Microwave-puffable half-products of starch-containing material and their production process

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FIELD OF THE INVENTION

The invention generally relates to expanded snack foods and in particular to a process for manufacturing unexpanded half-product employing an extruder and a subsequent production of expanded snack foods by exposing the half-product to microwave and/or convection heat energy.

DESCRIPTION OF THE PRIOR ART

Expanded snack foods are currently manufactured by several processes. Most expanded or puffed snacks are manufactured by a process known as extrusion cooking. In this process a starch-bearing material is mixed with water to a maximum moisture content of 30% and forced through a heated barrel by a helical screw towards a constriction or die. In transit, the starch becomes plastic and flows under pressure. Upon exiting from the die to atmospheric pressure, the sudden pressure reduction causes the rapid flashing-off of excess moisture with an accompanying expansion in product volume. The extent of volume expansion (bulk density), structural modification, product texture and functional characteristics can be controlled by the moisture content in the extrudate, speed and configuration of the screw and the amount of the heat applied externally to the barrel.

Alternatively, some snack products are made by expanding or "puffing" or what is known in the art as a "half-product". To prepare such a half-product, a starch-bearing material is either blended with a sufficient quantity of a precooked starch or is itself partially cooked and kneaded to form a dough-like mass. The mass is then formed or shaped without heating into dough pieces which are subsequently dried to lower the content of moisture from about 35% to about 8 to 10%, usually in a forced air convection tunnel dryer. The half-product is next puffed or expanded by causing the remaining moisture trapped within the pieces of the half-product to expand from the liquid state to the vapour phase by rapid heating or rapid depressurizing, which results in both cases in expansion of the half-product. The rapid heating is usually conducted by deep fat frying under high temperature-short time conditions or by heating in a convection oven. The rapid depressurizing technique is known as "gun puffing" and is commonly used to prepare ready-to-eat cereals. The whole cereal grains or preformed pieces of the half-product are rotated in a sealed, heated drum. As moisture is released from the pieces, the pressure in the drum increases. At the appropriate moment a pressure release gate is opened, the pressure in the drum drops rapidly to atmospheric pressure which causes the expansion of the particles.

The known processes of manufacturing of expanded snack foods require high energy inputs (mechanical, electrical, and thermal). It is estimated that energy accounts for around 30% of production costs. A substantial amount of this energy is lost as radiated heat. Moreover, the expansion by the deep fat frying results in a substantial absorption of fat by the product. In this case and also when flavor is developed by application of a fat-soluble flavoring, the caloric content of the product is increased. This contributes to the perception that snack foods consist of empty calories and are fattening.

Expanded products are packaged at point of manufacture and require warehousing prior to and following shipment at distribution centres. Some warehousing is also experienced at the retail level. Since most warehousing and shipping costs are expressed on a volume basis, handling of expanded products can be expensive. The number of different handling events may cause physical damage to the packaged product which detracts from its quality at the retail level. The shelf stability of such products particularly of deep fried snacks is also limited.

A method is known in which an extruder is employed for producing a half-product. Water is added to a starch-bearing material to an extent that water content reaches approximately from 35 to 45%. The resulting mixture is a dough which is then cooked in the barrel of the extruder. The cooked dough is extruded through a cooling end of the extruder in a form like spagetti. The wet extrudate is dried to a water content of about 10%. The drying typically takes about 24 hours and the temperature and the relative humidity must be very carefully controlled, otherwise uniformly puffable dried half-products cannot be produced. Another important disadvantage of this process is that the dimension of the half-product is restricted; namely, the thickness must be at most about 2mm in order to secure uniformly puffable half-products.

SUMMARY OF THE INVENTION

It has been found that these disadvantages may be reduced and eliminated by preparing a half-product by a low temperature extrusion of a starch mix having a predetermined moisture content. A starch melt produced in the process of extrusion hardens up into a densified, non-expanded, shelf stable half-product without the need for a drying step. This half-product can be expanded at will into a low density, ready-to-eat snack food by short exposure to microwave energy as the only heating source, to a convection heating, or to a combination of convection and microwave heating.

