Blanching works to keep vegetables crisp and green when cooked because
a. ppos are denatured at high temperature
b. acids keep colors bright
c. bases keep vegetables crisp
d. ice water reverses maillard reactions

a. PPOs are denatured at high temperature

Explanation:

Polyphenol oxidase, PPO, is one of the most studied enzymes in the food industry as it is responsible for enzymatic browning reactions in fruits and vegetables. One of the reasons why its study is important is because commercially it is undesirable, since it modifies the sensory, nutritional and in general quality properties that impair the commercialization of a product.

The importance of controlling polyphenol oxidase is that it largely determines the quality and economic value of fruits and vegetables harvested, stored and processed. Bruising, chopping and other mechanical procedures damage the walls of fruits and vegetables which allows oxygen to penetrate, resulting in darkening or enzymatic browning reactions.

Enzymatic browning can be controlled through the use of chemical and physical methods, often used in combination. The commonly used physical methods are the reduction of temperature, oxygen and the use of modified atmospheres or coating films. The use of chemical methods will depend on what you want to inhibit, be it the enzyme, the substrate (oxygen or phenolic compounds) or the products.

enzyme active site and substrate specificity

enzymes bind with chemical reactants called substrates. there may be one or more substrates for each type of enzyme, depending on the particular chemical reaction. in some reactions, a single-reactant substrate is broken down into multiple products

this age may represent the age of the earth's accretion, of core formation, or of the material from which the earth formed. this dating is based on evidence from radiometric age-dating of meteorite material and is consistent with the radiometric ages of the oldest-known terrestrial and lunar samples.