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Optimization of ultrasound-assisted enzymatic extraction conditions on yield, antioxidant
activity and gel strength of agar from Red seaweed (Gracilaria fisheri)

1 Omar Noorain Nasuha, Ibrahim Nor Hayati*, Mohamad Nizaha Juhaida
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1 Faculty of Fisheries and Food Science, Universiti Malaysia Terengganu, 21030, Kuala Nerus,
Terengganu, Malaysia.

* Corresponding author: yati@umt.edu.my

Abstract:

Agar from red seaweed (Gracilaria fisheri) is well-known for its gelling ability and has appreciable
antioxidant properties. However, the currently employed extraction procedure seems to adversely affect
these properties due to alkaline treatment and high extraction temperature. To overcome this drawback,
an ultrasound-assisted enzymatic extraction (UAEE) method is being proposed. This study investigated
the effect of three UAEE conditions namely solid: liquid ratio (1:50 – 1:100), ultrasonic power (296 –
506 W) and temperature (40 – 60 °C) on yield, DPPH antioxidant activity and gel strength (responses)
of G. fisheri agar. Twenty experimental runs were carried out based on a response surface methodology
(RSM) with face-centered central composite design (CCD), the yield, DPPH antioxidant activity and
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gel strength were found within the range of 3.31 – 34.50 %, 52.84 – 60.76 % and 12.88 – 77.61 g/cm ,
respectively. Experimental data for each response was best fitted with a quadratic model, having high
coefficients of determination (R = 0.80 – 0.95) and no lack-of-fit. The results revealed that significant
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(p < 0.05) non-linear (curvature) effects of the UAEE conditions were observed for all responses of
agar yield, DPPH antioxidant activity and gel strength with an exception for the quadratic effect of
ultrasonic power. At optimum conditions namely solid: liquid ratio of 1:98, ultrasonic power of 506 W
and temperature of 60 °C, the desirable agar extract of G. fisheri recorded 32.02 % yield, 58.05 % DPPH
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antioxidant activity and 83.48 g/cm gel strength. These experimental values were within the predicted
values given by the respective models which further validated the prediction capability of the models.

Keywords: G. fisheri agar, Ultrasound-assisted enzymatic extraction, Gel strength, Antioxidant
properties, Optimization