| Food Science: Courses: FS 532 |
9. Vacuum Concentration
Introduction
Evaporation is a unit operation in which solvent is removed from a fluid by vaporization. For food fluids which are not particularly heat sensitive, this operation is carried out at atmospheric pressure and temperatures employed will be slightly in excess of 100° C. However, for liquid foods containing heat-sensitive components, the evaporation may be conducted at pressures less than atmospheric thereby reducing the boiling point of the solution. When conducted under vacuum, this operation is called vacuum concentration.
The function of this laboratory exercise is to introduce the student to the unit operation of vacuum concentration and to observe the effects of concentration (moisture removal) on a fluid food product.
References
1969. Food Engineering Operations. Chapter 11. Elsevier Publ. Co., Ltd. N.Y.
1967. Concentration by evaporation. Chapter 16 in Fundamentals of Food Processing Operations. AVI Publ. Co., Inc., Westport, CT.
1975. Food Process Engineering. AVI Publ. Co., Inc., Westport, CT.
Purpose
To demonstrate changes brought about in food materials by concentration under reduced pressure.
Materials
- fresh oranges sufficient to yield three liters of juice
- laboratory vacuum concentration unit
- 0.1 N NaOH
- phenolphthalein, refractometer, dry ice, and acetone
Procedure
Prepare the orange juice and determine its Brix value and acidity (expressed in grams on citric acid per 100 ml).
Grams of citric acid/100 ml = ml NaOH x NaOH x 0.064 x 100 ml of sample Calculate the Brix-acid ratio:
Brix-acid ratio = Brix value grams of citric acid/100 ml Store one pint of fresh juice under refrigeration until needed.
Concentrate the orange juice in the laboratory vacuum concentration unit using the following procedure:
a. Fill the product chamber of the concentrator approximately 1/3 full with juice. Pack the distillate collection flask in dry ice and acetone. Concentrate the juice under reduced pressure and admit any remaining juice as soon as it can be accommodated. Record the pressure and product temperature.
b. After collection of the first 150 ml sample of distillate, remove the collection flask, seal it and store it in dry ice and acetone. Collect the remaining distillate in a flask packed in ice.
c. Determine the Brix value and acidity at Brix values of approximately 20, 30 and 40°. Estimate the point to interrupt the operation from the amount of product in the concentrator and amount of condensate in the collection flask. Remove samples from the vacuum concentration unit via the sampling port on the down flow tube.
Product Evaluation.
a. Divide the concentrate into two equal portions.
b. Reconstitute one portion to the original Brix value by adding the distillate collected in the dry ice/acetone bath and water.
c. Reconstitute the other portion to the original Brix value by adding water.
d. Determine the Brix-acid ration of each reconstituted product and evaluate the reconstituted juices for flavor, color and turbidity.
Questions
1. List the 5 basic components of a vacuum concentrator.
2. Explain the function of a condenser and an entrainment separator.
3. Briefly explain the operating principle of a multiple effect vacuum pan.
4. List the advantages of concentrating heat sensitive products under reduced pressure as compared to normal pressure.
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Copyright © 2003, Department of Food Science, University of Wisconsin-Madison. |
Updated January 16, 2003. foodsci@facstaff.wisc.edu |