EP2421379A1

EP2421379A1 - Method for drying rice

Info

Publication number: EP2421379A1
Application number: EP10718129A
Authority: EP
Inventors: Eliana Zamprogna
Original assignee: Buehler AG
Current assignee: Buehler AG
Priority date: 2009-04-24
Filing date: 2010-04-26
Publication date: 2012-02-29
Also published as: WO2010122166A1; CH700876A1; CN102438459A; CN102438459B; US9060534B2; US20120076916A1

Abstract

The invention relates to a method for drying rice, in which, during the drying process, the grains of rice pass through surface states which have various pairs of values of temperature (T) of the surface and moisture (U) of the surface, the surface of the grains of rice remaining in a viscoelastic state during the drying process, and in which, in a diagram comprising temperature (T) of the surface of the grains of rice and moisture (U) of the surface, a) the temperature (Tv) of the surface of the grains of rice lies no more than 40°C above the temperature (Tg) on the glass transition curve of the rice at the point of the same moisture of the surface and/or b) the moisture (Uv) of the surface of the grains of rice lies no more than 20% above the moisture on the glass transition curve of the rice at the point of the same temperature of the surface.

Description

Process for drying rice

The invention relates to a method and an apparatus for drying rice, in particular paddy.

When drying rice, it must be ensured that the grains of rice during the drying process have as few adverse effects as possible. B. experience cracking and / or experience excessive discoloration.

Too strong and rapid drying at high temperature, low relative humidity and high air circulation (high drying performance) causes shrinkage stresses, which usually lead to cracks or cracks in the rice grains during or after their drying. In this way, dried rice then decays easily when grinding in individual pieces.

In addition, by such "aggressive" drying Maillard reactions on the rice grains during their drying favors, causing the mentioned discoloration arise.

In addition, the previously achieved drying times in the rice drying are still unsatisfactory long.

The invention has for its object to avoid the disadvantages of the prior art and in particular dried rice of the highest quality, in particular without unwanted cracks and excessive discoloration, in the shortest possible time, especially with significantly shortened drying process to produce. This object is achieved according to the independent patent claims.

The process according to the invention comprises the step of drying rice, wherein the drying conditions are chosen in such a way be that at least a portion of the surface, preferably the entire surface of a rice grain remains in a viscoelastic state at least during a part-time period of the entire duration of the drying process. As a result, it is not only possible to reduce the extent of shrinkage stresses and Mailard reactions during the drying process, so that the dry rice produced in this way has no cracks and no discoloration, but also achieves a significant reduction in the duration of the drying process.

A viscoelastic state is also referred to as a rubbery, viscoelastic or plastic state.

This method thus has the advantage that the drying time is reduced and / or the yield at Head Rice, i. the so-called Head Rice Yield, is increased. The yield on head rice is economically relevant because rice, which can not be classified as a head rice, has a high financial value, i. E. up to 70%, loses.

A Maillard reaction is understood to mean a so-called non-enzymatic browning reaction, in particular a discoloration reaction. For example, by a Maillard reaction a yellowish-red discoloration of the rice take place, which is disadvantageous, since the value of rice depends in particular on its white color.

For the purposes of the present application, Head Rice is understood to mean a rice grain which, after rice production, in particular the cleaning and / or peeling of rice, still has at least Η the original length of the rice grain, based on the longest extent of the rice grain. Expediently, at least a partial region of the surface, preferably the entire surface, passes through different temperature / humidity states, in particular with an increase in the temperature of the surface and a decrease in the moisture of the surface.

Preferably, the various temperature / humidity states of the surface are selected such that they are in a viscoelastic state above the glass transition temperature. In particular, in a graph of temperature and humidity of the surface a) the temperature of the surface is not more than 40 ⁰ C above the temperature on the glass transition curve in the same moisture point of the surface and / or b) the moisture of the surface not more than 20 % above the moisture on the glass transition curve at the same temperature point of the surface. The temperature / humidity conditions of the surface of the rice are thus always within a maximum bandwidth above the glass transition curve.

