Food Science: Courses: FS 532

11. Drum Drying

Introduction

One of the most economical drying methods is drum drying. In this operation, food slurry is contacted with a hot, revolving drum to form a thin layer on the surface. After sufficient residence time to allow the evaporation of water the product is removed from the drum by a scraper device (called a doctor knife) located usually 1/2 to 3/4 of a revolution from the point of application.

Factors affecting the rate of drying and final moisture content are:

  1. Residence time on the drum
  2. Surface temperature
  3. Film thickness

The method of dehydration can only be applied to food slurries or liquid food systems and the product must be able to withstand high temperature-short time exposures without undergoing severe quality changes. It has been most successfully applied to drying of potatoes and has been used to dry milk, soup mixes, baby foods and dried fruit purees (e.g. apple sauce).

Production of Potato Flakes

"Potato flakes" is a term used to identify mashed potatoes that have been dehydrated on a drum dryer. Desirable characteristics of factors which influence the quality of potato flakes include: (1) high starch content, (2) strong cell walls to avoid release of starch from the cell, (3) processing techniques which minimize cell rupture, (4) addition of antioxidants and other color stabilizers, and (5) low oxygen and water permeability packaging. The basic processing steps are: (1) wash, (2) steam or lye peel, (3) trim, (4) slice to slabs, (5) precook (e.g. 20 min at 160°F ), (6) steam cook until soft enough to rice, (7) add antioxidants and other stabilizers, (8) drum drying, (9) breaking and screening, and (10) packaging.

Factors affecting efficient operation of the drying process are: (1) drum clearance (avoid cell rupture), (2) drum speed, (3) solids content of potatoes (20-22%), (4) sheet density, (5) steam pressure, and (6) final product moisture.

Production of Apple Flakes

Special techniques are necessary for drum drying fruit purees or other food products containing large amounts of sugars. The sugars are concentrated in the liquid as drying proceeds and frequently the sugar mass is molten rather than crystallized as the sheet is removed from the drum. Under these conditions the product sheet wrinkles at the doctor blade forming sticks of product. These sticks are very hard to disperse and are unsatisfactory in physical appearance. Many devices have been developed to circumvent this problem but the most successful has involved careful control of the properties of the air at the doctor blades. The water vapor must be rapidly removed from above and below the drums and a steam of dry, cool air is directed at the product just before removal at the doctor blade. In addition take-off rolls are used to control the final thickness of the dried sheet.

Purpose

To familiarize the student with the drum drying operation.

To determine the effect of some of the variables in drum drying.

Materials:
Procedure
  1. Drum Drying skim milk

    Weigh 1000g of milk and place it in separation flask. The group should make sure that the steam pressure remains constant, record time and control the flowrate of the milk. Dry the milk by slowly dropping the milk onto the surface of the drum dryer. Record the time to dry the skim milk and temperature of the drum dryer.

    Sample no. Steam pressure Drum RPM* Gap Clearance
    1 40 psig 31/3 0.0025 in
    2 60 psig 31/3 0.0025 in
    3 40 psig 71/4 0.0025 in
    4 60 psig 71/4 0.0025 in

    *Setting of 5 = 31/3 rpm

    Setting of 9 = 71/4 rpm

    Weigh the dried product and report the time necessary to dry each batch. Calculate the drying rate as pounds of water/min/ft2. Store the powder in a ziplock bag with the date, sample name, the powder weight and group name written on it. (will be used for the drying evaluation lab)

  2. Drum Drying potatoes

    Make a potato slurry by peeling the potatoes (about 3-4 spuds). Cook them in boiling water for 20 min and mash the potatoes using the blender. Add 500g of water and blend the mix. Weigh about 500g of the mix and place it inside a separation flask. Dry the mix by slowly dropping it onto the surface of the drum dryer. Record the time to dry the slurry.

    Steam Pressure Drum RPM Gap Clearance
    60 psig 31/3 0.025 in

    Weigh the final product and record the drying time. Calculate the drying rate in lb water/min/ft2. Make observations on the color, flavor and rehydration characteristics of the product. Store the powder in a ziplock bag with the date, sample name, the powder weight and group name written on it. (will be used for the drying evaluation lab)

  3. Drum Drying Applesauce

    Dry 200g applesauce using the same settings as for potato. Report any problems and observations on the drying operation.

Questions
  1. Why do people in the baking industry sometimes prefer dried milk powder to higher quality spray dried milk powder?
  2. Tomato powder tends to cake during storage. How is this problem overcome?

Worksheet [PDF]

University of Wisconsin College of Agricultural & Life Sciences
Copyright © 2003, Department of Food Science,
University of Wisconsin-Madison. 		Updated March 12, 2003.
foodsci@facstaff.wisc.edu