Method and apparatus for drying fruit pulp and the like

A method and apparatus for producing a dried material from a mixture of solid particles colloidally suspended in a liquid, for example, for the production of fruit leather from a pulp and fruit juice mass, include floating an infrared-transparent film on the surface of a body of heated water and placing the material to be dried, such as the pulp and juice mix, on top of the film to absorb heat from the water and thereby cause drying of the material. The apparatus for carrying out the method includes a reservoir of water, a film floated on the water, and a heater for maintaining the temperature of the water at a predetermined level. In a preferred embodiment, the reservoir is in the form of an elongated trough and the film comprises an endless strip of polyester material, such as Mylar, to form a conveyor belt that floats on the surface of the water in the trough and carries the material to be dried on it.

BACKGROUND OF THE INVENTION 
This invention relates to the drying of fruit pulp and the like and, more 
particularly, to the efficient transfer of heat to the material being 
dried. 
Most presently used processes for drying fruit pulp to produce, for 
example, fruit leathers or concentrates of the pulp, involve placing the 
pulp on a carrier and then heating the carrier to dry the pulp. In these 
systems the entire carrier is heated, regardless of whether it contains 
pulp or not and a substantial heat transfer occurs from the carrier to the 
air in places on the carrier where there is no pulp, without any 
appreciable use of that heat in the drying of the pulp. For example, in 
the system of Shepard, shown in U.S. Pat. No. 2,301,589, issued Nov. 10, 
1942, a metallic carrier is provided and the pulp is laid on the metallic 
carrier. The carrier is then heated through electrical induction and the 
heat used to drive the moisture from the pulp. Since the entire carrier is 
heated, it is necessary to ensure that every portion of the carrier is 
covered with pulp in order to obtain the most efficient heat transfer from 
the carrier to the pulp. In actual practice, it is difficult if not 
impossible to obtain such complete coverage of the carrier and therefore 
heat is lost to the open air without doing any substantial work in drying 
the pulp, causing more heat to be consumed than is necessary simply to dry 
the pulp. In a system like that of Shepard's, the only way to obtain 100 
percent heat transfer from the carrier medium to the pulp is to have an 
inspection and control station to monitor the coverage of the carrier by 
the pulp and adjust the pulp so that it covers every open area of the 
carrier. This causes additional problems in labor and monitoring controls 
that may be enough to offset any gain in efficiency of heating. 
SUMMARY OF THE INVENTION 
The present invention provides a method and apparatus for drying fruit pulp 
and other like materials, in which the transfer of heat from the heated 
medium to the pulp occurs most efficiently and that substantially 
eliminates heat loss in areas of a carrier medium carrying the pulp that 
are open to the air and are not covered by the pulp. More specifically, 
the present invention provides a method of drying a product, such as pulp, 
that includes the steps of floating a film of transparent material on a 
body of water, heating the water to maintain the water at a predetermined 
temperature, and placing the product to be dried on the floating film. In 
order to carry out this method, an apparatus is provided that includes a 
reservoir of water and means associated with the reservoir of water for 
heating the water to the predetermined temperature and maintaining the 
reservoir of water at that predetermined temperature. A transparent, solid 
film is floated on the surface of the heated water and is adapted to 
receive the pulp or other material to be dried. 
In preferred forms of the invention the floating material can be a flexible 
plastic and can be arranged on a conveyor system so that it moves across 
the reservoir of water carrying the pulp from a loading station to a 
removal station with the timing of the conveyor belt and the parameters of 
the product being applied to the conveyor belt being such that the product 
is applied in a colloidal suspension of solid particles in a liquid base 
at one end of the conveyor and removed in its dried state at the second 
end of the conveyor. While certain preferred materials are known for use 
as the floating carrier film, it is sufficient if the carrier is 
transparent to infrared radiation and able to withstand temperatures in 
the range of the boiling point of water without shrinkage or deterioration 
of the material.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
In accordance with the present method, a reservoir of water, such as in a 
trough, is heated to some predetermined temperature. Preferably, the 
temperature of the water is maintained as close as possible to the boiling 
point of water without actually producing boiling. Therefore, at sea 
level, the temperature would be close to 100 degrees C. A thin film of 
transparent material, such as a polyester film, is floated on the water 
surface in the trough so that it covers substantially the entire open 
surface area of the water. Because of the reflective properties of the 
water surface and the film covering the water, there is little, if any, 
evaporative heat transfer from the water to the air above it. When a 
product to be dried, such as a puree of fruit pulp, is placed on the film, 
the portion of the film carrying the product is brought into intimate 
contact with the surface of the water. The contact between the film 
carrying the product to be dried and the surface of the water enables a 
transfer of heat directly from the water through the film into the product 
to be dried, causing the moisture in the product to be driven off. There 
is substantially no transfer of heat from the water at those surface 
locations not covered by the product and still covered by the film because 
of the reflective properties of the water surface and therefore the heat 
transfer from the water is confined to the product to be dried, such that 
the maximum efficient use of the heat is accomplished. 
