Finished mixtures for the food sector are often prepared with powdered flavourings. Especially if natural finished flavourings are called for, the use of freeze-dried products should be taken into consideration. The dry matter of a natural extract contains carbohydrates, proteins and other nitrogen compounds, fats and waxes, minerals, vitamins, acids and flavouring substances, which all have an impact on the drying behaviour.
It is advisable to perform a concentration step before drying. The less water has to be removed during the actual freeze drying process, the more economical is the processing. On the other hand, concentration can only be applied within certain limits. Apart from problems with viscosity, the freezing point will for physical reasons decrease with increasing concentration to such an extent, that already the freezing step can become uneconomical. For an excellent overview of processing options for concentration purposes see Pala and Bielig .
For the following discussion, we will chose the method of freeze-concentration, as it combines well with the ensuing freeze drying process. A solution of fructose in water will be selected as a practical example for illuminating the physical principles.
If a 20% fructose solution is selected, Young  shows in diagram (Fig. 2.63), that pure ice freezes out from this mixture at appr. -2.5°C; this is a result of the known phenomenon of freezing point reduction in solutions. The freezing-out of pure ice, however, causes an increase in the sugar concentration and thus a further freezing point depression. This continues until a mixture of ice and fructose dihydrate is present at -10°C. This lowest common solidification point is called 'eutectic point', the mixture is referred to as 'eutectic'. True freeze drying is only possible below the eutectic point, when all water is present as ice. A certain amount of pure ice will be present in the sugar solution before reaching the eutectic point, which can be removed by a simple separation process, such as centrifugation. This method is called ice- or freeze-concentration and can be performed in several stages. During the subsequent freeze drying less water has to be vaporized.
The frozen material is then exposed to vacuum in a chamber. The ice can now sublimate as described. However, it has to be taken into consideration that, as mentioned, the heat of sublimation of 680 kcal per kg ice has to be applied. For practical applications this means that the material has to be heated to prevent a decrease in temperature and thus a reduction of the sublimation speed. At the end of the drying process a porous product is obtained, the macro structure of which is basically the same as in the preceeding frozen state.
Problems can occur with sugar-containing solutions and fruit juices if, during freezing the solution, the sugar delays in crystallizing below the eutectic point and a supersaturated solution forms. The seemingly crystallized sugar then tends to foaming and splashing: the dried layer breaks down - a phenomenon which is described as collapse in the literature [1, 18-21]. In practical applications, the addition of water or carbohydrates can facilitate drying.
It remains to be added that the described model fructose/water can only be taken as an illustration of the principles. Natural extracts possess a far more complicated compo sition and trials in the laboratory have to be performed to determine the drying behaviour.
Beke, Bartucz-Kovacs and Degen  report on the combination of the two methods for coffee. As a result of its success in the market-place, freeze-dried coffee has become the generic notion for freeze drying in general. Its production is the showpiece for the combination of highly modern technologies. For an excellent summary with a detailed overview of literature see Sylla , Schweinfurt  and Kerkhof . The results depicted therein can be applied to the majority of questions concerning the technology of flavour production from natural products.
Apart from the good flavour characteristics, freeze-dried coffee also possesses an interesting structure. It consists of rather coarse, spongy, irregularly formed bits which show good dissolution properties. The granulated material can be obtained by performing a foaming-up process in the cold. This process can be directed within certain limits to vary the bulk density in order to meet the individual demands. The expectations of the consumer set the standard for dosage. It is obvious that one teaspoon of granular material should yield one cup of beverage.
Tea has encountered increasing importance. The various concentration and drying methods for tea are discussed by von Bomben, Bruin, Thijssen and Merson . With black tea, it is possible to dry the pure extract. The freeze-drying process is exhaustively treated in Deicke .
In the majority, herbal teas require a carrier matrix. Maltodextrines of various qualities are used. These carbohydrates are capable of retaining volatile compounds to a certain extent, as will be discussed in 184.108.40.206.3. These results can also be transferred to other extracts.
