WATER CONTENT OF FOOD

Field of the Invention

The present invention relates to food analysis, and particularly to methods and apparatus for determining water content of food.

Background of the Invention

Water content of food is a ratio that represents the quantity of water contained in food including chemically bound water and non-chemically bound "free" water. Water content plays an important role in food quality/safety monitoring, food storage conditioning and food processing control. It largely determines food texture, taste as well as appearance. A traditional method for measuring water content is to measure the weight of the food before and after all the water in the food is removed by, for example, evaporation and to have the difference of weight divided by the initial weight of the food.

Water activity is another ratio represents the quantity of non-chemically bound "free" water of food at a given temperature. It may be expressed as vapor pressure of water in the food divided by the vapor pressure of pure water at the given temperature. For the same food, there is a corresponding relationship between its water content and water activity, which may be described in a sorption isotherm curve. Under a mild heating condition, after water exchange between air and the food has reached an equilibrium state, water vapor in the air may be deemed as dominated by the non-chemically bound "free" water evaporated from the food, and therefore relative humidity of the air may be deemed as the water activity of the food at the given temperature, assuming initial water vapor in the air is negligible . Traditionally, water activity is obtained by means of capacitance or a dew point hygrometer.

However, both of these two methods require the water exchange equilibrium state to be achieved which takes minutes or even hours of time. Besides, such methods are mainly used in industry and research labs, which are either expensive or too complex for applications in consumer products such as kitchen appliance. Object and Summary of the Invention

Because of the issues stated above, it would be advantageous to achieve a more time efficient method and apparatus for determining water content of food. It al so would be advantageous to achieve a less expensive and less complex method and apparatus for determining water content of food suitable for consumer product applications.

In one aspect, one embodiment of the invention provides a method for determining water content of food contained in a container. The method comprises at a pre-determined temperature and at a position within a predetermined distance from a surface of said food to sense changes of said plurality of relative humidity values before water exchange in said container reaches an equilibrium state; calculating water activity of said food at the pre-determined temperature based at least on said plurality of relative humidity values; and determining water content of said food based at least on said water activity. By using such a method, water content of food may be determined before water exchange between food and air in the container is achieved, and therefore the method saves significant amount of time compared to traditional methods.

Advantageously, to increase the signal to noise ratio, said step of calculating water activity of said food in the method further comprises calculating a rate of change for said plurality of the relative humidity values; and calculating the water activity of said food at the pre-determined temperature based on said rate of change.

Advantageously, said step of measuring a plurality of relative humidity values is performed using ultrasound wave and comprises detecting a plurality of transmission velocities of the ultrasound wave transmitting in the container. To obtain accurate measurement of relative humidity values of air in the container, temperature is considered as a factor in measuring relative humidity values. Therefore, said step of measuring further comprises determining said plurality of said relative humidity values based at least on said plurality of transmi ssion velocities of the ultrasound wave and said pre-determined temperature.

Advantageously, said container is a closed space to avoid influence from air outside the container.

Advantageously, because relative humidity values and water activity are both temperature dependent, therefore the above method further comprises maintaining said food at the pre-determined temperature.

Advantageously, the method further comprises decreasing water in the air in said container prior to said step of maintaining. This is to reduce influence of water vapor in the air in the container prior to the measurement of relative humidity.

In one aspect, another embodiment of the invention provides a food processing method comprises determining at least one control parameter of said food processing based on the water content determined through one or more of the above described steps. By using this method, control parameters such as cooking time, cooking temperature, the amount of water to be added for cooking and so forth may be customized based on the water content which reflects the quality and status of food.

In one aspect, another embodiment of the invention provides an apparatus for determining water content of food in a container. The apparatus comprises a container, configured to contain food; a heater coupled with said container, configured to maintain said food in said container at a pre-determined temperature; a hygrometer coupled with said container, configured to measure a plurality of relative humidity values of air in said container before water exchange in said container reaches an equilibrium state, wherein said hygrometer is arranged at a position to sense changes of said plurality of relative humidity values; and a first processor coupled with said hygrometer, configured to calculate water activity of said food based at least on said plurality of relative humidity values, and to determine water content of said food based at least on said water activity. By using such an apparatus, water content of food may be determined before the equilibrium state of the water exchange between food and air in the container is achieved, and therefore significant amount of time may be saved. In addition, said method may be performed even if said container is not a closed space, which renders a very wide application of such method, simplifies requirements of the measurement and saves measurement time.

