Continuous food sterilization system with hydrostatic sealed treatment chamber

Apparatus and method for continuously cooking and sterilizing particulate food material are disclosed wherein the outlet of a pressurized steam treatment chamber is sealed by hydrostatic sealing means in which sterilized, particulate food material is simultaneously cooled and depressurized as it is removed from the treatment chamber.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention pertains to apparatus and method useful in the aseptic 
canning of particulate food material. More particularly, this invention 
relates to apparatus and method for continuously cooking and sterilizing 
particulate food material without substantially altering its texture, 
taste and nutritional value. 
2. Description of the Prior Art 
In the conventional process for canning foods, containers are first filled 
with food product and sealed. Thereafter, the sealed containers are heated 
in a pressure cooker or retort to sterilize the canned product. Inadequate 
preservation of the canned food material's organoleptic quality is one 
well-recognized problem associated with this approach. To insure that 
every food piece is adequately sterilized, unavoidable overheating of at 
least some of the food pieces occurs. This is particularly true when some 
of the canned food pieces require different heating times to reach 
sterilization conditions. In particular, those foods requiring less time 
for sterilization generally tend to be overheated. Heat transfer 
limitations with the conventional sterilization approach represents still 
another drawback. Sterilizing food material by transferring heat through 
both a container and a contained fluid requires more energy than would 
otherwise be consumed if the canned material were sterilized directly. 
Recognizing these limitations, the prior art has proposed procedures for 
aseptically canning food material. In aseptic canning, food material is 
sterilized before it is sealed in a container. Generally, food material is 
quickly heated to sterilization temperatures, typically in the range of 
250.degree. F. to 300.degree. F., by direct contact with pressurized 
steam. The food material is maintained at such temperatures for sufficient 
time to effect sterilization. Thereafter, the food material is rapidly 
cooled and the cooled, sterile material is filled into pre-sterilized 
containers and sealed within a sterile or aseptic environment. 
In a particularly efficient and convenient sterilization arrangement, food 
material is conveyed through a pressurized steam treatment chamber. In 
this arrangement, heat treatment is controlled simply by controlling the 
rate food material is passed through the treatment chamber. In order to 
insure the food material rapidly achieves sterilization temperatures, a 
temperature in the range of 250.degree. F. to 300.degree. F. must be 
maintained in the treatment chamber. This condition is obtained using 
super atmospheric pressure steam, at pressures of, for example, about 10 
to about 55 psig. 
To date, the basic arrangement for feeding solid particulate food material 
into and withdrawing it from a pressurized steam treatement chamber 
involves some type of rotary valve. Both inlet and outlet valves act as 
seals to maintain pressure within the treatment chamber. One problem with 
rotary valves, however, is that structural degredation of the product 
often occurs as a result of mechanical abrasion. This is particularly true 
at the outlet valve where the texture of the food product, to some extent, 
has been unavoidably impaired by sterilization and is consequently more 
suseptible to mechanical damage. Another drawback of rotary valves is that 
at the ever increasing processing speeds demanded by food processors, 
there often is insufficient residence time within the outlet valve for 
adequately cooling the food material to a temperature that avoids product 
flashing. Flashing is caused by rapid vaporization of hot liquid from 
within the food material caused by its sudden depressurization. Flashing, 
which tends to disintegrate the solid food particles, can only be avoided 
by cooling the food material below the atmospheric boiling point of 
absorbed liquid before depressurization. An improved apparatus and method 
for removing solid food material from pressurized steam treatment chambers 
would be very useful to the food processing industry. 
It is an object of this invention to provide apparatus and method for 
removing solid particulate food material from a pressurized steam 
treatment chamber. 
It is another object of this invention to provide apparatus and method for 
removing solid particulate food material from a pressurized steam 
treatment chamber while avoiding product flashing. 
