148
7 Agar
At concentrations between 0.5 and 2% w/v in water at around 80–100 °C, agar will
form a gel upon cooling (Rioux and Turgeon 2015). Gelatin, for example, will require
a higher concentration and much lower temperature to form a gel, and carrageenan
requires potassium or calcium salts in the solution in order to form a gel; agar forms
a gel at variable pH and without requiring the presence of cations. This makes agar
a preferred gelling agent in biotechnology applications.
Gels formed by polysaccharides are prone to syneresis. This is the release of water
from the gel as it loses its gel conformation. This is caused by the rearrangement of
the polymer chains due to different factors (Mizrahi 2010). The relationship between
degree of syneresis and agar concentration is quantified in Eq. (7.1) (Mizrahi 2010).
Degree of Syneresis = 1/Concentration
2
(7.1)
The gel strength is affected by the growth method and growth conditions. For
example, a gel strength of 505 g cm
−2 was measured for agar extracted from
Gracilaria grown from tissue-cultured seedlings using the broadcast method of seaweed cultivation, while 201 g cm
−2 was obtained from the same species grown using
the off-bottom method from tissue-cultured seedlings (Rejeki et al. 2018).
The lower sulfate content in agar compared to the other more sulfated seaweed
polysaccharides gives it a higher gel strength and gel melting point. Compared to
carrageenan, for example, which has a melting point between 50 and 70 °C, agar has
a melting point between 85 and 95 °C and while gel strength of agar varies between
700 and 1000 g/cm
2 for a 1.5% w/v concentration, that of carrageenan is between
100 and 350 g/cm
2 (Rhein-Knudsen et al. 2015).
7.3.3 Viscosity
Viscosity of polymers is related to the molecular weight. This relationship is quantified by the Mark–Houwink equation (Eq. 7.2), where [η] is the intrinsic viscosity, M
is the average molecular weight and K and a are constants which vary for different
polymers.
[η] = K M
a
(7.2)
The constants K and a are obtained experimentally by plotting the values of intrinsic viscosity against molecular weight and fitting to the equation (Wang et al. 1997).
The viscosity is therefore an important parameter which is used in the characterization of polymers. Agar has a lower viscosity (10–100 centipoise at 1.5%, 60 °C) than
carrageen (30–300 centipoise) Rhein-Knudsen et al. 2015). This lower viscosity is
attributed to the lower molecular weight of agar compared to carrageenan. While
number average molecular weight of agar is between 36 and 1144 kDa, that of carrageenan is usually at the higher end from 200 to 800 kDa (Weiner et al. 2017).
7 Agar
At concentrations between 0.5 and 2% w/v in water at around 80–100 °C, agar will
form a gel upon cooling (Rioux and Turgeon 2015). Gelatin, for example, will require
a higher concentration and much lower temperature to form a gel, and carrageenan
requires potassium or calcium salts in the solution in order to form a gel; agar forms
a gel at variable pH and without requiring the presence of cations. This makes agar
a preferred gelling agent in biotechnology applications.
Gels formed by polysaccharides are prone to syneresis. This is the release of water
from the gel as it loses its gel conformation. This is caused by the rearrangement of
the polymer chains due to different factors (Mizrahi 2010). The relationship between
degree of syneresis and agar concentration is quantified in Eq. (7.1) (Mizrahi 2010).
Degree of Syneresis = 1/Concentration
2
(7.1)
The gel strength is affected by the growth method and growth conditions. For
example, a gel strength of 505 g cm
−2 was measured for agar extracted from
Gracilaria grown from tissue-cultured seedlings using the broadcast method of seaweed cultivation, while 201 g cm
−2 was obtained from the same species grown using
the off-bottom method from tissue-cultured seedlings (Rejeki et al. 2018).
The lower sulfate content in agar compared to the other more sulfated seaweed
polysaccharides gives it a higher gel strength and gel melting point. Compared to
carrageenan, for example, which has a melting point between 50 and 70 °C, agar has
a melting point between 85 and 95 °C and while gel strength of agar varies between
700 and 1000 g/cm
2 for a 1.5% w/v concentration, that of carrageenan is between
100 and 350 g/cm
2 (Rhein-Knudsen et al. 2015).
7.3.3 Viscosity
Viscosity of polymers is related to the molecular weight. This relationship is quantified by the Mark–Houwink equation (Eq. 7.2), where [η] is the intrinsic viscosity, M
is the average molecular weight and K and a are constants which vary for different
polymers.
[η] = K M
a
(7.2)
The constants K and a are obtained experimentally by plotting the values of intrinsic viscosity against molecular weight and fitting to the equation (Wang et al. 1997).
The viscosity is therefore an important parameter which is used in the characterization of polymers. Agar has a lower viscosity (10–100 centipoise at 1.5%, 60 °C) than
carrageen (30–300 centipoise) Rhein-Knudsen et al. 2015). This lower viscosity is
attributed to the lower molecular weight of agar compared to carrageenan. While
number average molecular weight of agar is between 36 and 1144 kDa, that of carrageenan is usually at the higher end from 200 to 800 kDa (Weiner et al. 2017).