Through‐circulation drying of seaweed III.—Laminaria digitata frond and stipe; Laminaria saccharina frond

AbstractThe drying characteristics of Laminaria digitata frond and stipe have been studied and are compared with those of L. cloustoni frond and stipe.A striking similarity was found in the shapes of the curves of time versus air velocity for the fronds and stipes respectively of the two species. The empirical equations for fronds were similar in type and those for the stipes were also similar. It was concluded that the physical differences between the two parts of the same plant had a greater influence on the drying time than the differences between the three common sublittoral species.The close agreement between the species was most marked in the effect of bed depth on the drying time. This relationship was apparently independent of any particle‐shape factor since fronds (either minced, shredded, pre‐dried or wet) gave the same general type of curve, i.e. concave upwards. The bed‐depth curves for minced or sliced stipe were in general agreement, but were in contrast with the frond curves.These physical properties of stipes and fronds have been compared with those of other vegetable materials (hops, potatoes, carrots and brewer's spent grain) and it has been shown that the time versus bed‐loading curves for all these materials can be classified into three groups, depending on whether the material is compressible or rigid.The output versus bed‐loading curve for the first group (materials similar to fronds) reaches an optimum value, and mathematical considerations have demonstrated that the optimum drying time is about twice the minimum time. The minimum time for through‐circulation drying is the period required to dry a single layer of the material, and is represented by the intercept of the smooth curve (of time versus bed‐loading) on the time axis.The effect of agitation on the reduction of drying time was investigated for two‐inch beds of L. digitata frond. Agitation was carried out at equal time‐intervals and has been expressed as effective stages. In the first half of the run (T = 7 to T = 3, where T = total water content, lb. of water/lb. of bone‐dry solid) three effective stages reduced the time to only 95% of the corresponding static time, whereas a reduction to about 40% was obtained in the range 3 to 0.1.Bed‐depth experiments suggest that L. digitata plants dry faster than, and L. saccharina slower than, L. cloustoni. Internal temperatures of a 1/8‐in. slice of L. cloustoni stipe during a drying run have been measured. The effect on drying time of storage between harvesting and drying has been found for L. digitata frond.