Effect of the different drying methods on the drug quality

Freeze-drying or lyophilisation (Figure 9.5) is also an artificial drying technique, and it is becoming more widespread also in the case of medicinal and aromatic plants.

Figure 9.5 Freeze dryer (photo: Sárosi, 2012)

The price of a new freeze-dryer and its operation cost is high, and in several cases the process causes significant loss of the active compounds, or disadvantageous changes in the composition. Therefore, even if the lyophilised drugs are found to be the most preferable ones by the consumers based on their organoleptic parameters, their quality usually do not meet the requirements of the official formulas (e.g. pharmacopoeias).

Thus, more scientific results would be necessary to optimize this method, and to precisely describe in which cases it is advisable to be used.

Freezing is an alternative method that can be applied to the preservation of medicinal plants as well. The vitamin C content of the fast-frozen plant materials is almost equal to the fresh fruits; however the freezing process causes serious damage to the plant tissues and significant loss of other active compounds.

9.1.2 Effect of the different drying methods on the drug quality

In this chapter a summary is given referring to the main changes in the active compounds caused by the different drying-preservation techniques. In the case of sugar-containing plant parts (oligo-and polysaccharides, mucilage, etc.) the optimal temperature of drying is 50-60 C. However, not only the conventional drying methods have been tested, the possible usage of freeze-drying was also evaluated in the case of foxglove species (Digitalis lanata L., Digitalis purpurea L.) containing cardiac-glycosides (Figure 9.6).

Figure 9.6: Chemical structure of cardiac glycosides (according to Pellati et al., 2009)

Based on the type and number of sugar parts, the effect of the cardiac glycoside is different; therefore the decomposition of the initial cardiac glycosides can significantly change the therapeutic effect of the final material. If the drying process is fast (by using hot air drying), or carried out in aerobe circumstances the enzyme activity is successfully blocked, and the Lanatosid-C content can be preserved in much higher amounts. On the contrary, if the drying process is slow (cold air drying) in anaerobe conditions, the sugar parts are decomposed, and after drying the plant material will be rich in digoxin. Thus, the final purpose determinates the way of the primary processing. Freeze-drying can be taken in the second group, since it is a long (at least 72 hours) process, carried out at very low temperature. However, against the expectations, in the final plant material not digoxin, but its derivative – acetyl-digoxin – was found. Since acetyl-digoxin does not have the same effect as digoxin, the usage of lyophilisation is not advised in the case of foxglove species.

In the following group phenoloids are discussed. A lot of active compounds can be classified as phenolic compounds; among them the most important ones are flavonoids, tannins, phenol-glycosides, phenolic acids. Flavonoids are widely distributed in the plant kingdom, especially as plant pigments. They can be found as glycosides and as aglycones as well; therefore similar to the cardiac glycosides the type and number of the sugar parts can influence significantly the therapeutic effect. For example the ratio of quercetin (aglycone) (Figure 9.7) and its glycoside derivate rutin (quercetin-3-O-rutinoside) (Figure 9.8) can be affected by the different preservation methods in the case of onion. Quercetin is known as perspective anti-cancer (real clinical data is missing) and anti-viral agent, while rutin has a beneficial effect on the cardio-vascular system (it decreases capillary permeability, also know as vitamin-P). During the primary processing of onion warm air drying resulted in higher amount of quercetin, while freeze-drying was more advantageous referring to the rutin content.

Figure 9.7: Structure of quercetin (according to Wikipedia)

Figure 9.8: Structure of rutin (according to Wikipedia)

The bark of white willow contains salicin (phenol-glycoside) (Figure 9.9), known as the basic initial compound of Aspirin. It has an anti-inflammatory effect, and has been used in the folk medicine for centuries. The bark needs to be dried only when its moisture content is more than 12-14 %. Although the simplest way is natural drying, other preservation methods (warm-air drying, freeze-drying, and freezing) have also been tested on the quality parameters of the bark. Drying at 70 C caused significant loss in the salicin content, as well as freezing.

Freeze-drying can be regarded less damageable; however none of these artificial methods produced the same results as natural drying.

Figure 9.9: Structure of salicin (according to Wikipedia)

As it has already been mentioned the individual colour, taste and smell of black and green tea is due to the different primary processing. During black tea production before drying tea leaves are fermented (leaves are rolled, crushed, the liquids react with the oxygen), while the enzymes are blocked by steaming in the case of green tea production. Therefore the active compounds of these tea types are also different; green tea is rich in catechins (Figure 9.10), while black tea contains tannins (Figure 9.11). Even if these active constituents are similar, their effect is different; catechins, especially epigallocatehin gallate (EGCG) are one of the strongest and most stable natural antioxidants. Tannins also have antioxidant effect; however, they are better known for their beneficial cardiovascular properties. Of course not only the fermentation can cause changes, the way of preservation can also influence significantly the quantity of these compounds. Too high temperature (above 60

C) and freezing have disadvantageous effect; however, natural and cold air drying, furthermore lyophilisation causes less damage.

Figure 9.10: Structure of catechin (according to Wikipedia)

Figure 9.11: Structure of tannins (according to Wikipedia)

Rosmarinic acid (Figure 9.12), also known as a strong and stable natural antioxidant, is also sensitive to the high temperature and freezing; while freeze-dried spices (rosemary, sage and thyme) preserve their initial rosmarinic acid content in a high percentage.

