Food Biochemistry and Food Processing (2 edition)

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564 Part 5: Fruits, Vegetables, and Cereals

R

O

1 μ = 1000 nm

Microparticles

Colloids

Ions and molecules

10 μ

0.1 nm

2 nm

0.3 μ M

F

F

U

F

Figure 28.5.Membrane filtration. RO, reverse osmosis; UF,

ultrafiltration; MF, microfiltration; F, filtration.

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 fil-

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

production, but also for the quality of the end product. Both

pressure and vacuum filters have been used with success in

juice production (Nelson and Tresler 1980). A recent develop-

ment in the juice industry is the membrane ultrafiltration. Ul-

trafiltration, based on membrane separation, has been used with

good results to separate, clarify, and concentrate various food

products. Ultrafiltration of apple juice will not only clarify the

products, but depending on the size of the membrane, will also

remove the yeast and mold microorganisms common in apple

juice (Fig. 28.5).

Apple Juice Preservation

Preservation of apple juice can be achieved by refrigeration, pas-

teurization, concentration, chemical treatment, membrane filtra-

tion, or irradiation. The most common method is pasteurization

based on temperature and time of exposure. The juice is heated

to over 83◦C, held for 3 minutes, filled hot into the container

(cans or bottles), and hermetically sealed. The apple juice is held

for 1 minute at 83◦C and then cooled to less than 37◦C. When

containers are closed hot and then cooled, vacuum develops,

reducing the available oxygen that also aids in the prevention of

microbial growth. After the heat treatment, the juice may also

be stored in bulk containers. Aseptic packaging is another com-

mon process where, after pasteurization, the juice is cooled and

packed in a closed, commercially sterile system under aseptic

conditions. This process provides shelf-stable juice in laminated,

soft-sided consumer cartons, bag-in-box cartons, or aseptic bags

in 200–250 liter drums.

Apple juice concentration is another common method of

preservation. The single-strength apple juice is concentrated

by evaporation, preferably to 70◦or 71◦Brix. By an alternate

method, the single-strength juice is preconcentrated by reverse

osmosis to about 40◦Brix, then further concentrated by evapo-

ration methods. The reduced water content and natural acidity

make the final concentrated apple juice shelf stable at room

temperature. There are several evaporators used in apple juice

production including rising film evaporators, falling film evap-

orators, and multiple effect tubular and plate evaporators. Due

to the heat sensitivity of the apple juice, the multiple-effect

evaporator with aroma recovery is the most commonly used.

The general method in a multiple-effect evaporator is to heat the

juice to about 90◦C and capture the volatile (aroma) components

by cooling and condensation. This is followed by reheating the

20–25◦Brix juice concentrate in the first stage to about 100◦C

and then concentrate it to about 40–45◦Brix. Another stage of

heating and evaporation at about 50◦Cto60◦Brix, and final heat-

ing and concentration in the fourth stage to 71◦Brix provides

fully concentrated juice. The warm concentrate is then chilled

to 4–5◦C prior to adjusting the Brix to 70◦before barreling or

bulk storage.

Chemicals such as benzoic acid, sorbic acid, and sulfite are

sometimes used to reduce spoilage of unpasteurized apple juice,

either in bulk or as an aid in helping to preserve refrigerated

products. Application of irradiation and ultrasonic sound are

new emerging methods of preservation with high potential even

though not fully accepted by consumers at this time.

Apple essence is recovered during the concentration of ap-

ple juice. The identification of volatile apple constituents, com-

monly known as essence or aroma, has been the subject of

considerable research. In 1967, researchers at the USDA iden-

tified 56 separate compounds from apple essence. These com-

pounds were further refined by organoleptic identification, using

a trained panel of sensory specialists. These laboratory eval-

uations revealed 18 threshold compounds identified as “De-

licious” apple compounds consisting of alcohols, aldehydes,

and esters. Three of the eighteen compounds had “apple-like

aromas” according to the taste panel. These were 1-hexanal,

trans-2-hexenal, and ethyl, 2-methyl butyrate (Flath et al. 1967,

Somogyi et al. 1996b).

Processed Apple Products

The popularity of blended traditional/tropical juice products has

pushed the consumption of traditional fruit juices such as apple,

orange, and grape juices to 25 liters in 2002, an increase of more

than 24% from 1992 (Statistics Canada 2003). Apples are nor-

mally processed into a variety of products, although apple juice

is the most popular processed apple product. With a produc-

tion of 465,418 metric tons of apple in 2001, Canada exported