Abstract:
Method for pasteurizing almonds and other nuts with steam at atmospheric pressure. The amount of condensation on and the consequent water uptake of nuts is limited by pasteurizing the nuts in a heating chamber with a gaseous atmosphere including a steam mixture and forming a forced-convection path through the nuts. The limited water uptake maintains the quality of the nuts. A forced-convection steam cooker conveying food products on a foraminous conveyor belt forces the steam mixture through the nuts. A preheater preheats the nuts so that they can be pasteurized in the steam cooker at a temperature of between 85° C. and 99° C. to reduce the dwell time and increase throughput.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation-in-part of co-pending U.S. patent application Ser. No. 15/122,824, filed Aug. 31, 2016, which is a 35 U.S.C. § 371 application of PCT/US2014/019789, filed Mar. 3, 2014. The disclosures of both applications are incorporated into this application by reference. 
     
    
     BACKGROUND 
       [0002]    The invention relates generally to steam cooking and more particularly to methods for steam-heating nuts with forced convection at temperatures below 100° C. (212° F.). Nuts, such as almonds, are often pasteurized by immersion in hot water or an air-steam environment. Steam pasteurizers conventionally use the heat of condensation to heat the outer surfaces of nuts to temperatures high enough to deactivate enough micro-organisms to meet acceptable pasteurization levels. The nuts enter the steam pasteurizer at a temperature below the temperature of the steam. The steam condenses on the outer surface of the nuts and raises their temperature. But the condensation can wrinkle and loosen the outer skins of the nuts. In the case of almonds and other nuts to be sold in their skins, the nuts&#39; uptake of water should be limited. One approach to limiting condensation in almonds is described in International Patent Publication No. WO 2013/171336. That document teaches pre-heating low-moisture foods to a temperature above or slightly below the condensation temperature of the water vapor in the heating chamber to limit condensation. Another approach, described in U.S. Patent Application Publication No. 2013/0040030, steams nuts at a pressure below atmospheric to limit water uptake. But pre-heating requires an additional heater, and a vacuum system requires batch and not continuous handling. 
       SUMMARY 
       [0003]    One version of a method embodying features of the invention for pasteurizing nuts comprises: (a) conveying nuts along a conveying path through a preheating chamber; (b) conveying the nuts preheated in the preheating chamber along a conveying path through a heating chamber; (c) forcing a substantially homogeneous gaseous atmosphere comprising a steam mixture through the nuts in the heating chamber along a connection path intersecting the conveying path to heat the outer skins of the nuts and limit the amount of water condensation enrobing the nuts; (d) maintaining atmospheric pressure in the heating chamber; and (e) controlling the temperature of the gaseous atmosphere in the heating chamber to a heating temperature of greater than 85° C. and less than 99° C. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0004]    These features and aspects of the invention, as well as its advantages, are described in more detail in the following description, appended claims, and accompanying drawings, in which: 
           [0005]      FIG. 1  is a side view of a portion of a steam cooker, with its facing sidewall removed for clarity, embodying features of the invention; and 
           [0006]      FIG. 2  is an axial cross sectional view of the steam cooker of  FIG. 1 ; 
           [0007]      FIG. 3  is a side-elevation cross sectional view of the steam cooker of  FIG. 1 ; and 
           [0008]      FIG. 4  is a block diagram of a steam cooker as in  FIG. 1  with a preheating zone. 
       
    
    
