Food Biochemistry and Food Processing

526 Part V: Fruits, Vegetables, and Cereals

hydrating 1% gelatin in warm 60°C water. Gelatin
can be added either in combination with the enzyme
treatment, bentonite, or midway through the enzyme
treatment period. The positively charged gelatin
facilitates the removal of the negatively charged sus-
pended colloidal material from the juice. Bentonite
can be added in the range of 1.25–2.5 kg of rehy-
drated bentonite per 4000 L of juice. Bentonite is
also added to increase the efficiency of settling, pro-
tein removal, and prevention of cloudiness caused

by metal ions. For cloudy and natural apple juice,

enzymes are usually not used. The enzyme-treated,

refined, and settled apple juice is then pumped from

the settled material (lees) and further clarified by fil-

tration.

The filtration of apple juice is done with or with-

out a filter aid. The major types of filters are pres-

sure leaf, rotary vacuum, frame, belt, and Millipore

filters. To obtain the desired product color and clari-

ty, most juice manufacturers use a filter medium or

Figure 22.3.Pectin degradation by pectin methylesterase, polygalacturonase, and pectin transeliminase.