Processing of Horticultural Crops

Clarification of Juice

Clarification of Juice: Fruit and vegetable juices are clarified by using different methods like straining or screening, settling or sedimentation and filtration.
a. Straining or screening: Unclarified fruit juices contain varying amounts of suspended matter consisting of broken fruit tissue, seed, skin, pectic substances and protein in colloidal suspension. Seeds and skin which adversely affect the quality of juice are removed by straining through a muslin cloth or sieve. The fruit juices are strained or screened by muslin cloth or stainless steel mesh sieves manually to remove coarse particles in a small-scale industry. But in large industries power operated screening system or filter press is used.

b. Finishing: Citrus juices need finishing for separating cloudy but otherwise clean juice from pulp, rag and seeds. The finisher separates the pulpy matter from the juice by the action of a rotating auger inside a cylinder screen. Screen hole size range from approximately 0.020 to 0.030 inch in diameter, depending on the condition and softness of the fruit. Finishing is judged by the pulp content in the orange fruit juice.

c. Decantation: Decantation is the simplest method of clarification, in which the juice containing solids is allowed to settle down and then clear juice is decanted or siphoned out. Keep juice at low temperature for long periods also helps in setting of solid to allow clarification.

d. Centrifugation: The clouding particles can be separated by centrifugal action. The juice containing solids is fed into a basket or disc type centrifuge, where the centrifugal force separates the light and dense components in each layer. The clear juice is collected and unwanted solids are separated.

e. Enzymes: The plant carbohydrates, pectin, starch and proteins make the colloidal suspension in the freshly extracted fruit juice. The pectinol enzyme is widely used for better juice recovery as well as clarification of fruit juices as it breaks pectin into soluble form thereby freeing the suspended particles which settle down and leaves the juice clear. Similarly, proteolytic and starch liquefying enzymes i.e. amylases are used to remove protein and starch from fruit juices. Pectinol is more effective in the case of acidic juices. Fruit juices can be clarified in about 1-2 hours at 40-50⁰C but requires 20 hrs at 20⁰C.

f. Physical finings: Certain fining agents, which have physical or mechanical action are kaolin, diatomaceous earth, Spanish clay, bentonite or china clay and are known as filter aids. Generally 0.5 to 0.1 percent earth is mixed with fruit juice and then passed through the filter press. Ultra filtration is a process that separates particles based on molecular weight and has better retention of the nutrients in the juice. It is necessary to degrade the pectin enzymatically before ultra-filtration, to reduce viscosity and allow a satisfactory juice.

g. Chemical finings: Gelatin and casein are used to clarify the fruit juices and act partly to neutralize the electrical charged particles and partly by forming insoluble precipitate with the constituents of the juice. The gelatin combines with tannins and casein with acid of the juice. The gelatin may cause juice cloudy if used in excess. Depending on the tannin content of the fruit juice, gelatin solution is mixed and allowed to stand for 18 to 24 hrs to ensure that the precipitated matter clots together and settles down. The clarified juice is then siphoned off. Albumin (egg white) can also be used in clarification of juices.

h. Clarification by freezing: Grape juice contains cream of tartar or potassium hydrogen tartarate along with pulp and skin which is removed by freezing and thawing the juice or by refrigeration for a long storage. Apple juice can also be clarified using this method after freezing is precipitated on thawing.

i. Clarification by heating: During heating, the colloidal material in fruit juices coagulates and settles down on cooling which can be separated by using a filter press. For clarification of apple and pomegranate juice, the juice is heated to 80-85⁰C for few minutes and cooled immediately followed by filtration by passing the juice through a filter press.

j. Addition of sugars: All juices are sweetened by adding sugar, except those of grapes and apple. Sugar can be added directly to the juice or as syrup made by dissolving it in water. Fruit squash, cordial, syrups are made by adding appropriate quantity of sugar into the pulp or juice using cane sugar. Similarly ready to serve drinks and fruit nectar are sweetened by using cane sugar.