Thus, the present invention provides a process for manufacturing a half-product in a shelf stable, glass-like rope or piece form having a moisture content of not more than about 25% by weight, which half-product upon heating can be expanded into expanded snack foods, which process comprises the steps of:

a) providing a starch-containing material having a moisture content of not more than about 25%;

b) extruding the moisture-tempered material at a barrel temperature of from about 80.degree. to 110.degree. C. and at an extruder die temperature of less than 100.degree. C. without causing any expansion, thereby producing a hot melt extrudate in a rope or piece form; and

c) leaving the extrudate at room temperature, without substantially lowering the moisture content, for the time necessary to harden up, thereby producing the ready to be expanded half-product.

Alternatively to room temperature cooling, the hot melt extrudate may be subjected to assisted cooling such as is achievable with forced air, carbon dioxide or the like, as far as the moisture content is substantially maintained. According to another aspect of the invention there is provided a starch-based half-product in a shelf-stable glass-like rope or piece form having a moisture content of not more than about 25% by weight, which half-product upon heating with convection and/or microwave energy can be expanded into expanded snack foods. According to still another aspect of the invention there is provided a process for manufacturing of expanded snack foods, which comprises the steps of:

(a) providing a starch-containing material having a moisture content of not more than about 25%;

(b) extruding the material at a barrel temperature of from about 80.degree. to 110.degree. C. and at an extruder die temperature of less than 100.degree. C. without causing any expansion, thereby producing a hot melt extrudate in a rope or piece form;

(c) leaving the extrudate at room temperature or alternatively by force-cooling it, without substantially lowering the moisture content, for the time necessary to harden up, thereby producing a ready-to-be expanded half-product in a shelf-stable glass-like rope or piece form having a moisture content of not more than about 25% by weight; and

(d) exposing the hardened extrudate to convection or microwave energy or a combination of both for the time necessary to expand the extrudate into expanded snack foods.

Preferably the extrudate is divided into discrete pieces of appropriate or selected length at the die face of the extruder and prior to hardening.

According to yet another aspect of the invention there is provided expanded snack foods produced by the above process.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The possible raw, starch-containing materials generally include any cereal flour or starch or any root flour or starch without regard to starch composition. The flours should be composed of predominantly starch. Preferred raw materials useful in the present invention include cereal and root starches, oat and potato flour or any composite mixture thereof. These raw starch-containing materials usually have a moisture content of less than 14%, typically less than 12%. In our experimental runs, wheat starch, cornmeal, potato flour, wheat flour and substantially bran-free oat flour and a mixture (1:1) of potato flour and substantially bran-free oat flour were tested. All resulted in fairly good products, though the degree of puffing and the appearance vary to some extent reflecting the composition. It appears that certain types of flours containing a gluten type protein somewhat resist post-extrusion expansion. However, this does not necessarily mean that such flours are totally unsuitable or unacceptable to consumers.

Moisture-tempered material is usually prepared by admixing water and the starch-containing material. Where the starch-containing material has a relatively high moisture content, for example, about 14% and a high-powered extruder is employed, additional water may not be necessary. The water content in the moistened material should be kept in the range of not more than about 25%. In a preferred embodiment, the water content is 19 to 25%, particularly from 19 to 21%, by weight. In another preferred embodiment, particularly where a powerful extruder is employed, the water content may be from 12 to 19% by weight. This mixing, where necessary, can be carried out either outside the extruder, or if appropriately equipped, inside the extruder.

The moisture-tempered starch containing material is distinguished from a dough which is a moistened mass having a consistency high enough to be kneaded or rolled. The moisture-tempered material according to the invention is still granular or powdery even after the moisture tempering because of the relatively low moisture content (i.e., not more than 25%).

The moisture tempered starch material is heated within the extruder barrel at a temperature of from about 80.degree. to about 110.degree. C., preferably from about 100.degree. to about 110.degree. C. and then extruded from the extruder die at a temperature of less than 100.degree. C, preferably about 60.degree. to 70.degree. C. It is under the influence of the extrusion conditions that the moistened starch-containing material becomes plastic within the extruder barrel. The type of extruder is not important according to the invention. Both single and double screw types can be employed. The extruder and/or its operation conditions should not be such that the extrudate expands on exit therefrom, because otherwise the products would not puff in a subsequent heating step.

If the above ranges of the process parameters are observed, an unexpanded starch melt produced in the process of extrusion hardens into a densified, glass-like (i.e., hard and transparent or translucent) half-product within approximately 1 hour of exit from the extruder at room temperature, without substantially lowering the moisture content.

The half-product so obtained has a water activity less than 0.6 and is shelf stable for greater than twelve months without requ