It is still another object of this invention to provide apparatus and 
method for removing solid particulate food material from a pressurized 
steam treatment chamber while preventing its disintegration, attrition and 
mushing. 
Other objects and advantages of the invention will become apparent from the 
following description. 
SUMMARY OF THE INVENTION 
In one aspect, the present invention relates to apparatus for continuously 
heat treating particulate food material with steam at super atmospheric 
pressure of the type having a steam treatment chamber with pressure-tight 
inlet and outlet openings through which particulate foods are fed into and 
discharged from the treatment chamber. 
The invention particularly relates to an improvement wherein a hydrostatic 
sealing means seals the outlet opening of the treatment chamber. The 
hydrostatic sealing means comprises a column of liquid in communication 
with the treatment chamber's outlet opening as well as with a region of 
lower pressure, whereby the liquid column balances the steam pressure in 
the treatment chamber. The liquid in the hydrostatic sealing means 
constitutes a broth for the treated particulate food material. The 
apparatus also includes means for cooling liquid in the hydrostatic 
sealing means and means for conveying particulate food material discharged 
from the treatment chamber through the hydrostatic sealing means to the 
region of lower pressure. By this arrangement, hot food products 
discharged from the treatment chamber are cooled and gradually 
depressurized so as to avoid flashing and loss of structural integrity 
while extraction of valuable components such as vitamins, minerals and 
flavors from the food material is simultaneously prevented. 
In another aspect, the present invention relates to a method for cooling 
particulate food material discharged from a pressurized steam treatment 
chamber having pressure-tight inlet and outlet openings comprising: 
(a) providing hydrostatic sealing means to seal the outlet opening of the 
treatment chamber, said sealing means comprising a column of liquid in 
communication with the outlet opening and with a region of lower pressure, 
the liquid in the liquid column balancing the steam pressure in the 
treatment chamber, the liquid constituting a broth for the treated 
particulate food material; 
(b) discharging particulate food material from said treatment chamber into 
said liquid column; 
(c) cooling the liquid in the hydrostatic sealing means; and 
(d) conveying said discharged particulate food material through said liquid 
column to the region of low pressure so that the temperature of absorbed 
liquid in said food material is always below its boiling point at the 
pressure prevailing around the food material; whereby the particulate food 
material is cooled and gradually depressurized as it is conveyed through 
the liquid column.

DETAILED DESCRIPTION 
The phrase "particulate food material" is intended to embrace a wide 
variety and size of solid food materials from small vegetable pieces 
commonly used as garnish in soups, such as carrot, celery, onion and 
potato dices and corn kernals, to whole food items such as broccoli and 
asparagus spears and cauliflower florets. Fruits, meats and seafoods are 
also included within the intended meaning of food material. Generally, the 
size of the food material treated with this invention is determined by 
final product considerations. This phrase is intended to exclude liquid 
and semi-solid food materials such as tomato sauce. 
Referring to FIG. 1, particulate food material is delivered to pressurized 
steam treatment chamber 10 through line 1. Particulate food material is 
temporarily stored in surge bin or feed hopper 2 and is controllably 
metered into chamber 10 through valve assembly 3. Generally, valve 
assembly 3 is a rotary type valve typically having a cylindrical, 
closed-end shell which in combination with a driven internal rotor defines 
moving pockets which serially advance metered amounts of particulate food 
material from hopper 2 into treatment chamber 10. 
Pressurized steam treatment chamber 10 can be of any conventional design. 