It is necessary that the film covering the water be able to withstand the 
heat and the temperature of the water without deteriorating or shrinking. 
It is also necessary that the film be transparent to infrared heat 
radiation, which, in turn, means that it will typically be optically 
transparent as well, although, it is the transparency to the infrared 
wavelengths that is important and not the passage of visible light. 
An embodiment of an apparatus useful in a continuous drying process to 
carry out the method described above is shown in FIGS. 1 and 2. A trough 
10 is formed of some nonporous medium, such as ceramic tile, and acts as a 
receptacle for the heated water used in the drying process. The ceramic 
tile trough is found to be desirable because of its low heat-conducting 
properties and also its ability to be easily cleaned, for use with edible 
foostuffs; however, any suitable rigid trough will suffice, for example, 
one made of stainless steel. In order to minimize the heat loss of water 
in the trough, the trough is additionally insulated by insulation layer 
12, positioned below the trough. The heated water 14 is pumped into the 
trough from a source 15 of heated water, which can be an electric water 
heater, or a boiler fueled by any suitable fuel source, such as natural 
gas, wood, coal, et cetera. In practice, the water flows into the trough 
at a first end and the trough is preferably tilted slightly along its 
length so that the water runs by the force of gravity to a second end of 
the trough from which it is pumped back to the hot water source, reheated 
to the desired temperature, and then pumped back into the trough so that 
the water in the trough is always maintained at the predetermined, desired 
temperature. In the illustrated embodiment, a polyethylene sheet 16 lines 
the inner surface of the trough 10 and overhangs the edges of the trough 
to provide an additional moisture and heat barrier between the water and 
the trough. In practice, it has been found that a moisture layer of 
condensation builds up on the underside of the polyethylene sheet 16 
between the sheet and the interior of the trough, which acts as a further 
heat barrier, thereby minimizing heat loss from the water to the walls of 
the trough 10. A flexible sheet of infrared transparent material 18 floats 
on the surface of the water 14 in the trough. A suitable material has been 
found to be a 3-mil-thick sheet of polyester film available from E. I. 
Dupont De Nemours Co. under the name 300A Mylar. The 300A Mylar meets all 
of the requirements of the invention in that it shows very little 
distortion or shrinkage in the operable temperature range. It is flexible 
so that it lends itself to a conveyor belt type of application and also 
has the strength required to operate for long periods of time without 
breakage. While the polyester sheet 18 only needs to float on top of the 
water in the illustrated embodiment, the polyester sheet is wider than the 
trough 10 so that it not only overlies the water but also rides slightly 
up the sides of the trough. The extra width of the polyester sheet 18 
ensures a complete coverage of the water so that there is no evaporative 
exposure of the water to the open air above the trough. The overwide sheet 
18 also provides a channel that keeps the product to be dried, typically a 
fruit pulp puree consisting of solid particles in suspension in a fruit 
juice, from running off of the polyester sheet into the hot water. The 
material to be dried, for example, fruit pulp 20, is placed on top of the 
polyester sheet 18 and remains on the polyester sheet until it reaches the 
proper consistency or dryness, that is, until a predetermined amount of 
the moisture is driven off the mixture by the heat transferred from the 
hot water to the fruit pulp through the polyester sheet. 
Depending on the final product sought, the thickness of the fruit pulp will 
vary; however, it has been found that a thickness of approximately 
one-eighth to three-sixteenths of an inch is capable of producing a 
satisfactory fruit leather. In the illustrated embodiment, a thin sheet 22 
of material such as polyethylene, for example, 0.5 to 1.5 mils thick, is 
placed between the polyester sheet 18 and the fruit pulp 20. The 
polyethylene sheet 22 does not enter directly into the workings of the 
process of the present invention, but, rather, supplies a convenient 
medium upon which to place the fruit pulp to prevent adherence of the pulp 
to the polyester sheet 18. The polyethylene sheet 22 is removed from the 
polyester sheet 18 with the fruit pulp to maintain the polyester sheet 18 
in a clean condition suitable for use with edible footstuffs. Also, the 
polyethylene sheet separates easily from the Mylar sheet 18 so that the 
Mylar sheet is not damaged as it might be if it was necessary to scrape 
the dried fruit leather directly off of the polyester sheet. The 
polyethylene sheet 22 and dried fruit leather can be rolled simultaneously 
from the polyester sheet to provide a convenient method by which the fruit 
leather is transported and/or stored after its removal from the dryer. 
