Food Biochemistry and Food Processing

22 Biochemistry of Fruit Processing 527

filter aid in the filtration process. The filter media
include diatomaceous earth, paper pulp pads, cloth
pads or socks, and ceramic membranes, to name a
few. The filter aid helps prevent the blinding of the

filters and increases throughput. As the fruit ma-

tures, more filter aid will be required. Several types

of filter aids are available; the most commonly used

are diatomaceous earth or cellulose-type materials.

Additional juice can be recovered from the tank bot-

toms or “lees” by centrifugation or filtration. This

recovered juice can be added to the raw juice before

filtration.

Diatomaceous earth, or kieselguhr, is a form of

hydrated silica. It has also been called fossil silica or

infusorial earth. Diatomaceous earth is made up of

the skeletal remains of prehistoric diatoms, single-

celled plants that are related to the algae that grow in

lakes and oceans. Diatomaceous earth filtration is a

three-step operation:(1) building up a firm, thin pro-

tective precoat layer of filter aid, usually of cellu-

lose, on the filter septum (which is usually a fine

wire screen, synthetic cloth, or felt); (2) use of the

correct amount of a diatomite body feed or admix

(about 4.54 kg/3000 cm^2 of filter screen); and (3) the

separation of the spent filter cake from the septum

prior to the next filter cycle.

Prior to the filtration, centrifugation may be used

to remove high molecular weight suspended solids.

In some juice plants, high-speed centrifugation is

used prior to the filtration. This centrifugation step

reduces the solids by about 50%, thus minimizing

the amount of filter aid required. The filtration

process is critical, not only from the point of view of

production, but also for the quality of the end prod-

uct. Both pressure and vacuum filters have been

used with success in juice production (Nelson and

Tresler 1980).

A recent development in the juice industry is

membrane ultrafiltration. Ultrafiltration, based on

membrane separation, has been used with good re-

sults to separate, clarify, and concentrate various

food products. Ultrafiltration of apple juice not only

will clarify the product, but also, depending on the

size of the membrane, will remove the yeast and

mold microorganisms common in apple juice (Fig.

22.5).

Apple Juice Preservation

Preservation of apple juice can be achieved by re-

frigeration, pasteurization, concentration, chemical

treatment, membrane filtration, or irradiation. The

most common method is pasteurization based on

temperature and time of exposure. The juice is heated

Figure 22.4.Apple juice and concentrate flowchart.