The fraction of soluble starch that contribute to staling: Amylose or Amylopectin?
There are many studies and researches being conducted to study whether amylose or amylopectin that is responsible for the staling of bread. The amylose which is the linear fraction that tends to more readily retrograde, was first claimed to be the reason for bread staling. This was due to the evidence from Katz (cited in Gray & Bemiller 2003) which discovered the linear side-by-side associations of starch molecules forming in the B-type x-ray patterns of both retrograded starch and staled bread. Another research Alsberg (cited in Gray & Bemiller 2003) focused on the character of staled bread that can be restored to freshness by heating above 50 °C. However, it is found that retrograded amylose does not melt at this temperature (Knightly cited in Gray & Bemiller 2003). This brings to the study of amylopectin as the responsible fraction of starch for bread staling. Further discovery showed that at 30°C, the predominant fraction of soluble starch that was being leached out from the bread crump is amylopectin. It was being hypothesized that the firming of bread is caused by spontaneous progressive aggregations of amylopectin molecules (Schoch & French cited in Gray & Bemiller 2003).
Moreover, the contribution of amylose in the staling of bread was suspected to be insignificant, since the amylose was believed to retrograde only during cooling. Upon baking, amylose may have been retrograded or insolubilized making amylose unable to leach out and bind moisture thus cannot contribute to the staling process. Mainly, as studied by Schoch and French (cited in Lee & Lee 2012), the firming of bread is due to the swollen starch granule surrounding the amylopectin that becomes rigid resulted by the amylopectin molecules in the swollen starch granule as they aggregate with each other. However, technically, in order to approach its compressibility, a staled bread need to be heated to about 100 ºC, but as the temperature reached 60 ºC, the retrograded amylopectin should have melted beforehand. Therefore, it is impossible to claim that retrogradation of amylopectin to be the only factor responsible for bread firming.
A study has found that amylopectin Arabic bread produced using waxy barley starch and cross-linked waxy barley starch staled at a faster rate than did Arabic bread made with normal wheat starch, but then reached equal firmness after a few days of storage. The same study also contradicts in which starch with a low degree of crosslinking was found to promote retrogradation of amylopectin possibly by keeping polymer chains in close proximity to one another, and that starch with a higher degree of crosslinking decreased the staling rate possibly by restricting granule swelling and separation of polymer chains (Toufeili cited in Gray & Bemiller 2003). This shows that there is a slight correlation between amylopectin retrogradation and staling, even though both events may not occur in the same process. Concisely, it is agreeable to point out that amylopectin retrogradation is part of the staling process, but is not exclusively responsible for the observable hardening in the bread texture.
We now look at the role of amylose in bread staling. While Schoch and French (cited in Gray & Bemiller 2003) believed that the linear fraction of starch had a minor influence on bread staling, there was evidence to state that amylose is involved in the process. Kim and D’Appolonia (cited in Gray & Bemiller 2003) found that there was about 5 to 24 times more amount of soluble amylopectin in fresh bread in comparison to the amount of soluble amylose. This indicates that only a slight amount of amylose was leached from granules and possibly as the bread was cooled to room temperature, significant amount of the amylose had been insolubilized by retrogradation. An experiment by Ghiasi and others (cited in Gray & Bemiller 2003) whereby the ratio of amylose to amylopectin in flour was changed by using waxy barley starch discovered that there was involvement of amylose fraction in staling of bread but in the duration of 1 day only.
Interestingly, under microscopic examination, Hug-Iten and others (cited in Gray & Bemiller 2003) reported that, a separation of amylose and amylopectin was observed during baking where accumulation of amylose occurred at granule centers, yet upon aging, birefringence had been regained by the gelatinized granules. In fact, it was observed that the granule centers that were rich in accumulated amylose attained the most intense birefringence. The agreed hypothesis was that the starch granules’ rigidity during staling was enhanced by the reorganization of intragranular amylose. Numerous researchers have agreed with the fact that retrogradation of starch is responsible for bread staling. Nevertheless, the ability to specifically measure and distinguish the degree of effects that amylose and amylopectin have on the staling of bread is currently limited. Thus, it can be summarized that amylopectin retrogradation plays a significant role in the hardening of bread especially during storing period, but it is not solely the cause of it since amylose is also an important component that can contribute and enhance the staling process mostly during the first day of storage.