The glass transition temperature is understood here to mean the temperature at which a material has the greatest change in the deformability. Below the glass transition temperature, the material behaves brittle and above viscoelastic. The viscoelasticity is characterized by a partially elastic, partially viscous behavior. The material relaxes only incompletely after removal of the external force, the remaining energy is dissipated in the form of flow processes (retardation). The term glass transition temperature as used herein refers to the rice as a whole and not to individual microscopic constituents thereof. If the moisture, ie the water content, decreases from rice, its glass transition temperature increases. If the glass transition temperature is measured with rice at different humidities and these are entered in a temperature / humidity diagram, a glass transition curve is obtained for this rice.

When the ambient air of a grain of rice has a different temperature and / or humidity than the grain of rice, equilibrium temperature and humidity are established on the surface of the grain of rice. This equilibrium temperature and humidity of the surface can be adjusted to minimize the drying time. This minimization is achieved by aiming for the lowest possible equilibrium moisture content of the rice surface as quickly as possible. This is done by the appropriate choice of Trocknungslufttemperatur- and moisture, provided that the surface remains in the viscoelastic state, in particular so not embrittled.

Preferably, a glass transition curve in the temperature / humidity (T / U) diagram of the rice is provided for the process. The measurement of the glass transition curve is carried out by means of known measuring methods, such as DMTA (dynamic mechanical thermal analysis); also a measurement by means of DSC (differential scanning calorimetry) is possible. It is important that the measurement reproduces as much as possible the deformability of the entire grain of the rice and not just individual microscopic constituents, therefore the measurement by means of DMTA is preferred.

Preferably, the equilibrium moisture U is monitored at the surface of the rice grain during drying. It is particularly advantageous if, during drying, the temperature / moisture value pairs of the surface of the rice grain are adjusted in such a way that the glass transition curve in a graph of temperature of the surface and moisture of the surface is not fallen below into the glassy region.

It is particularly advantageous when drying (only) the equilibrium moisture U is monitored at the surface of the rice grain. This is the moisture on the surface of the rice grain. This surface moisture is impressed on the rice grains by the surrounding drying climate and forms one of the boundary conditions (gas temperature, water vapor partial pressure) for the moisture gradient which occurs during drying inside the rice grain. The humidities U are given as (mass of the water in the product) / (total mass of the hydrous product).

The drying climate is preferably an air atmosphere with defined air temperature and defined relative humidity. If required, it is also possible to work with other gases, in particular oxygen-free or oxygen-poor inert gases, as drying climate. Advantageously, the gases are nitrogen or carbon dioxide and mixtures of these with a defined partial pressure or molar fraction of water vapor contained therein.

The rice is preferably rice from the harvest, ie paddy, which typically has a moisture content of 17-25% wb before drying. If desired, also parabolized rice (also called parboiled rice) may be dried, which typically has a moisture content of 25% wb-35% wb before drying. The viscoelastic state (T; U) occurring during drying of the rice at a temperature T and a moisture U should have a minimum distance ΔU _MIN = U-Ug parallel to the moisture axis U from a state at the glass transition (T _g ; U _g ) of the rice (Wt.% Water / total weight) of the temperature / humidity diagram.

During drying "you ride on the glass transition curve" in the T / U diagram. Surprisingly, it has been found that this short drying times can be achieved with low energy consumption and without affecting the product properties mentioned above.

This can be done by continuous control or regulation of the drying conditions. Control or regulation of the drying conditions can also be carried out in stages, for example by stepwise control or regulation of the temperature and / or the humidity.

Preferably, the minimum distance is in the range 0.5% <ΔU _MIN <5%, more preferably in the range 1% <ΔU _MIN <3.5% and most preferably in the range 1.5% <ΔU _MIN <2.5%. This ensures in the temperature / humidity diagram (T / U) a safety margin to the glass transition curve, whose transgression to the glass state is to be prevented, at least during a part-time period of the drying process. If necessary and at the right time during drying, a controlled and temporary transfer to the glass state may possibly be made possible.

The viscoelastic state (T; U) occurring during drying of the rice at a temperature T and a humidity U should be a minimum distance from a state at the glass transition (T _g ; U _g ) ΔT _MIN = T - have T _g parallel to the temperature axis T (degrees Kelvin) of the temperature / humidity diagram.

