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of product quality. Product temperature in the layers on bands one to three does not reach air temperatures due to the cooling effect of the water evaporating from the surfaces.

Examinations carried out by Schmitt [17] show the influence of an uneven drying within a lot on the product quality. Samples were taken from fast-drying spots of an industrial static dryer, which were overdried and exposed over a longer period of time to a higher temperature nearby the air temperature of 55-60°C (up to 8 hours); the samples had an essential oil content of 0.5% instead of 0.8%. This value corresponds to a related additional loss of 35% [17]. On the other hand, the microbial counts and the fungus contamination were lower than the values of other normally dried spots (factor 30 for microbial counts and factor 6 for fungus contamination).

8.2.3.1.2 Influence of Relative Humidity of Air

Samples of chamomile flowers dried at 60°C and 0.2 m/s showed at r.H. of drying air above 50% higher losses in total content of essential oils. Losses increased from 25% in the region of r.H. below 50% to 30% at a r.H. of 60% and to 50% at 70% r.H. [13].

The losses of single components of essential oil reach their maximum in the case of chamazulene at a relative humidity of 40%, for bisabolol at 30%, for bisabololoxide A at 50%, and for bisab-ololoxide B at 50% (air temperature 60°C, air velocity 0.2 m/s). Therefore it can be concluded that at relative air humidities above 30% increased losses for the four main components of the essential oil occur. It is recommended that the portion of recirculated air flow has to be limited so that the relative humidity of the drying air does not exceed 30% at a temperature of 60°C and an air velocity of 0.2 m/s [13].

8.2.3.1.3 Influence of Air Velocity and Load of Dryer

Low air velocities and high dryer loads (in kg raw material per m2 of drying area) result in a high air humidity of drying air and even condensing of water vapor especially in the top layers of a drying bed. Water vapor uptake capacity of drying air is limited, and air is cooled when coming into contact with product in cool top layers. High air humidity or even condensing of water vapor leads to a micro water vapor distillation of essential oils out of the gland chambers [16]. This reduces the total content of essential oils in the dried product and deteriorates the oil components as stated in Section 8.2.3.1.2. Therefore, the air flow per drying area or air velocity through a bed has to be coordinated with load of bed and applied air temperature. Drying chamomile flowers in big layers and at low air velocities even limits the admissible value of the relative humidity to 10% [14].

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