Seasonal changes in carrageenan yield and gel properties in three commercial eucheumoids grown in southern Kenya

Abstract

Three eucheumoid strains (brown Eucheuma denticulatum, green and brown Kappaphycus alvarezii growing in fixed, off-bottom seaweed farms) were examined for carrageenan properties at three sites (Gazi, Kibuyuni and Mkwiro) in Kenya, monthly for one year; these properties were related to a suite of environmental factors and seaweed growth condition. The mean carrageenan yield was significantly higher for green K. alvarezii (59.1% dry wt) than both brown E. denticulatum (56.6% dry wt) and brown K. alvarezii (56.5% dry wt). Thalli at Gazi had a higher carrageenan yield (58.0% dry wt) than both those at Kibuyuni (57.1% dry wt) and Mkwiro (57.3% dry wt), although this small difference does not appear meaningful commercially. Both green and brown K. alvarezii exhibited much higher gel strengths (1042.1 g cm-2 and 1053.7 g cm-2, respectively) than brown E. denticulatum (100.8 g cm-2). Thalli at Kibuyuni had higher gel strengths (783.0 g cm-2) than those at Gazi (690.1 g cm-2), while those at Mkwiro had intermediate values (747.8 g cm-2). In both green and brown K. alvarezii, the gel strengths were positively correlated with photon fluence rate, while the gel strengths of brown K. alvarezii showed an inverse correlation with both relative growth rate and percentage “ice-ice” syndrome. Brown E. denticulatum had carrageenan with higher viscosity (81.7 mPa s) and sulphate content (29.1% dry wt) than both green and brown K. alvarezii. The gel viscosities of all the strains were higher during the southeast monsoon (April–September: 67.3 mPa s) than during the northeast monsoon (October–March: 46.3 mPa s), and were positively correlated with gel strengths. The results show that the three strains produced carrageenans of commercial quality.