinone polymers which have a distinctive dark color.  Oxygen is also required for this reaction.  Inside the fruit, the chemicals are protected from oxygen.  But when the skin is pierced, oxygen enters the fruit and the oxidation reaction occurs.  Blanching is a cooking technique where the fruit is exposed temporarily to high heat.  This causes the enzyme to denature, or fall apart.  When the enzyme unfolds and comes apart, it loses its activity and the browning oxidation reaction is halted.

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You would like to add some decorative slices of apple to the top of a baked dessert. Whenever you’ve tried this, unfortunately, the apples brown before you have the chance to serve them to your dinner guests. While watching a cooking show, you learn that the browning of fruits such as apples is a chemical reaction. It is catalyzed by an enzyme called polyphenol oxidase. To prevent browning, the cooking show suggests that you blanch the fruit by placing it in boiling water or steam for a short time. Explain how blanching affects the enzyme, polyphenol oxidase, to stop the browning process.

The short answer is that under the high-temperature conditions of the boiling water or steam, the polyphenol oxidase enzyme is denatured. Most likely your instructor wants you to give an explanation of what denaturation is, how it occurs, and how it relates to enzyme structure and function.

Biological enzymes such as polyphenol oxidase are proteins—long chains of amino acids joined together by peptide bonds. There are a variety of different amino acids that differ in their R groups. The important thing about these differences is that some are polar, meaning that they have one atom with a slight positive charge and another with a slight negative charge. The chain of amino acids is somewhat flexible and tends to settle into an arrangement where a positive part of one amino acid is near a negative part of another. This arrangement is known as a “configuration,” and protein configurations are generally stable (meaning that they don't change) at room temperature.

The configuration of a protein is important because the “chain” of amino acids is wrapped and condensed into a three-dimensional shape. This shape is essential to a protein’s function. In the case of an enzyme, the shape brings reactants in a chemical reaction together in an orientation that allows the reaction to occur at biological temperatures (many reactions would only occur at much higher temperatures—unsuitable for life—in the absence of enzymes).

The shape of a functional enzyme, however, is only one of many possible configurations that a particular amino acid chain could potentially adopt. Proteins fold as they emerge from ribosomes during synthesis, taking on their one and only possible configuration that is actually functional. If the configuration of an enzyme were disrupted, without breaking any chemical bonds, the chance that it would return to its functional configuration, instead of some other random configuration, would be infinitesimal.

This is what denaturation is. Heating the enzyme causes the atoms in it to vibrate and move, and the energy and motion can overcome the electrostatic attractions between different R groups. We envision the amino acid chain flopping around. When cooled, it settles into one of the millions of possible configurations available to it—but not into the functional configuration. It no longer has the shape needed to promote the oxidation reaction that produces the brown color, so browning does not occur.

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