220.127.116.11.3 Flavour Preservation in Natural Products and Extracts (1) Natural Products
Experience has shown that freeze drying results in qualitatively superior products when compared to other drying and preservation methods. Herbs such as basil, chervil, dill, parsley, garlic, marjoram, oregano, rosemary, sage, tarragon, thyme and watercress are especially suitable. Economical reasons can also play a role. Freeze-dried products are, in contrast to fresh produce, constantly available all year round at rather stable prices .
In the majority, good flavour preservation can be observed if the raw material's structure undergoes few changes. The volatile flavour constituents are well encapsulated and can hardly be perceived in the dried products; it possesses a hay-like smell. The natural flavour reappears with remarkable expressiveness only after rehydration and swelling. Moreover, the natural colour of the fresh products reappear. The colour of the dried product can be influenced by adjusting freezing temperature and pressure, as Poulsen and Nielsen demonstrate for parsley and chives .
Detailed investigations on the behaviour of flavour components in herbs are described in Huopalathi and Kesaelathi . Tschogowadse and Bakhtadze  have per formed a comparison of thermal drying and sublimation drying for the constituents of coriander.
Green pepper is ideally suited for freeze drying. It possesses a different flavour profile than the usual white or black product. The fresh, unripe fruits constitute the raw material. During freeze drying, the berries hardly undergo any volume contraction and the green colour is largely retained. The freeze-dried berry can be easily reconstituted with water and it regains its original softness. On the other hand, it can also be well used when dry, as the peppercorns can be easily crumbled with the fingers, no peppermill is required.
To maximize the preservation of the volatile flavour constituents in natural products, it is advisable to freeze out as much free water as possible, as the flavour constituents then bind more strongly to the remaining structure. The single products show retention maxima at varying amounts of frozen-out free water. Therefore, the optimal processing and drying program for each product to be freeze-dried should to be determined individually through trials. This also applies to raw material composed of several varieties or coming from different growing areas.
Maelkki Nikkilae, Aalto and Heinonen [31, 32] stress the importance of the raw material quality of onions. Stieger  has investigated the suitablity of various strawberry varieties. Processes for flavour preservation have been thoroughly examined with cultivated mushrooms, leading to an illumination of the most important freeze drying parameters .
Even the best freeze drying process can not compensate poor prior preparation. (2) Extracts
What applies to natural products, is also valid for extracts. The latter possess the advantage that the optimal matrix for freeze drying can be assembled independently. As mentioned above, good raw material, gentle extraction and an optimal freezing process are prerequisitive. Fruit juices have been investigated by Capella, Lercker and Lerici .
It also has to be mentioned that certain flavour losses always occur in the course of freeze drying. In the majority, these are water vapour volatile constituents which escape during the removal of water. A number of authors had dealt with this topic and have investigated the possibility of flavour preservation through various absorbents [36-40]. Studies with model character on the flavour retention of terpenic and non-terpenic essential oils have been performed by Smyri and LeMaguer .
Maltodextrines constitute a good carrier matrix. They are characterized through the range of starch degradation products with varying molecule size. A characteristic number in this context is the so-called DE-value = dextrose-equivalent. Generally the rule applies that the larger the molecules (low DE-value), the higher, and thus the more advantageous, is the eutectic temperature. Mixtures of simple, reduced (high DE-value) sugars are unsuitable. The investigations of Saint-Hilaire and Solms  of orange juice give an overview.
The preservation effect for flavours, however, proceeds oppositely. Kopelman, Mey-dav and Wilmersdorf  have demonstrated with freeze-dried citrus flavourings that the flavour preservation increases with rising DE-value. For practical applications, analysis of the raw material and pre-trials with various carrier matrixes have, therefore, to be employed to find the optimum. The groups of Thijssen as well as Flink and Karel [45-56] have performed exhaustive basic studies on this problem.
The structure of carbohydrates can also play an important role for flavour sorption. Studies of Niediek and Babernics  deal with the flavour sorption properties of amorphous saccharose and lactose. So far, they were able to confirm that the sorption capability in the amorphous state is considerably higher than in the crystalline state.
Grinding processes also play a role for product quality, as Grinberg et al. [59-60] report for apricots and apple purée.
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