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8 Ulvans
The key challenge to overcome in the extraction process is that ulvan has relatively low solubility in water, and the cell wall has a rather stable structure with
strong intermolecular bonding between the cell wall polymers within which ulvan
is entangled (Robic et al. 2009). These challenges are addressed by altering the pH
which in turn affects the solubility of the ulvan and the intermolecular interactions
between ulvan and the cell wall polymers as these parameters are pH-dependent. In
other words, the intermolecular bonds between ulvan and the other molecules making up the green algae cell wall are weakened, while its tendency to be isolated into
the solution is increased.
A further challenge in extraction of ulvan is the isolation of ulvan while excluding
the other soluble polysaccharides and proteins present in the green algae biomass.
The solubility of these other polymers is also pH- and temperature-dependent. Glucuronan, for example, has higher solubility in alkaline condition. This is addressed
by selecting the right pH which favors only ulvan, making the extraction process
more selective toward ulvan. The presence of salt also affects the extraction process
as salts cause aggregation of ulvan. This is addressed by treatment with warm water
to dissolve off the salt and increase osmotic pressure prior to extraction (Glasson
et al. 2017).
At lower pH, the extraction is more selective toward ulvan as the solubility of
other polysaccharides such as glucuronan and proteins is reduced at this condition.
For example, protein impurities in ulvan extracted from U. ohnoi are significantly
lower when hydrochloric acid extraction is used compared to when sodium oxalate
extraction is used. The protein content of the extract increased from 4 to 7 µg protein
per mg extract to 114–162 µg protein per mg extract. The drying and milling process
is also important to minimize the particle size and optimize contact surface between
biomass and extractant. Degradation during extraction results in an ulvan extract
with shorter chains and less option for depolymerization to obtain more versatile
fractions. While lower pH favors more selective extraction with less impurities,
under acidic conditions and high temperature the ulvan extract is likely to undergo
more degradation during extraction. The processor is therefore faced with the choice
of higher yield at the expense of a more degraded product having lower degree of
polymerization.
Recommended extraction conditions based on that used in various extractions
reported in the literature are the extraction at a pH between 2 and 4.5, temperature
between 80 and 90 °C and extraction period of 1–3 h. Some studies carry out repeat
extraction on the same biomass to further extract any ulvan still left in the sample.
Additional cost of energy and time should be considered for the second extraction
which would have lower yield than the first and a lower concentration of ulvan requiring more evaporation per gram of ulvan extracted. Furthermore, since the structure
of ulvans varies for different sources and also varies with harvest season, for extract
with most uniform chemical structure, biomass from the same source and harvest
should be used in the batch.
Unit operations which follow the extraction process are required to isolate and
purify the product. These processes vary depending on availability and quality or
form of desired final product. The first step after extraction is the separation of
8 Ulvans
The key challenge to overcome in the extraction process is that ulvan has relatively low solubility in water, and the cell wall has a rather stable structure with
strong intermolecular bonding between the cell wall polymers within which ulvan
is entangled (Robic et al. 2009). These challenges are addressed by altering the pH
which in turn affects the solubility of the ulvan and the intermolecular interactions
between ulvan and the cell wall polymers as these parameters are pH-dependent. In
other words, the intermolecular bonds between ulvan and the other molecules making up the green algae cell wall are weakened, while its tendency to be isolated into
the solution is increased.
A further challenge in extraction of ulvan is the isolation of ulvan while excluding
the other soluble polysaccharides and proteins present in the green algae biomass.
The solubility of these other polymers is also pH- and temperature-dependent. Glucuronan, for example, has higher solubility in alkaline condition. This is addressed
by selecting the right pH which favors only ulvan, making the extraction process
more selective toward ulvan. The presence of salt also affects the extraction process
as salts cause aggregation of ulvan. This is addressed by treatment with warm water
to dissolve off the salt and increase osmotic pressure prior to extraction (Glasson
et al. 2017).
At lower pH, the extraction is more selective toward ulvan as the solubility of
other polysaccharides such as glucuronan and proteins is reduced at this condition.
For example, protein impurities in ulvan extracted from U. ohnoi are significantly
lower when hydrochloric acid extraction is used compared to when sodium oxalate
extraction is used. The protein content of the extract increased from 4 to 7 µg protein
per mg extract to 114–162 µg protein per mg extract. The drying and milling process
is also important to minimize the particle size and optimize contact surface between
biomass and extractant. Degradation during extraction results in an ulvan extract
with shorter chains and less option for depolymerization to obtain more versatile
fractions. While lower pH favors more selective extraction with less impurities,
under acidic conditions and high temperature the ulvan extract is likely to undergo
more degradation during extraction. The processor is therefore faced with the choice
of higher yield at the expense of a more degraded product having lower degree of
polymerization.
Recommended extraction conditions based on that used in various extractions
reported in the literature are the extraction at a pH between 2 and 4.5, temperature
between 80 and 90 °C and extraction period of 1–3 h. Some studies carry out repeat
extraction on the same biomass to further extract any ulvan still left in the sample.
Additional cost of energy and time should be considered for the second extraction
which would have lower yield than the first and a lower concentration of ulvan requiring more evaporation per gram of ulvan extracted. Furthermore, since the structure
of ulvans varies for different sources and also varies with harvest season, for extract
with most uniform chemical structure, biomass from the same source and harvest
should be used in the batch.
Unit operations which follow the extraction process are required to isolate and
purify the product. These processes vary depending on availability and quality or
form of desired final product. The first step after extraction is the separation of