Generally, treatment chamber 10 will comprise a horizontal or slightly 
inclined cylindrical shell adapted to withstand an internal pressure 
between about 10 to about 55 psig. Chamber 10 will normally be operated at 
a pressure of about 15 psig. High temperature, pressurized steam is fed 
into treatment chamber 10 through conduit 6 which typically includes a 
pressure control valve for regulating the supply pressure independently of 
a higher source pressure. Spent steam is withdrawn through pressure 
release valve-controlled conduit 7. Chamber 10 also includes means for 
transporting particulate food material from inlet opening 4 to outlet 
opening 5. This can be done simply by inclining the chamber 10 and 
rotating or oscillating it on its axis to thereby cause particulate food 
material to tumble through the chamber. Preferably, chamber 10 is provided 
with means for positively urging food material therethrough, such as a 
helical conveying screw or conveying paddles. These latter arrangements 
allow better control of the food material's residence time in the 
treatment chamber and also tend to cause less abrasion. Adequate control 
of residence time is particularly important if continuous sterilization is 
to be obtained without seriously impairing the food materials organoleptic 
quality. The food material is normally subject to a steam temperature 
above about 121.degree. C. for at least about 20 minutes. In the broad 
practice of this invention, however, steam temperatures from above about 
100.degree. C. are contemplated. 
Heat treated, particulate food material is discharged from treatment 
chamber 10 into the liquid column 8 of hydrostatic sealing means 20. 
Obviously, the steam treatment temperature will determine the temperature 
of the discharged food material. Typically, the temperature of the food 
material will be about 120.degree. C. In this preferred embodiment, liquid 
column 8 of hydrostatic sealing means 20 comprises an inner portion 11 
interconnected with an outer portion 12. The inner portion communicates 
with outlet opening 5 of treatment chamber 10, while the outer portion 
communicates with low pressure region 15. The height of the liquid column 
8 is sufficient to balance the steam pressure in treatment chamber 10. In 
order to seal a treatment chamber operating at pressures between about 10 
to about 55 psig., liquid column heights between about 20 and about 130 
feet are required. The liquid height needed to seal the treatment chamber 
at any particular operating pressure is readily apparent to one skilled in 
the art. 
Particulate food material descends through inner portion 11 and is 
collected by endless conveyer 13. Conveyer 13 can be of a conventional 
design and transports the particulate food material upwardly through outer 
portion 12. Other arrangements for conveying the food material through the 
hydrostatic sealing means will be apparent to those skilled in the art. 
For example, a screw conveyor could be substituted for endless conveyor 
13. The food material is cooled and gradually depressurized as it is 
conveyed upwardly through outer portion 12. 
Generally, the treated food material will be conveyed through the 
hydrostatic sealing means to an atmospheric pressure region. Consequently, 
to avoid product flashing the temperature of absorbed liquid in the 
treated food material must be less than 100.degree. C. as it exits the 
hydrostatic sealing means. More precisely, to prevent product flashing at 
every point in the food material's travel through the hydrostatic sealing 
means, the temperature of absorbed liquid in the treated food material 
must be below its boiling point at the pressure prevailing around the food 
material. Liquid in the sealing means must be maintained at an appropriate 
temperature to provide this result. Since hot food material is 
continuously discharged into and conveyed through the hydrostatic sealing 
means, the liquid in hydrostatic sealing means 20 must be cooled. While a 
wide temperature distribution is possible depending upon such factors as 
the nature of the food material being treated, the food material's exit 
temperature from treatment chamber 10, the speed of conveyor 13, etc., the 
liquid in at least the upper section of outer portion 12 is generally 
maintained at about 95.degree.-98.degree. C. while the liquid at the 
bottom of outer portion 12 is generally maintained at about 
115.degree.-125.degree. C. At these conditions, the speed of conveyor 13 
is generally regulated so that it takes a few minutes, e.g., about 2-6 
minutes, for the food material to be conveyed through the hydrostatic 
sealing means. 