Preferably, the minimum distance is in the range IK <ΔT _M i _N <1OK and more preferably in the range IK <ΔT _MIN <5K. This also ensures a safety margin in the temperature / humidity diagram (T / U) to the glass transition curve, which is to be prevented from entering the glassy state at least during a part-time drying process, if necessary and at the right time during drying, if necessary controlled and timed limited trespassing in the glass transition can be made possible.

The viscoelastic state (T; U) occurring during drying of the rice at a temperature T and a humidity U should be at a maximum distance ΔU _MAX = U - Ug parallel to the moisture axis U (wt) from a state at the glass transition (T _g ; U _g ) % Water / total weight) of the temperature / humidity diagram does not exceed.

Preferably, the maximum distance is in the range 5% <ΔU _MAX <20%, and more preferably in the range 5% <ΔU _MAX <10%.

The viscoelastic state (T; U) occurring when the rice is drying at a temperature T and a humidity U should have a maximum distance ΔT _Ma χ = T - T _g parallel to the temperature axis T from a state at the glass transition (T _g ; U _g ) (Degrees Kelvin).

Preferably, the maximum distance is in the range 1OK <ΔT _Ma χ <4OK and more preferably in the range 1OK <ΔT _Max <30K. Preferably, the glass transition curve in the temperature / humidity (T / U) plot of the rice is provided by measurements on samples of the rice before and / or during drying. The necessary sampling and measurements can be carried out online or offline. Moistures and / or temperatures are determined on the samples, which are the homogeneous moisture or homogeneous temperature in the entire volume of the sample taken, after the moisture and temperature gradients within the sample at the time of sampling are reduced to have .

Alternatively or complementarily, the required glass transition curve in the temperature / humidity (T / U) plot of the present rice can be provided from a library providing glass transition data and moisture content data in equilibrium with the ambient air.

The determination of the glass transitions in the samples can be carried out by DSC measurements or DMTA measurements familiar to the person skilled in the art.

Preferably, during at least part of the drying operation, the relative humidity and / or temperature of the drying environment of the rice is controlled or controlled so as to prevent glass transition from the viscoelastic state to the glassy state, at least in partial areas of the surface of the rice. In the areas in which the rice to be dried is not in the glassy state, water molecules can diffuse faster (about 5 to 10 times faster), so that the removal of water and thus the drying of the rice grains takes place faster overall. Ideally, the passage into the glass area is controlled by controlling the relative Moisture of the drying climate at any temperature during the drying process prevented.

It is particularly advantageous if the rice grains are prevented from passing into the glass region by controlling the relative humidity and / or the temperature of the drying air during the entire drying process. Preferably, this is prevented for at least 90% and more preferably for at least 95% of the total duration of the drying process. This should be the case for at least 80% of the total surface of the rice grains. It is particularly advantageous if the partial surface not present in the glassy state is at least 90% and more preferably at least 95% of the total surface area of the rice grain.

Conveniently, the relative humidity of the drying environment is maintained below 98%, preferably 95%, more preferably 92%, and most preferably below 90%. This reduces the risk of condensation effects, which can lead to undesirable sticking of the rice during the drying process. In addition, this also has a positive influence on the drying kinetics.

During drying, most of the volume, preferably the entire volume, or at least the entire surface of the rice grain is expediently in a viscoelastic state beyond the glass transition of the rice. It is particularly preferred if, during the entire duration of the drying process, the rice has a viscoelastic state beyond the glass transition. Preferably, the glass transition to the glassy state should only be reached below the rapid cooling of the dried rice to ambient temperature at the end of the drying process. The temperature T is expediently kept below 150 ^° C. and preferably below 120 ^° C. during the drying. This prevents Maillard reactions in the rice grains and thus to strong discoloration during drying.

Conveniently, the total duration t _TO τ of the drying is kept below 300 minutes, more preferably below 240 minutes, even more preferably below 200 minutes, and preferably below 180 minutes. Even with a total duration t _TO τ under 120 min, good results are possible. In the process according to the invention, this is sufficient for complete drying from an initial moisture content content before drying to a final moisture content after drying and enables energy-saving drying.