Figure 9.12: Structure of rosmarinic acid (according to Wikipedia)

In the following group of active compounds fatty acids (Figure 9.13) will be discussed. Usually, they are found in fruits and seeds as important nutrients feeding the sprouts. As in all drug types, the prescribed maximum water content of the fruits and seeds are 14 %. If the plant material contains higher amounts of water, it needs to be dried. Unsaturated fatty acids are sensitive to the air temperature and the length of the drying.

Therefore it is advisable to apply lower temperature (not more than 40 C); however natural drying needs to be avoided, since it can take several weeks (too long period), and the decomposition of fatty acids is initiated.

Figure 9.13: Structure of a well-known unsaturated fatty acid – gamma-linolenic acid (according to Wikipedia) Volatile fatty acids play an important role during the fermentation of cocoa beans. However, their content needs to be reduced during the drying process, because their presence in the final product can be detrimental to the cocoa quality. The easiest way of their elimination is to apply natural drying (sun drying) (Figure 9.14). According to the literature data this method cannot be substitute by artificial drying methods (warm-air drying), because the decomposition of the volatile fatty acids is not acceptable.

Figure 9.14: Sundried cocoa beans in Africa (http://www.flickr.com/photos/19998197@N00/3010666896/)

Several active compounds can be found among terpenoids, such as volatile oil components. As we have already mentioned plant parts containing essential oil are more sensitive to the drying temperature, therefore it is advised not to apply more than 40 C during the primary processing. Freezing and freeze-drying can also cause significant damages to the essential oil glands, which are presented by Figure 9.15 in the case of garden thyme.

Figure 9.15: Essential oil content of differently preserved garden thyme samples (according to Novák, 2011) Not only the quantity parameters of essential oils are influenced by the primary processing methods, the essential oil composition can also change. Referring to the above mentioned sample – garden thyme – the ratio of the main compound (thymol), which is mainly responsible for the characteristic taste and smell of the drug, will decrease, if the drying takes too long time (cold-air drying, natural drying). On the contrary, fast drying on higher temperature, freezing and lyophilisation preserve more effectively the initial proportion of thymol in the distilled essential oil, that is comparable even to the fresh samples (Figure 9.16). However, it needs to be emphasized that huge differences can emerge among the plant species, even if they are members of the same plant family.

Figure 9.16: Ratio of thymol in the essential oil of differently preserved garden thyme samples (according to Novák, 2011)

Carotinoids (e.g. beta-carotene, Figure 9.17) giving orange colour to carrot, pumpkin or marigold (Calendula officinalis), and the red coloured licopin found in the tomato, are also terpenoids (they are tetraterpenes). It is easy to be noticed, that during cooking, baking, their colour intensity is decreasing, especially compared to the fresh plant materials. Because of the heat, significant decomposition of carotinoids can be detected. Therefore, in the case of marigold it is advised to apply fast drying at 70-80 C, or sun drying.

Alternatively freeze-drying can be also applied, although the decomposition of carotenes is more significant and in the final product mainly lutein is found.

Figure 9.17: Structure of β-carotene (according to Wikipedia)

Finally, let‟s see the N-containing active compounds, where several alkaloids are found. Capsaicin (Figure 9.18), which gives pungent taste to chilly paprika, is a well-known proto-alkaloid constituent. It has several therapeutic properties, it is used as a topical analgesic, it has tonic, antiseptic and carminative effect; it is useful against rheumatism, arthritis, neuralgia, lumbago and spasm. Conventionally, Hungarian paprika is dried in the harvesting territory, under the direct sunlight (Figure 9.20). According to the literature data it is useless to apply artificial drying, because the primary processing cannot affect significantly the capsaicin level of the fruits (Figure 9.19); the cultivation techniques and the genetic background (different cultivars) are the main influencing factors.

Figure 9.18: Structure of capsaicin (according to Wikipedia)

Figure 9.19: Effect of the different drying methods on the capsaicin content of pepper (according to Yaldiz et al., 2010

Figure 9.20: Natural drying of Hungarian paprika (http://www.panoramio.com/photo/19529119)

Among poppy alkaloids morphine (Figure 9.21) is the most well-known for its strong analgesic effect.

Although, generally alkaloids are less sensitive to the temperature during drying, the amount of morphine can decrease by 10-11 % above 40 C. Also natural drying of the capsules is advised to be avoided, since it is a too long process causing significant loss in the alkaloid content.

Figure 9.21: Structure of morphine (according Wikipedia)

Summarizing the results it is obvious that the too high drying temperature can significantly damage the active compounds; also lyophilisation cannot be used in all cases, even if it is becoming more widespread in the case of medicinal and aromatic plants. The possible usage of freeze-drying according to the literature data is demonstrated in Table 9.1.

Table 9.1: Possible usage of lyophilisation in the case of medicinal and aromatic plants

Lyophilisation can be advised Lyophilisation need to be avoided Plant parts containing flavonoid glycosides Plant parts containing cardiac glycosides Plant parts containing phenol glycosides Plant parts containing unsaturated fatty acids Plant parts containing tannins Plant parts containing low amount of essential oil Plant parts containing rosmarinic acid

Plant parts containing colouring agents