Citrus pectic enzymes

The quality of extracted citrus juices depends on enzyme reactions that occur not only in the fruit during the development period, but also in the juice during processing. When juice is extracted from citrus fruit, enzymes are released from their normal restraint in the cell. Several of these enzymes catalyze reactions that adversely affect taste and appearance of the juice. Unless the reactions are controlled, the juice products will not meet the standards of quality set up by the USDA Food Safety and Quality Service. The two reactions of commercial importance are the hydrolysis of pectin to pectic acid, which clarifies juice, and the lactonization of limonoic acid A-ring lactone to the bitter compound, limonin. Research efforts to identify and characterize the reactions, to isolate and purify the enzymes, and to develop methods to control the reactions are described in this review. 

Also he speculated that citrus juices contain two cloud-coagulating enzymes of different thermostabilities. One enzyme, most active at low pH and temperature, appeared to be destroyed by heating the juice at 65 to 70°C (149 to 158°F). The second enzyme, most active at pH 3.0 to 3.3 and about 35°C (95°F), appeared to require heating to 888C (191°F) for inactivation (12). Stevens (13) described a rapid test for pectic enzymes in citrus juice. It involved adding pectin under controlled conditions of temperature and sample preparation, and measuring the time required for flocculation. Stevens, and coworkers (14) further elaborated on the patent work (12, 13). They produced a trend curve of the... 

A quantitative objective measurement of citrus juice turbidity was used by Loeffler (15, 16) to show that pectic enzyme changes occurred so rapidly after the juice was reamed from the fruit that at least a partial coagulation of the cloud occurred before the juice could be screened, deaerated and heated to a pasteurization temperature. He showed that juice turbidity was increased by flash-pasteurization and also by homogenization of the juice before pasteurization. Loeffler (15, 16) presented data on turbidity of flash-pasteurized citrus juices (heat exposure for 16 to 18 sec) after storage at several temperatures. He found that "samples pasteurized at 918C (196°F) lost their cloud when stored at 35°F (95°F) but others pasteurized at 93-95°C (199-2038F) retained their cloud almost indefinitely". 

Pectin is a long chain of pectic acid and pectinic acid molecules. Because these acids are sugars, pectin is categorized as a polysaccharide. It is prepared from citrus peels and the remains of apples after they are squeezed for juice. In the plant, pectin is the material that joins the plant cells together. When fungus enzymes break down the pectin in fruit, the fruit gets soft and mushy. 

Similarly, there are present in citrus juices active enzyme systems that destroy the important pectic constituents. These enzymes have to be rapidly inactivated to protect the desirable cloud (turbidity) which is apparently stabilized by pectic substances. ... 

Pectolytic enzymes are described in section 4.4.5.2. Pectic acid which is liberated by pectin methylesterases flocculates in the presence of Ca + ions. This reaction is responsible for the undesired cloud flocculation in citrus juices. After thermal inactivation of the enzyme at about 90 °C, this reaction is not observable. However, such treatment brings about deterioration of the aroma of the juice. Investigations of the pectin esterase of orange peel have shown that the enzyme activity is affected by competitive inhibitors oligogalacturonic acid and pectic acid (cf. Fig. 2.51). Thus, the increase in turbidity of citrus juice can be prevented by the addition of such cort5)ounds. 

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