In a particularly advantageous embodiment of the method according to the invention, the time integral of the temporal temperature profile T (t) (in ^O C) over the total drying time t _TO τ is less than 20 × 10 min ⁰ C and preferably less than 15 × 10 min ⁰ C. This also contributes to the prevention of discoloration caused by drying and makes it possible to keep the drying energy consumption low and still avoid falling below the glass transition to the glassy state during drying.

If the condition of the surface of the rice briefly (a few seconds to a few minutes) below the glass transition to the glassy state, this is the more harmless the earlier this takes place in the drying process. In particular, a short time falls below the glass transition during the transfer of rice to the drying stage harmless. The rice grains may be moved relative to each other during the drying process and / or kept relatively spaced.

Ideally, in the invention, both the surface and the interior of the rice grains remain to the midpoint during the entire drying process, in the viscoelastic, i. not glassy state. As a result, the transport of moisture from the interior or from the center of the rice grain to its surface and from its surface into the surrounding drying air takes place rapidly. Thus one achieves a higher drying speed or a faster drying saturation. Since there is no glass transition, there is no risk of cracking in the rice grains.

The surface and interior or center of the rice grains, when dried, reach the moisture content in equilibrium with the surrounding air.

The object mentioned above is therefore further achieved with a device having the features of the independent device claim.

An apparatus according to the invention for drying rice according to a method described above comprises at least one temperature sensor and at least one moisture sensor for determining the temperature and the humidity of the drying climate.

Preferably, the at least one temperature sensor and humidity sensor associated with a control unit or control unit, which is programmed or programmable in that drying of rice is allowed according to a method described above.

It is particularly advantageous if a library is connected or connectable to the control unit or the control unit, in which glass transition data and rice data, in particular glass transition data of rice are provided, particularly preferably data on the moisture content in equilibrium with the surrounding air.

Preferably, the drying means is associated with a control unit or control unit with which the humidity and the temperature of a drying climate can be controlled or regulated.

Preferably, the desiccants have a device for determining a glass transition of the rice.

The control unit or control unit for controlling the drying climate is preferably associated with a library in which glass transition data and / or moisture content data are provided in equilibrium with the surrounding air.

The invention further relates to a method for operating a device as described above, wherein the device is controlled or regulated such that when intended use of the device for drying rice a total time t _{tot of} the drying process of less than 300 min, preferably less than 240 min , more preferably less than 200 minutes, and most preferably less than 180 minutes. Particularly preferably, a total duration t _{tot of} the drying operation of less than 120 minutes results. Further advantages, features and applications of the invention will become apparent from the following examples with reference to Table 1 and 2 and Figures 1 and 2.

Table 1 shows drying conditions that can be applied in a drying device according to the invention.

Table 1:

The rice grains listed in this example were harvested at a water content of 20 g / 100 g total weight (20% wb). The rice was transferred to the drying apparatus in which drying was carried out by means of convective drying with conditioned air. The water content of the rice after drying was about 12% wb. The conditions of this drying air (temperature T and relative humidity RH) were applied over the total drying time of 180 minutes according to the nominal values given in Table 1. The total drying time is the sum of the residence times t (see 4th column of the table) for a particular climate (T / RF combination, see column 2 and column 3 of the table). Table 2 shows alternative drying conditions that can be applied in a drying device according to the invention.

Table 2:

The rice grains listed in this example were harvested at a water content of 20 g / 100 g total weight (20% wb). The rice was transferred to the drying apparatus in which drying was carried out by means of convective drying with conditioned air. The water content of the rice after drying was about 12% wb. The conditions of this drying air (temperature T and relative humidity RH) were applied over the total drying time of 120 minutes according to the nominal values given in Table 2.