Advantageously, the first processor of the apparatus is further configured to process said plurality of relative humidity values to generate a rate of change and calculate the water activity based at least on said rate of change. Therefore, signal to noise ratio may be increased.

Advantageously, the hygrometer of the apparatus comprises an ultrasound wave detector configured to detect a plurality of transmission velocities of the ultrasound wave transmitting in the container; a second processor configured to obtain the plurality of relative humidity values based at least on the plurality of transmission velocities of said ultrasound wave; and a thermometer coupled with said container, configured to detect one or more temperature of the air in said container; wherein said second processor is further configured to obtain said plurality of relative humidity values based at least on said one or more temperature detected by said thermometer. By considering temperature as a factor, the accuracy of the measurement of relative humidity values is accordingly increased. Such an apparatus does not require an absolute dry air working as a reference air in order to obtain the relative humidity values of the air in the container. Also, the measurement of the relative humidity values does not require the temperature in the container to reach an equilibrium state.

Advantageously, said container is a closed space so that influence of air outside the container may be avoided.

In one aspect, another embodiment of the present invention provides a food processing device. The device is configured to determine at least one control parameter for processing of said food based on water content determined by any of the above described apparatus. By using this device, food processing control factors such as cooking time, cooking temperature and the amount of water to be added when cooking may be customized based on the water content which reflects the quality and status of the food.

In one aspect, another embodiment of the present invention provides an apparatus for determining water content of food in a container. The apparatus comprises a hygrometer, configured to measure a plurality of relative humidity values of air in said container containing said food before water exchange in said container reaches an equilibrium state, wherein said hygrometer is arranged at a position to sense changes of said plurality of relative humidity values; and a processor coupled with said hygrometer, configured to calculate water activity based at least on said plurality of relative humidity values, and to determine water content of said food based at least on the water activity. Such an apparatus may enable traditional cooking device to determine water content by plugging the apparatus into the container of the cooking device.

Advantageously, said processor of the apparatus is configured to determine quality of said food or adjust storage conditions of said food based on the water content. Such a device enables a more accurate analysis of food quality and a more efficient adjustment of the storage conditions of the food. Brief Description of the Drawings

The above and other objects and features of the present invention will become more apparent from the following detailed description considered in connection with the accompanying drawings, in which:

FIG. 1 shows an apparatus for determining water content of food in a container in accordance with one embodiment of the present invention;

FIG. 2 shows another apparatus for determining water content of food in a container in accordance with another embodiment of the present invention;

FIG. 3 is a flowchart of a method for determining water content of food in a container in accordance with one embodiment of the present invention;

FIG. 4 is an example of water desorption isothermal curve;

FIG. 5 is a flowchart of another method for determining water content of food in a container in accordance with another embodiment of the present invention; and

FIG. 6 shows an ultrasound wave hygrometer in accordance with one embodiment of the present invention.

Detailed Description

Reference will now be made to embodiments of the invention, one or more examples of which are illustrated in the figures. The embodiments are provided by way of explanation of the invention, and are not meant as a limitation of the invention. For example, features illustrated or described as part of one embodiment may be used with another embodiment to yield still a further embodiment. It is intended that the invention encompass these and other modifications and variations as come within the scope and spirit of the invention.

Figure 1 illustrates an apparatus 100 for determining water content of food. Food 101 may be contained in a container 102 of apparatus 100. F ood 101 may be various kinds of food such as rice, bananas and so forth. Container 102 may be made of glass, metal, plastic or other materials with good thermal conductivity and stability. In one embodiment, food 101 and container 102 may be kept in an isothermal environment. Apparatus 100 may also comprise a heater 104 configured to maintain container 102 and food 101 at a temperature T forming the isothermal environment. In various embodiments, heater 104 may be a thermal bath or heated air flow.

Apparatus 100 may also comprise a hygrometer 106 configured to measure relative humidity values of the air in container 102. Various kinds of hygrometer may be used herein to measure the relative humidity values. In one embodiment of the present invention, an ultrasound wave hygrometer may be used for the measurement. Detailed introduction of ultrasound wave hygrometer is provided in view of Figure 5.

