# Effects of Heat Treatment Processes: Health Benefits and Risks to the Consumer
## 5 Rehydration and Cooking  
As with most consumable dried products, there is the need for rehydration of the products prior to consumption, in order to restore the fresh properties of the dried product, usually through contact with a liquid phase. In general, this process can be divided into three phases: (1) water absorption by the dried material, (2) swelling of the dried product and (3) loss or diffusion of the soluble components [10].  
Rehydration and blanching are performed to increase palatability but can cause a significant loss of phenolic content, which leaches to the boiling water. [PERSON] et al., when studying the rehydration kinetics of _H. elongata_, observed that the rehydration time decreased as temperatures increased the magnitude of absorbed water [10]. Additionally, it was also observed that for _H. elongata_, rehydration of biomass resulted in a decrease in total phenolic compounds in the dried seaweed, from 1.21 \(\pm\) 0.02 g GAE/100 g db to 0.2 \(\pm\) 0.009 g GAE /100 g db when the rehydration was finished [10]. Phenolic compounds are heat-labile, and when exposed to boiling water through blanching or cooking, there can be a significant loss of such compounds. It is generally accepted that rehydration is dependent on the degree of cellular and structural disruption. When irreversible damage has been caused, large structures within the cells can collapse and reduce the hydrophilic properties, hence reducing the rehydration properties [10].  
The hydration of dehydrated seaweed food products is usually applied before consumption [10]. This rehydration is a complex method used to restore, in the best possible way, the quality and properties of the original fresh product using a liquid phase, normally an aqueous solution. This method is based on three steps: absorption of water by the dried biomass; swelling of the rehydrated product, which becomes similar to the original fresh product; and loss or diffusion of the soluble compounds (co-extraction of the compounds by the liquid phase) [23; 24]. In dried raw seaweed material, the rehydration time is longer at lower temperatures. On the other hand, when the temperature is high (above 80 \({}^{\circ}\)C), rehydration is quick and the dry biomass regains the original moisture level more quickly; however, the compounds have greater losses in the yield and quality, mainly the bioactive compounds, such as phenolic and flavonoid compounds, which are oxidated and destroyed [10]. [PERSON] et al. [9] demonstrated that the heat treatment at 100 \({}^{\circ}\)C for 40 min caused losses of about 83% and 93% in the total phenolic and flavonoid content, respectively, in _H. elongata_. Furthermore, rehydration velocity is not a constant, because in the initial stages, the rehydration is very rapid and there is a decline until equilibrium in the system is reached. In this process, as demonstrated above, the water temperature is a key factor of the rehydration process, where hot water can rehydrate the seaweed rapidly but causing a major change in the tissue structure (mainly cell wall destruction and denaturation of thermolabile compounds) and composition of the seaweed (the blanching technique used in the kitchen) [10]. Blanching is used mainly to make sea and terrestrial vegetables more palatable; however, this technique also leads to co-extraction (leaching) and denaturation of heat-sensitive compounds [25].  
Every thermal process is detrimental to the integrity of plant tissues, particularly cellular membranes. Temperatures above the optimal point can lead to damage which will consequently lead to variations in response to the process applied [10].