(194m) Effect of Drying Temperature, Humidity and Time on the Physico-Chemical Properties of Sugar Kelp (Saccharina latissima)

Sugar kelp (Saccharina latissima) is marine macro-algae and is a rich source of fibers, vitamins, minerals and antioxidants. Due to high moisture (~92%), it is highly susceptible to microbial attack and enzymatic deterioration and is either conventionally sun dried or hot air dried for extending its shelf life. Sun drying is one of the oldest techniques for food preservation, it is very slow, requires clear weather conditions which makes it an unreliable technique for Maine seaweed farmers. Hot air drying induces faster drying rate, but also leads to deterioration of texture and reduction in heat sensitive nutrients including vitamin C, antioxidants, phytochemicals, total flavonoid content and total phenolic content. Drying also creates void space and stress at the cellular level, leading to shrinkage that affect the quality of the dried seaweed. Since, shrinkage is governed by glass transition temperature (T_g). The aim of this research is to optimize the drying parameters by studying the effect of glass transition phenomenon on shrinkage of sugar kelp and hot air drying temperature, humidity and time on the physico-chemical properties (water activity, moisture content, pH, color, water holding capacity (WHC), oil holding capacity (OHC), ash content, fat content, vitamin C, antioxidant capacity and total phenolic content) of sugar kelp.

Freeze-dried sugar kelp was placed in air tight jars with different water activities (0.1–0.9). Moisture sorption isotherm was fitted non-linearly to BET and GAB model. Equilibrated samples were analyzed using a differential scanning calorimeter for T_g and freezing point (Tf ). Samples were scanned from -90 to 100°C at the rate of 2°C/min. State diagram was developed based on Gordon-Taylor and Chen’s model. Fresh sugar kelp samples of approximately 450g were dried at an air temperature of 30°C, 40°C, 50°C, 60°C and 70°C with relative humidity levels of 25% and 50% and air velocity of 10.0 m/s in the convective dryer. Dried sample were packed in air tight zip lock bags and stored at -80°C for further analyses of physicochemical properties.

The temperature gradient between air temperature and glass transition temperature had a clear impact on the shrinkage rate of sugar kelp. Drying temperatures below T_g resulted in lower shrinkage rates as compared to higher drying temperatures. At higher drying temperatures, the amorphous form of water continued to stay in the rubbery state, exhibiting a more flexible solid matrix. As the drying was entirely in the rubbery state region, the diffusion mechanism did not deviate from ideal Fick’s law, which can thus be applicable for modeling thin layer diffusion for the falling rate period. The ash content of the samples were found to be in the range of 23.32% - 33.05% (w.b) and are inversely correlated to the water holding capacity (r = -0.84) and oil holding capacity (r = -0.84), which indicate the textural properties are highly dependent on the ash content irrespective of the drying temperature and humidity conditions. Heat sensitive nutrients such as vitamin C showed a positive correlation with respect to the drying temperature (30°C to 70°C) indicating drying time have significant effect (p<0.05) and it increases as the drying temperature increases corresponding to drying humidity of 25% and 50%, respectively.

This is the first study to report the effect of T g on shrinkage during drying of sugar kelp. This study also demonstrated the potential application of dehumidified drying in food industry as it might preserve the heat-sensitive nutrients. Results from this study will also help in estimating storage and processing conditions for the seaweed industry. This important information is essential for developing an innovative technology focused on clean, energy efficient and closed drying system for producing top-notch and local finished products for American consumers.