in comparison to microwave, ultrasonic and unpretreated variants
of the experiment, what suggests that application of relatively
low-temperature and short time enzymatic hydrolysis (unlike in
the SHF process) of waste bread for direct conversion to ethanol
using GSHE could be used in fermentation of media at higher solids
loading than in present study (150 g kg1) avoiding the problem of
water binding capacity of raw material. Montalbo-Lamboy et al.
[23] studied the effect of batch and continuous ultrasonic pretreatment
of corn slurry prior to GSHE aided fermentation. They discovered
that both batch and continuous method increased the initial
concentration of glucose in raw material slurries, what was probably
caused by starch gelatinization during pretreatment. Sadeghi
and Shawrang [24] studied the effect of microwave irradiation on
rumen degradability of corn meal. They found that irradiation of
raw material for 3 to 5 min at 800W power input increased starch
degradation rate. In present experimental conditions no effect of
ultrasonic and microwave pretreatment on initial sugar release
and liquefaction of waste bread was observed. This was probably
caused by the physico-chemical alterations of bread ingredients
during baking and staling where the formation of gluten-lipid
network and starch retrogradation occurs which are difficult to
reverse using physical treatment [31].
3.2. Effect of pretreatment method on fermentation course of waste
bread with GSHE The fermentation dynamics assessment shown that carbon
dioxide liberation from fermentation media was more advantageous
for enzymatically pretreated waste bread and for the SHF
process in comparison to other variants of experiment (data not
shown). The changes in reducing sugars concentration in microwaved,
sonificated and unpretreated samples was similar during
the fermentation process (Fig. 1a). Its concentration ranged ca.
53, 10 and 0.2 g L1 after 24, 48 and 72 h of fermentation respectively.
In enzymatically pretreated samples sugars concentration
after the first day increased by ca. 12 g L1 in comparison to initial
state. After subsequent days of fermentation the reducing
sugars content in this variant of experiment was below