Why sorghum and biscuits have similar texture : the roles of protein starch and sugar

Abstract

To help improve food security in sub-Saharan Africa, producing products such as biscuits from locally grown sorghum represents a viable alternative to using imported wheat. Sorghum biscuits are also valuable option for coeliac patients due to their gluten-free nature. Unlike the inferior quality that characterizes many gluten-free products, sorghum biscuits have similar texture to wheat biscuits. Although gluten is present as part of the wheat biscuit matrix, there are debates about its actual state and role in the texture of wheat biscuits. The objective of this work was to determine why sorghum biscuits have similar texture to wheat biscuits despite the absence of gluten. Biscuits were made from sorghum flour and their level of hardness and brittleness were compared with laboratory made sugar-snap and commercial Marie wheat biscuits. The roles of protein, starch and sugar in biscuit texture were investigated. SEM and TEM showed that in sorghum biscuits, the kafirins (prolamin storage proteins) remained encapsulated in their protein bodies and were unlikely to contribute to the texture of sorghum biscuits due to their confined state. However, a continuous gluten sheet was observed to envelope intact starch granules in sugar-snap and Marie wheat biscuits. Starch granules were observed to be intact in both sorghum and wheat biscuits. Stereomicroscopy indicated that starch granules were embedded in the sorghum biscuit matrix with visible strands of starch gel suggesting some starch granules, although minute were gelatinized, while vast majority of the starch granules remained intact and few swollen. Polarizing light microscopy (PLM) confirmed that most of the starch granules were ungelatinized as they remained birefringent, while some were non-birefringent. Increasing dough water level increased the hardness and brittleness of sorghum biscuits and the texture was similar to that of Marie wheat biscuit at both 35% and 40% water on a flour basis, while it was similar to sugar-snap wheat biscuit at 30% water on a flour basis. Addition of different levels of pre-cooked sorghum flour reduced the hardness and brittleness of sorghum biscuits. As the level of pre-cooked sorghum flour was increased, biscuits became softer, less cohesive and more crumbly. This indicates that gelatinization resulted in weakening of the biscuit structure and could not have been responsible for the increase in the hardness of sorghum biscuit when water level was increased. PLM also indicated that the starch granules were embedded in the sorghum biscuit matrix through the formation of networks with a sugar glass.. Increasing sugar level also increased the hardness and brittleness of sorghum biscuits with biscuits containing 20% sugar on a flour basis having a similar texture to sugar-snap and Marie wheat biscuits, while biscuits containing no sugar were extremely soft and crumbly. This indicated that the sugar was responsible for holding other components of the biscuit together. DSC and XRD confirmed that the sugar in sorghum biscuit was present as a glassy amorphous matrix, as was also the case in wheat biscuits. It appears the sugar glass becomes more continuous as sugar level is increased, thereby allowing the formation of stronger networks with the starch granules which caused an increase in biscuit hardness. This study shows that even though gluten is not present in sorghum biscuits, sorghum biscuits of similar texture to wheat biscuits can be produced by optimizing the level of sugar and water used in dough formation. On the other hand, the texture and role of gluten in wheat biscuits depends on the type of wheat biscuit being investigated as they differ in formulation. It appears due to the inability of kafirins to form a continuous network in sorghum biscuits, an oil emulsified sugar glass infiltrates the biscuit matrix embedding starch granules, protein matrix and cell wall materials. The structure formed through the interaction of the sugar glass with other components of the sorghum biscuit, especially starch seems to produce a functionality that compensates for the absence of gluten and this is responsible for the similar texture of sorghum to wheat biscuits.