In Figure 1, a glass transition curve in a temperature / humidity diagram is exemplified in Table 1. Plotted is the temperature T of the surface of the rice grain against the equilibrium moisture U of the surface of the rice grain, expressed in weight percent based on the total weight of the rice grain. As the humidity increases, the glass transition temperature decreases. In the inventive method, rice grains are brought into a state A at a temperature in the device of 60 ⁰ C and dried there for 60 min in this state A, ie at least the rice grain surface has this temperature during drying in the state A. Subsequently, the rice grains are brought into a state B. at a temperature in the device of 70 ⁰ C and there for 60 min in this state B dried, ie at least the rice grain surface has this temperature during drying in the state B. Then, the rice grains are brought into a state C at a temperature in the device of 80 ⁰ C and dried there for 60 min in this state C, ie at least the rice grain surface has this temperature during drying in the state C. The total drying time is thus 180 min.

In Figure 2, an alternative inventive method is shown in Table 2. Here, rice grains are brought into a state A and there for 60 min in this state A brought at a temperature in the device of 90 ⁰ C and there for 60 min in this state A. dried. Subsequently, the rice grains are brought into a state B at a temperature in the device of 60 ⁰ C and dried there for 60 min in this state B. The total drying time is thus 120 minutes.

The step of drying in the state B is used to standardize the moisture over the entire grain of rice. When drying in the state A drying of the surface of the rice grains under a target moisture, but the surface remains in the viscoelastic state. The interior of the grain of rice has increased moisture relative to surface moisture. So it creates a moisture gradient in the rice grain. When drying in the state B substantially no loss of moisture of the rice grain is more to the environment, i. the total moisture content of the rice grain remains substantially constant. There is a moisture transport from the inside of the rice grain to the surface until an equilibrium has been established and, in particular, the selected target moisture is established.

Claims

claims

A process for drying rice in which the rice grains undergo conditions at the surface during the drying process which have different value pairs of temperature (T) of the surface and moisture (U) of the surface, and wherein at least a portion, preferably the entire surface of the rice grains remains during at least part of the drying process, preferably during the entire Trock ^¬ drying process in a viscoelastic state, wherein in a graph of temperature (T) of the surface of the rice grains and humidity (U) of the surface

a) the temperature (T _v ) of the surface of the rice grains is not more than 40 ⁰ C above the temperature (T _g ) on the glass transition curve of the rice at the same moisture point of the surface, preferably not more than 30 ⁰ C, ^{¬ ¬} particularly preferred not more than 20 ^° C .;

and or

b) U _v), the moisture (the surface of the rice grains is not more than 20% of the moisture on the glass ^¬ transition curve of the rice at the point of the same temperature of the surface, preferably not more than 15%, particularly preferably not more than 10%.

2. The method according to claim 1, characterized in that the surface of the rice grains pass through during the drying process to ^¬ stands, different temperature tur / moisture conditions (T; U), wherein the through ^¬ these states run in particular by increasing the temperature- temperature of the surface and decrease in the moisture content of the surface.

3. The method according to any one of claims 1 to 2, characterized in that the temperature and humidity of the surface of the rice grains are controlled or regulated during the drying process such that the time required for the entire drying process is minimized.

4. The method according to any one of claims 1 to 3, characterized in that for controlling or regulating the process, a glass transition curve in a diagram of temperature (T) and moisture (U) of the surface of the rice is used.

5. The method according to any one of claims 1 to 4, characterized in that during drying, the equilibrium moisture is monitored on the surface of the rice grains.

6. The method according to any one of claims 1 to 5, characterized in that during the drying process of the rice grains undergone viscoelastic states in which the rice grains temperatures (T _v ) of the surface and humidities (U _v ) of the surface, of states at the glass transition with glass transition temperatures ( _Tg ) and glass transition humidities

(Ug) Minimum distances ΔU _min = U _v - U _g parallel to the moisture axis U in a graph of temperature (T) and moisture (U) of the surface of the rice grains, where for ΔU _min : 0.5% <ΔU _min <5 %.