     DETAILED DESCRIPTION 
       [0009]    A steam cooker that operates according to and embodies features of the invention is shown in  FIG. 1  with its facing side wall removed to better illustrate its components. The cooker  14 , which is open to the atmosphere, has an enclosure  16  that is supported on legs  17  and extends from an entrance end  18  to an exit end  19 . A foraminous conveyor belt  20  is trained around drive and idle sprockets  22 ,  23  at opposite ends of an upper carryway  24  that traverses the cooker. Diverting rollers or drums  26  guide the endless belt loop along a returnway  28  below the cooker. A network of steam pipes  30  injects steam supplied by a boiler or other steam source into the cooker through the bottom of the enclosure. The injection of steam is regulated by valves  31  (in  FIG. 2 ) in the steam network. 
         [0010]    The cooker shown is modular with at least two identical heating modules  32 ,  32 ′. More modules may be connected in series to lengthen the total low-temperature heating region. A single module could be used for food products that require only a brief heating time. Each module is individually controlled with its own steam valves. A feedback signal from a temperature-sensing probe  34  in each heating module is used by a controller, such as a programmable logic controller, to control the opening of the steam-injector valve to maintain a predetermined heating temperature in each module. The probe, the controller, and the valve provide a means for maintaining a pre-selected temperature in each module. Air circulators, such as fans  36  or blowers, draw air  37  into the cooker through one of the side walls  38 , as also shown in  FIG. 2 . The fan also draws steam  40  injected into the cooker through openings in a plenum  42 , in which the air and steam are mixed. The fan blows the air-steam mixture through openings in the top of the plenum. The air-steam mixture then circulates along a convection path, indicated by arrows  44 , that intersects nuts  46  being conveyed atop the conveyor belt  20  along the carryway. The belt is foraminous to allow the air-steam mixture to pass through and also to allow any condensation to drain. Other features of such a forced-convection cooker as described thus far are given in U.S. Pat. No. 6,274,188, “Method for Steam-Cooking Shrimp at Reduced Temperatures to Decrease Yield Loss,” Aug. 14, 2001, incorporated herein by reference. One example of such a cooker is the CoolSteam® cooker manufactured and sold by Laitram Machinery, Inc., of Harahan, La., U.S.A. Because of the thoroughness of the forced-convection heat treatment described, heat-treating at temperatures of less than 100° C. (212° F.) at atmospheric pressure is possible. In fact, temperatures in the heating region below 85° C. (185° F.) and preferably in the range from 62° C. (144° F.) to 79° C. (175° F.) are effective in blanching or pasteurizing almonds without blistering or loosening their skins because the low temperatures minimize the uptake of moisture by the almonds. 
         [0011]    In operation, nuts, such as peanuts or almonds and other tree nuts, are conveyed into the steam cooker  14  by the conveyor belt  20  along a conveying path  56 . The nuts are heated in a low-temperature cooking region  58  that may include one or more identical forced-convection heating modules  32 ,  32 ′. Air is drawn into the modules and mixed with steam to form a substantially homogeneous gaseous atmosphere of air (or other gas, such as nitrogen) and steam or water vapor. This steam mixture is circulated by an air circulator, such as a fan, in a convection path that intersects the food product. In this example the convection path is perpendicular to the conveying path  56 , but it could intersect or cross the conveying path from other directions. Along with the low-temperature heat treatment, the forced-convection flow through the nuts shears condensation enrobing the nuts and inhibits the uptake of moisture. The duration of the heating—the dwell time—is set by one or more of: (a) the length of the low-temperature heating region  58 , (b) the speed of the conveyor belt  20 , (c) the temperature of the heating region, (d) the size and kind of nut, and (e) the thickness of the mat of nuts on the conveyor belt. For almonds, the dwell time may range from 4 to 9 minutes in order to achieve sufficient lethality, e.g., a 6 log reduction in a target organism, such as salmonella. The temperature of the heating region is measured by a temperature probe and controlled by the amount of steam introduced into the cooker in each module. 
         [0012]      FIG. 3  shows the steam cooker  14  with two heating modules  32 ,  32 ′. In the first module  32 , the convection path through the foraminous conveyor belt  20  and the conveyed nuts  46  is downward. In the second module  32 ′, the convection path is upward through the nuts. Subjecting the nuts or other food products to both upward and downward convection flows results in a more uniform heat treatment. 
         [0013]    Low-temperature pasteurization as described is effective in minimizing the deleterious uptake of water by the nuts. But pasteurizing nuts at lower temperatures requires a longer dwell time in the cooker to achieve the desired kill of pathogens. And longer dwell times lower product throughput. 
         [0014]    Preheating nuts in a low-humidity, dry-air preheating chamber  60 , or preheater, before they are conveyed into a pasteurizing region  62 , as shown in  FIG. 4 , reduces the uptake of moisture during subsequent pasteurization at higher temperatures, such as temperatures between 90° C. (194° F.) and 99° C. (210° F.) instead of below 85° C. (185° F.). For example, instead of pasteurizing room-temperature nuts in the low-temperature pasteurizing region for 4 min at 79° C. (175° F.), nuts preheated to a surface temperature of about 54° C. (130° F.) at a relative humidity of between 1% and about 60% can be pasteurized at 95° C. (203° F.) in just 1.75 min. The preheater heats the surfaces of the nuts to a temperature below the dew point, or condensation temperature, to ensure that condensation forms on the nuts&#39; outer surfaces during pasteurization. Testing has shown that by preheating nuts in the preheater  60  for between about 1 min and about 6 min at a temperature of between about 57° C. (135° F.) and about 85° C. (185° F.) to raise the surface temperature of the nuts to between about 38° C. (100° F.) and about 70° C. (158° F.), nuts can be pasteurized in the forced-convection steam pasteurizing region  62  of the cooker at a temperature of between 85° C. (185° F.) and 99° C. (210° F.) for a dwell time of between about 1 min and about 6 min. This reduced pasteurization time improves throughput. Pasteurization can also be performed down to a temperature of 74° C. (165° F.) with longer dwell times. 
         [0015]    Although the preheating chamber  60  has been described as a low-humidity, dry-heat heater, in other versions, the preheating chamber  60  can be a separate forced-convection steam cooker, an additional forced-convection heating module preceding the two modules  32 ,  32 ′ of  FIG. 1 , or the first module  32  of  FIG. 1 .