Sisal/Agave - Industrial Crops: Breeding for BioEnergy and Bioproducts - page 338

alkaline pretreatment, when combined with enzymatic hydrolysis, has been dem-
onstrated recently as an apparently more efficient way to yield reducing sugars from
the cellulosic material in Agave [ 37 ].

Saccharification by Hydrolysis

A recent study that compared acid hydrolysis efficiency with enzymatic hydrolysis
found that Viscozyme (Novozymes; Bagsvaerd, Denmark) yields the greatest
amount of reducing sugars when compared to acid hydrolysis and other enzyme
catalysts [ 37 ]. The study evaluated pathways for hydrolysis of bagasse from
A. tequilana that included raw lignocellulosic material from the leaf bases and
outer pi˜a that were trimmed before the pi˜a was crushed to extract sugars (called
metzal) as well as the fibers that remain after the pi˜a is processed (called
metzontete). Using acid hydrolysis of bagasse in dilute solution of HCl and bagasse
at a pH of 5.0, only 4-5 % of the metzal mass was converted to reducing sugars, and
3.5-10 % of the metzontete mass was converted to reducing sugars.
In a recent comparison, the combination of alkaline treatment with enzymatic
hydrolysis was more effective in releasing reducing sugars from Agave bagasse
than from sugarcane bagasse [ 37 ]. A 0.005:1 ratio of NaOH and bagasse (15 ml
0.25 M NaOH for 1 g bagasse) was used for the alkaline treatment, and then a
solution with 6 % bagasse and 1.33 % enzyme were kept at 55 C with the pH
adjusted to the optimum required by the enzyme catalyst (5.5-7.0). Percent sugar
conversions for each enzyme test are shown in Table 15.3 [ 37 ], with Viscozyme
apparently the most efficient hydrolytic catalyst in this experiment, converting
58 % of metzal to reducing sugars, of which 47 % was glucose and 24 % xylose.
Recent advances in yeast engineering would allow complete fermentation of this
sugar mix at industrial rates [ 38 , 39 ]. Metzal is the bagasse component that is
comparable to the raw lignocellulosic material in leaves that could serve as
bioenergy feedstock. These conversion rates are similar to what can be expected
for conversion of Agave leaf material.

Table 15.3 Percentage of Agave bagasse converted to sugars by contrasting enzymes after
treatment in alkaline solution

% Conversion efficiency from biomass to reducing sugars
Metzal (raw) Metzontete (post pi ˜ a processing) Enzyme

Pulpzyme 22 12

Cellubrix 28 14

Novozyme 38 12

Celluclast 43 26

Viscozyme 58 36

Reprinted from Ref. [ 37 ]. With permission from Elsevier