Gun-puffing

Reachons such as these usuahy take place in continuous reactors such as chip (memory or potato) processing lines, retorts, kilns, and conveyer belts in which the sohds pass through an oven with reactive gases such as air or perhaps in a fluidized sohd bed reactor as in the shot from guns puffed rice and other grain cereals or in a slurry reactor such as a vegetable oil or fat fryer. Some typical sohds reactors are sketched in Figure 9-2. 

Oven-puffed breakfast cereals constitute one of the most important categories. Undoubtedly, the most popular is expanded rice. These products have less expansion compared to products expanded with gun-puffing. The preferred rice is medium or short with intermediate amylose content (15%-20%). Waxy rice is not recommended, nor is milled rice with a high incidence of microfissures or stress cracks and fat content (>0.25%), because it expands less and yields low-quality products. Generally, the kernel expands 2 to 5 times its original volume. The traditional process for oven-puffed rice is depicted in Figure 11.3. The unit operations are practically... 

FIGURE 11.4 Flowchart of processes commonly used to manufacture gun-puffed wheat and other cereals. 

Breakfast cereals are typically manufactured using direct expansion or by forming dense pellets. Direct-expanded products are usually flavored and packaged, whereas pellets are usually transformed into their final shape by flaking, oven puffing, or gun puffing. 

Most pellets are first cooked in an extruder and then formed using a second unit. The aim is to produce a cooked peUet with high density that is further dehydrated to yield an intermediate shelf-stable product. After drying, pellets can be stored for long periods of time, or after a small tanpering or equilibration time, they can be processed into final products. Generally, pellets are gun-puffed to yield final products. 

FIGURE 11.9 Flowchart of the extrusion process to manufacture pellets further processed into gun-puffed cereals. 

As an alternative to the above method, cereal puffing can also be accomplished by a puffing gun. The process relies on the immediate transfer of the heated cereal containing superheated steam from a high to a low pressure allowing water condensation and consequent expansion. The suddenness at which pressure transition occurs is the key parameter determining the porosity (Kent and Evers 1994). 

The gun used in batch model explosion puffing was essentially a rotating cylindrical pressure chamber that was fitted with a quick-release lid, and was heated externally. The rotational speed of the gun was fixed to give an optimal tumbling action of the charge. This speed (33 rpm) was about 40% of the critical speed, that is, the speed at... 

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