7. The method according to claim 6, characterized in that for the minimum distance ΔU _M IN is: 1% <ΔU _M IN <3.5%.

8. The method according to claim 6, characterized in that for the minimum distance ΔU _M IN is: 1.5% <ΔU _M IN <2.5%.

9. The method according to any one of claims 1 to 8, characterized in that in the drying process of the rice grains undergone viscoelastic states in which the rice grains temperatures (T _v ) of the surface and humidities (U _v ) of the surface of states at the glass transition with glass transition temperatures ( _Tg ) and glass transition humidities

(Ug) Minimum distances ΔT _M i _N = T - T _g parallel to the temperature axis T have in a diagram of temperature (T) and moisture (U) of the surface of the rice grains, where for ΔTMIN: IK <ΔT _M IN <1OK ,

10. The method according to claim 9, characterized in that for the minimum distance .DELTA.T _M IN: IK <.DELTA.T _M IN <5K.

11. The method according to any one of claims 1 to 10, characterized in that in the drying process of the rice grains undergone viscoelastic states in which the rice grains temperatures (T _v ) of the surface and humidities (U _v ) of the surface, of states at the glass transition with glass transition temperatures ( _Tg ) and glass transition humidities

(Ug) Maximum distances ΔU _MAX = U - U _g parallel to the moisture axis U in a graph of temperature (T) and moisture (U) of the surface of the rice grains, where for ΔUMAX: 5% <ΔUMAX <20%.

12. The method according to claim 11, characterized in that for the maximum distance ΔUMAX applies: 5% <ΔUMAX <10%.

13. The method according to any one of claims 1 to 12, characterized in that in the drying process of the rice grains undergone viscoelastic states in which the rice grains temperatures (T _v ) of the surface and humidities (U _v ) of the surface of states at the glass transition with glass transition temperatures ( _Tg ) and glass transition humidities

(Ug) Maximum distances ΔT _Max = T - T _g parallel to the temperature axis T in a graph of temperature (T) and moisture (U) of the surface of the rice grains, where ΔT _Ma χ holds: 1OK <ΔT _Max <4OK.

14. The method according to claim 13, characterized in that for the maximum distance .DELTA.T _Hax applies: 1OK <.DELTA.T _Hax <30K.

15. The method according to any one of claims 4 to 14, characterized in that the glass transition curve was determined by measurements on samples of rice before and / or during drying.

16. The method according to any one of claims 4 to 14, characterized in that the glass transition curve is taken or derived from a library of glass transition data and / or rice data.

17. The method according to any one of claims 1 to 16, characterized in that during at least a part, preferably during the entire drying process, the relative humidity of the drying environment and / or the temperature of the drying environment of the rice are controlled or regulated so that at least a portion of the Surface, preferably the entire surface of the rice is not embrittled, but remains in a viscoelastic state.

18. The method according to any one of claims 1 to 17, characterized in that the time integral of the temporal temperature profile T (t) in ⁰ C over the total drying time t _TO τ is less than 20 x 10 ³ min ⁰ C.

19. The method according to claim 18, characterized in that the time integral of the temporal temperature profile T (t) in ⁰ C over the total drying time t _TO τ is less than 15 x 10 min 0C.

20. The method according to any one of claims 1 to 19, characterized in that the rice grains are moved during the drying process relative to each other and / or kept relatively spaced.

21. An apparatus for producing rice, in particular using a method according to one of claims 1 to 20, comprising at least one temperature sensor and at least one moisture sensor for determining the temperature and the humidity of the drying climate.

22. The device according to claim 21, characterized in that the at least one temperature sensor and humidity sensor is associated with a control unit or control unit, which is programmed or programmable such that the implementation of a method according to one of claims 1 to 20 is possible ,

23. The device according to claim 22, characterized in that connected to the control unit or the control unit is a library or connectable, in the glass transition data and / or data on the moisture content in the same weight are provided with the surrounding air.

24. A method for operating a device according to one of claims 21 to 23, characterized in that the device is controlled or regulated such that, when the device is used according to the purpose of drying rice, in particular rice grains, a total duration t _{tot of} the drying process of less than 300 minutes, preferably less than 240 minutes, more preferably less than 200 minutes, most preferably less than 180 minutes and most preferably less than 120 minutes.

25. Rice prepared by a process according to any one of claims 1 to 20 and / or using a device according to any one of claims 21 to 23, which was operated in particular by a method according to claim 24.