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Convective drying of chamomile flowers causes losses of essential oils of approximately 25% in a wide range of drying parameters [13].

FIGURE 8.2.2 Drying of chamomile flowers in a high layer (air temperature 60˚C, height of layer 40 cm) [2].

0 10 20 30 40 50 60 70 80 90 0 6 10 12 Drying time in h Water content in % w.w.b. 0,2 m/s 0,3 m/s (5,5 h)/0,2 m/s (4h) 4 2 8

8.2.3.1.1 Influence of Air Temperature

Older publications on drying of chamomile recommended a maximum drying temperature of 45˚C to limit the losses of essential oils. By 1969 Buschbeck [2] assumed, due to his experiments, that a higher air temperature of 60˚C could be applied without having to accept decisively higher quality losses. Müller [13] confirmed this statement by intensive examinations. Both made their tests at an air velocity of 0.2 m/s. Drying at temperatures above this limit causes remarkably higher losses in essential oils according to Reference 2, as shown in Figure 8.2.4.

Müller [13] could not find a sharp increase in losses even up to 90˚C. No clear link between drying temperature and the composition and losses of the four main components of the chamomile oil (chamazulene, bisabolol, bisabololoxide A, bisabololoxide B) for this temperature range could be discovered [13].

In Hungary quality examinations were carried out on samples from industrial drying units in 1966. In the case of a band dryer the content of essential oils decreased by 27% at temperatures of 80–90˚C in comparison to a sample dried on a loft [18]. There were no remarkable differences in essential oil contents at air temperatures between 40–50˚C and 60–70˚C. The chamazulene content even increased at 40–50˚C by 6% and at 60–70˚C by 21%. Only the high-temperature drying with 80˚C resulted in a loss of 6%.

Drying of chamomile flowers in a modern five-band dryer can be carried out with a staged temperature profile. The temperature under the top band can be adjusted to 60–70˚C, the temperature under the second and third band to 50–55˚C, and the temperature for the fourth and fifth band to 45–50˚C. Thus the drying capacity of the band dryer is increased without additional deterioration FIGURE 8.2.3 Sorption isotherms of chamomile flowers [2].

FIGURE 8.2.4 Related content of essential oils of chamomile flowers in dependence of drying air temperature (basis sample naturally dried in a shaded place), air velocity was 0.2 m/s for convective drying experiments [2].

0 5 10 15 20 25 0 20 40 60 80 100

Relative humidity of air in %

Water content in % w.w.b. 25°C 45°C 0 20 40 60 80 100 35 45 60 70 80

Drying air temperature in °C

Related content of essential oils in %

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].

8.2.3.2 Energy Consumption and Energy Costs

The specific energy demand per kg of dried chamomile flowers depends on: • Maximum drying temperature and resulting drying time

• Applied air flow in m3_{/h per m}2 _{of drying area}

• Number of product turning during drying period • Initial and final water content

• Height of product layer or load per drying area

• Kind of raw material (sieved flowers or mixed with herbs)

• Portion or recirculated air flow (e.g., relative humidity of air max. 30% at 60˚C and 0.2 m/s) The energy consumption of a static dryer for chamomile drying with partial recirculation of air flow can be estimated according to the assumptions stated below. Changing conditions will change the energy consumption, too.

Height of layer is 30 cm, bulk density is approximately 200 kg/m3_{, area charging with raw}

flowers is approximately 60 kg/m2_{, the initial water content is 80% (w.w.b.), final water content is} TF4015_book.fm Page 193 Wednesday, April 6, 2005 12:17 PM