7.5 Extraction of Agar from Red Seaweed
153
7.5.1 Alkali Extraction
Commercial agar extraction is more commonly carried out using alkali treatment.
The alkali treatment is more commonly used for the Gracilaria species to obtain
strong gels (Bixler and Porse 2011). The goal of the alkali treatment is the disruption of the cell walls of the red algae biomass and dissolving off of the non-agar
impurities such as proteins, minerals and polyphenols. An example of such method
involved treating dried algae Gracilaria in 80% v/v NaOH at 27 °C (±3 °C) for 12 h
at a mass-to-volume ratio of 1:10 (Rejeki et al. 2018). Under this high pH and temperature, the proteins and other smaller compounds are removed and the cell wall is
much more pervious. After the alkali treatment, the biomass is washed with water
followed by water extraction by heating at 90 °C in water for 1 h under continuous
stirring. At this point, the agar is released into the aqueous media. The agar solution is then filtered followed by gel formation upon cooling. The agar can also be
recovered using alcohol precipitation. The next stages involve water removal. This is
commonly achieved using gel pressing using high-pressure membrane presses followed by drying to obtain agar in powdered form which is better for storage and
packaging. Another example of optimum condition for alkali extraction of agar from
the red algae Gracilaria cliftonii is: pretreatment time of 1 h at 30 °C using an alkali
concentration of 5% with a mass-to-volume ratio of 1:150, extraction time of 3 h in
water at 100 °C (Kumar and Fotedar 2009).
Alkali extraction could result in the degradation of part of the agar which could
reduce the yield. However, alkali extraction method could result in agar with higher
molecular weight, crystallinity and purity which are desirable properties for commercial agar application (Martinez-Sanz et al. 2019). When alkali method was used
for the extraction of agar from Gelidium sesquipedale using hot water and alkali in
combination with sonication, the yield using only hot water and sonication reduced
from 10–12% to 2–3% when alkali was used (Martinez-Sanz et al. 2019). Figure 7.3
summarizes the extraction process in a flow chart.
7.5.2 Acid Extraction
Here we look at a typical extraction process used in the extraction of agar from the
red seaweed of the Gelidium genera (Hernandez-Carmona et al. 2013). This method
is commonly used for Gelidium red algae to obtain agar with superior gel strength
(Bixler and Porse 2011). In this method, the alga is cooked at 100 °C in acidic water
at pH between 6.3 and 6.5. The time of extraction varies depending on the conditions
and the red algae used. Under these conditions, the agar in the red algae biomass is
dissolved in water. The liquid containing the dissolved agar is then separated from the
solid residue by filtration. Upon cooling the agar dissolved in water forms a gel. This
gel contains 99% water and must be dehydrated for easy storage and longer shelf life.
Dehydration can be achieved by thawing the gel and drying in oven or through high
153
7.5.1 Alkali Extraction
Commercial agar extraction is more commonly carried out using alkali treatment.
The alkali treatment is more commonly used for the Gracilaria species to obtain
strong gels (Bixler and Porse 2011). The goal of the alkali treatment is the disruption of the cell walls of the red algae biomass and dissolving off of the non-agar
impurities such as proteins, minerals and polyphenols. An example of such method
involved treating dried algae Gracilaria in 80% v/v NaOH at 27 °C (±3 °C) for 12 h
at a mass-to-volume ratio of 1:10 (Rejeki et al. 2018). Under this high pH and temperature, the proteins and other smaller compounds are removed and the cell wall is
much more pervious. After the alkali treatment, the biomass is washed with water
followed by water extraction by heating at 90 °C in water for 1 h under continuous
stirring. At this point, the agar is released into the aqueous media. The agar solution is then filtered followed by gel formation upon cooling. The agar can also be
recovered using alcohol precipitation. The next stages involve water removal. This is
commonly achieved using gel pressing using high-pressure membrane presses followed by drying to obtain agar in powdered form which is better for storage and
packaging. Another example of optimum condition for alkali extraction of agar from
the red algae Gracilaria cliftonii is: pretreatment time of 1 h at 30 °C using an alkali
concentration of 5% with a mass-to-volume ratio of 1:150, extraction time of 3 h in
water at 100 °C (Kumar and Fotedar 2009).
Alkali extraction could result in the degradation of part of the agar which could
reduce the yield. However, alkali extraction method could result in agar with higher
molecular weight, crystallinity and purity which are desirable properties for commercial agar application (Martinez-Sanz et al. 2019). When alkali method was used
for the extraction of agar from Gelidium sesquipedale using hot water and alkali in
combination with sonication, the yield using only hot water and sonication reduced
from 10–12% to 2–3% when alkali was used (Martinez-Sanz et al. 2019). Figure 7.3
summarizes the extraction process in a flow chart.
7.5.2 Acid Extraction
Here we look at a typical extraction process used in the extraction of agar from the
red seaweed of the Gelidium genera (Hernandez-Carmona et al. 2013). This method
is commonly used for Gelidium red algae to obtain agar with superior gel strength
(Bixler and Porse 2011). In this method, the alga is cooked at 100 °C in acidic water
at pH between 6.3 and 6.5. The time of extraction varies depending on the conditions
and the red algae used. Under these conditions, the agar in the red algae biomass is
dissolved in water. The liquid containing the dissolved agar is then separated from the
solid residue by filtration. Upon cooling the agar dissolved in water forms a gel. This
gel contains 99% water and must be dehydrated for easy storage and longer shelf life.
Dehydration can be achieved by thawing the gel and drying in oven or through high