In one preferred form of the invention, the polyester sheet 18 is formed in 
an endless belt and is carried by a pair of rollers 24 and 26, 
respectively, mounted at either end of the trough to form a conveyor belt. 
The fruit pulp 20 is introduced onto the polyester sheet at a first end of 
the trough from a discharge means 28. The polyethylene sheet 22 is fed 
from a roll 30, also at the first end of the trough 10, so that the pulp 
20 lies on the polyethylene sheet 22. The pulp moves over the hot water 14 
on the polyester sheet conveyor belt 18 to the second end of the trough 
10. The speed of the conveyor belt is regulated so that the time that it 
takes for the fruit pulp to travel from one end of the trough to the other 
is sufficient to produce the proper drying of the fruit pulp so that it 
can be removed along with the polyethylene sheet 22 at the second end of 
the trough. The polyester belt 18 continues on under the trough 10 in a 
typical conveyor fashion, while the dried pulp is moved away on a second 
conveyor 32. In one form of the invention, which has been tested, it has 
been found that a fruit pulp of one-eighth to three-sixteenths inch 
thickness placed on a Mylar sheet 3 mils thick over a trough of water in 
which the water depth is approximately one inch and the water is heated to 
just below the boiling point, that is, just below 100 degrees C., takes 
approximately two and one-half hours to dry to a suitable fruit leather 
dryness. It is necessary to keep a slack in the polyester conveyor belt 18 
so that the film floats on the water and is not under such tension that it 
resists contact with the water surface when the pulp is placed on the 
belt. Typically, the fruit pulp will be open to the air during its drying. 
It is not desirable to direct any heated or dried air onto the fruit pulp 
as it dries because this forms an undesirable skin on the top of the pulp 
and actually impedes drying of the pulp by evaporation. An airflow into 
and out of the room in which the dryer is located is desirable to maintain 
the room air at a humidity that allows evaporation of the moisture in the 
pulp to the air and, in fact, dried heated air can be pumped into the room 
to lower the ambient humidity and increase the rate of absorption of water 
vapor by the air. In an apparatus described above with a water depth of 
one inch in the trough 10, and a trough of approximately 12 meters in 
length, it has been found that the temperature drop of the water from the 
inlet at one end of the trough to the outlet from the trough is only 
approximately 3 degrees C. Therefore, the hot water source does not have 
to expend much energy in raising the temperature of the water back to the 
desired temperature prior to reintroduction of the water into the trough. 
It will be understood that while one application of the invention described 
herein is in the production of fruit leather from a puree of fruit pulp 
and juice, the invention is not limited to the use with such fruit pulps. 
It also can be used for drying vegetables, or even soups and stews that 
have been blended to an even consistency prior to drying. Also, the 
invention, rather than completely drying a mixture, can be used to produce 
juice concentrates very rapidly; for example, in the 12-meter trough 
above, it has been found that introduction of tomato juice on one end will 
produce a 50-percent concentrate in approximately one and one-half hours' 
time from the exit end of the trough. It is important that the temperature 
of the fruit or other mixture be kept below the boiling point so that 
there is no disruption of the material on the polyester sheet and, 
typically, the fruit pulp is kept at a temperature of between 80 and 90 
degrees C. Also, the water temperature, of course, should be kept below 
boiling in order to prevent bubbling and evaporation of the water from the 
trough. 
While a preferred embodiment of the invention has been described and 
illustrated, it is clear that many changes can be made to the illustrated 
and described embodiment without exceeding the scope of the invention. For 
example, while particular materials such as the DuPont 300A Mylar have 
been described as being a suitable and even a desirable medium upon which 
to place the material to be dried, other media are also workable with the 
invention and, in fact, even a sheet of glass could be used, except for 
its undesirable properties of inflexibility. Also, while a conveyor system 
has been described, it is possible simply to utilize the invention by 
floating an infrared-transparent film on a body of hot water and then 
simply placing the material to be dried on the film, allowing it to sit in 
a stationary location on the reservoir of heated water without the use of 
a conveyor. The use of a conveyor does not affect the basic concept of the 
invention, but, rather, adds to the production capacity of a dryer made in 
accordance with the principles of the present invention. Since many 
changes can be made to the illustrated and described embodiment without 
exceeding the scope of the invention, the invention should be defined 
solely with reference to the claims that follow.