RVA and the degree of starch gelatinization in liquorice extrusion

Chris Hannaford (1) and Dr Paul Whalen (2)

(1) Hardware and Design, Inc., Golden Valley, MN, USA
(2) Whalen Consulting, Inc., Rapid City, SD, USA

This is a summary of an article and is reprinted here by kind permission of Kennedy’s Publication Ltd UK.

The desired sensory aspects of liquorice such as texture, mouth feel and appearance are related to processing parameters such as thermal input and shear from extrusion or die effects.

Modern extruded liquorice has evolved from the traditional black logs to the rainbow of multicoloured, appealing products seen today. The RVA is a powerful analytical technique to measure the starch cooking relative to liquorice quality.

Commercial liquorice is cooked and extruded using alternative methods including batch or continuous scraped surface heat exchanger (SSHE)/low-pressure extrusion and twin screw (TS) cooker-extrusion.

Liquorice Production Methods

Batch-cooking in a heavily built, scraped kettle is the oldest method of producing single color/flavor extruded liquorice. The process starts with cooking a wheat flour slurry, containing around 40–50% moisture, both swelling and gelatinizing the starch granules. The sugars, colors and flavors are added after the starch is gelatinized. The moisture is gradually reduced and after the mass has formed it is extruded to form ropes which are finished by oven drying. Batch-cooking is inconsistent, labour-intensive, messy and slow, but produces an excellent short-biting product with a glossy finish. (Figure 1. Samples A and B.)

The continuous-cooking method with SSHE continuously cooks and extrudes single color/flavor liquorice at approximately the final moisture, reducing the manufacturing time from days to minutes, and producing a more consistent product than the batch method. The product contains more fractured starch granules than batch product. It is initially satin and soft and undercooked product tends to harden after packaging. (Figure 1. Sample C.)

Powerful TS extruders deliver high shear and heat which can cause considerable damage to starch, fracturing granules and breaking down starch chains (dextrinization). Cooling is often undertaken by applying a vacuum to the dough, part of the way down the barrel. Cooking time in the extruder is a matter of seconds. These extruders can produce multiple colored/flavored liquorice, but products tend to be dough-like in texture with a dull, sticky finish that adheres to teeth. These qualities are indicative of too much shear during manufacture. (Figure 1. Sample D.) Emulsifier is almost always added to the formulation in an attempt to mitigate shear damage.

Low shear extrusion can be used to produce multi-colored products cooked in batch or continuous cookers in a manner akin to a TS extruder system, however, it does not result in massive shear damage to starch. This method can be used to extrude higher viscosity formulations than the TS method, producing a glossy, short-biting final product.

 Figure 1. RVA profiles of liquorice using different cooking processes

Discussion

The best quality liquorice contains a fairly high proportion of starch gelatinization with a high molecular weight. It takes a long cooking time to swell and gelatinize the starch granules, exerting minimal shear so the chains remain intact to produce the desired texture. Undercooked product contains a large amount of residual crystalline starch and tends to be dull in appearance, hardening after packaging. Overcooked product tends to be shiny, limp and elastic.

The RVA determines the degree of cooking and shear damage to the starch in a product by forming it into a slurry with water. The RVA gradually increases the temperature to completely cook the starch, while monitoring the torque required for constant stirring.

Sample A

Commercial batch-cooked liquorice which was extruded when hot. The sharp single peak represents even treatment in the extruder and demonstrates its homogeneity. There is no broadening of the peak or other peaks, and the low setback and final viscosity confirm that the product is not raw starch or partially cooked starch. There is no evidence of retrogradation (starch recrystallization). It is a very good product.

Sample B

A fairly raw, batch-cooked commercial liquorice that was permitted to gel and cool before extrusion. The cooling hold time has allowed the starch to extensively recrystallize (retrograde). The extended peak around 95°C is retrogradation and is likely to result from multiple bonding of chains from both amylose and amylopectin. This is indicated by continued gelling (i.e. that never peaks) in the 95°C region. The first peak, early in the heating profile, is a shear product.

Sample C

A SSHE and low shear forming die product. The peak is higher and broader than sample A, which is indicative of lower shear and preserved structure. This is confirmed by the higher final viscosity and setback. This continuously processed product is excellent liquorice. equivalent in quality to the hot extruded batch product (sample A). The RVA profile displays the key characteristics that contribute to the product’s desired attributes.

Sample D

A TS extruded product with emulsifiers. There is significant cold viscosity at around 25°C indicative of dextrins. This profile is strongly influenced by the emulsifier used as a processing aid. Emulsifiers are used to tie up dextrins and reduce the effects of shear. They preserve starch integrity, yet do not permit starch swelling. Emulsifiers are strongly reflected in the low RVA curve. They also tend to have an unpleasant flavour.

Conclusion

RVA analysis is an excellent method for determining degree of cooking, shear damage and starch retrogradation. The RVA enables the producer to measure, observe and control the degree of starch gelatinization in the liquorice to optimise the product.