An appropriate temperature distribution for cooling food material can be 
maintained in hydrostatic sealing means 20, for example, by delivering 
cooled liquid thereto through conduit 16 and withdrawing hot liquid 
therefrom through conduit 17. Alternatively, liquid in hydrostatic sealing 
means 20 can be cooled by locating appropriately designed indirect heat 
transfer surfaces; e.g., cooling coils, within liquid column 8, preferably 
in outer portion 12. In any event, the speed of conveyor 13 is controlled 
such that as particulate food material is gradually depressurized, and 
simultaneously cooled, the temperature of absorbed liquid is always below 
its boiling point at the pressure prevailing around the food material. In 
this way, product flashing is prevented. The combination of 
high-efficiency direct liquid cooling and the extended path traveled by 
conveyor 13 through hydrostatic sealing means 20 permits high-speed 
continuous operation without creating a product flashing problem. 
An important aspect of this invention is its use of broth as the liquid in 
hydrostatic sealing means 20. As used herein, the term "broth" broadly 
refers to an aqueous liquid having a concentration of vitamins, minerals, 
flavor constituents, etc., that retards or prevents the extraction of 
water soluble components from a particulate food material as it passes 
through the hydrostatic sealing means. By employing broth as the cooling 
liquid, the taste and nutritional value of food material can be better 
preserved. The necessary broth can be conviently developed by 
recirculating liquid in the hydrostatic sealing means through a closed 
loop, as will be more fully described hereafter in connection with FIG. 2. 
Endless conveyor 13 is designed so that liquid can drain therethrough as it 
emerges from the outer liquid column 12. In this way, free liquid is 
permitted to drain from the particulate food material before it is 
discharged from conveyor 13 into duct 21. 
Cool and depressurized, treated food material is discharged from the upper 
end of conveyor 13 falling through duct 21 to sterile or aseptic filling 
zone 30. Duct 21 can be provided as a column and relatively dry, sterile 
gas can be flowed upwardly therethrough if a dry product is desired for 
canning. Sterilized containers are fed to zone 30, as indicated at 31, and 
filled containers are discharged therefrom as indicated at 32. If desired 
the filled cans can then be sealed, either before or after filling with 
additional ingredients. 
A preferred way of cooling liquid in hydrostatic sealing means 20 is 
illustrated in FIG. 2. Hot liquid withdrawn through conduit 17 from liquid 
column 8 at inner portion 11 is passed through conduit 117 to pump 118. 
The liquid is pumped through filter 119 to remove large solids and makeup 
liquid is then added through valved-conduit 120 as required to maintain 
the hydrostatic seal. The liquid is then sterilized by indirect heat 
exchange with steam in heater 121. The liquid is held at the required 
sterilization temperature in holding tube 122 and is thereafter cooled by 
indirect heat exchange with cooling fluid in cooling zone 123. The degree 
of cooling is controlled by temperature recorder-controller 140 which 
senses the temperature of the cooled liquid through sensing means 141 and 
appropriately adjusts the cooling fluid flow rate via line 142 and valve 
143. A quantity of liquid is diverted as required from the cooling circuit 
through valved-conduit 124 to prevent the excessive buildup of certain 
constituents in the cooling circuit. Additionally, if desired, broth can 
be continuously diverted through conduit 124 for use as a liquid filler or 
sauce in canning the particulate food material. Cooled liquid is then 
returned to the hydrostatic sealing means through conduit 116 and is 
injected into the sealing means through conduit 16. By this arrangement, 
valuable constituents, for example flavors, vitamins and minerals, 
extracted from a particulate food material are equilibrated in the liquid. 
This condition prevents further leaching of such components from the 
particulate food material. As another important feature, the circulating 
liquid can be monitored for salt level and pH and appropriately adjusted 
as desired. 
While preferred embodiments of this invention have been discussed herein, 
those skilled in the art will appreciate that changes and modifications 
may be made without departing from the spirit and the scope of this 
invention, as defined in and limited only by the scope of the appended 
claims. For example, while the present invention has been specifically 
described with respect to the sterilization of particulate food materials, 
and has been shown to have particular utility therein, it is anticipated 
that the invention may also have applicability to the continuous heat 
treatment of food materials solely to inactivate enzymes and destroy molds 
which is done at somewhat lower temperatures and pressures.