In various embodiments, hygrometer 106 may be positioned close enough to the surface of food 101 in order to measure instantaneous relative humidity values and changes of the relative humidity values of the air in container 102 during the process of water exchange between the food and the air In one embodiment, the lower limit of the distance between ultrasound wave hygrometer 106 and the surface of food 101 may be at least 1 cm to avoid touching of the food surface by the ultrasound wave. The upper limit of the distance between hygrometer 106 and the surface of food 101 may depend on propagation speed of water molecules in the air in container 102 and hygrometer 106' s own properties, such as dimensions of the hygrometer 106 and so forth. Because the propagation speed of water molecules is very fast, for example up to 400m/s, dimensions of container 102 may become the dominant constraint of the upper limit of the distance between hygrometer 106 and the surface of food 101. In other embodiments, the position of hygrometer 106 may be adjusted to meet different measurement requirements.

Apparatus 100 may also comprise a processor 110 configured to process data received from hygrometer 106, to calculate water activity of food 101 and to consequently determine water content of food 101. In one embodiment, processor 110 may comprise a driver 112 configured to send out signals to control hygrometer 106 in measuring the relative humidity values. Processor 1 10 may also comprise a storage medium 114 configured to store data received from hygrometer 106. In one embodiment, processor 110 may also comprise an output unit or a display unit 116 configured to output the determined value of water content of food 101. In other embodiments, output unit of processor 1 10 may be configured to control one or more control parameters for cooking of food 101. The control parameters may be, for example, the amount of water to be added for cooking, cooking time, or cooking temperature and so on.

Figure 2 illustrates another embodiment of apparatus 200 for determining water content of food. Apparatus 200 may comprise a hygrometer 220 and a processor 210 which are configured to perform substantially similar operations as performed by the above described hygrometer 106 and processor 110. In one embodiment, apparatus 200, in particular hygrometer 220, may be constructed to be plugged into a container of a cooking device to determine water content of food enclosed in the container of the cooking device.

Figure 3 is a flowchart for apparatus 200 to determine water content of food in a container. At block 302, hygrometer 220 may be configured to measure relative humidity values of the air in the container. In one embodiment, the container of food is covered with a lid to form a closed space to avoid influence of air outside the container. In other embodiments, the container may not have to be a closed space as long as the measurement can be done within very short period of time, for example 5 seconds

At block 304, processor 210 may be configured to calculate water activity of the food based on data received from hygrometer 220. According to water dynamics theories and the assumption that water released from the food under a mild heating condition is dominated by the non-chemically bound "free" water in the food, instantaneous relative humidity value RH of the air in container may be expressed in terms of the water activity a _w :

wherein t is the time at which relative humidity value is measured, and a and β are empirical factors which are food type and temperature dependent. Generally speaking, a is a factor far less than 1 whereas β is usually close to 1. For example, for bananas, a is 0.04 at 50°C, a is 0.031 at 60°C, a is 0.021 at 70°C, and in all cases β is about 1.

In one embodiment, to increase signal to noise ratio, a rate of change DR for the relative humidity values at different time may be calculated as follow:

DR{t) = tit = £^ . -½i- ^ffit ¾ ^\ . (2)

Considering that a is far less than 1, and β is usually about 1, and the measurement time t is only up to a few seconds, the following simplifications may be made

Equation 2 may be further simplified as follow:

DR(t) » ¾G ½

Therefore, water activity a _w may be expressed as follow based on the rate of change for at least two relative humidity values:

wherein ti and t ₂ are two different measurement time. In various embodiments, ti and t ₂ are within a preliminary stage of the isothermal heating of the container and the food, for example ti and t ₂ may be within the first 5 seconds since the isothermal heating is started.

In other embodiments, hygrometer 220 may be configured to measure more than two relative humidity values at different time to further increase the signal to noise ratio. In another embodiment, a nonlinear regression may be performed based on Equation 1 or 2, when the humidity values measured cannot be fitted in a linear relationship with the water activity.

At block 306, processor 210 may be configured to determine water content C _w of the food based on the water activity a _w calculated at block 304 and a corresponding relationship between water activity a _w and water content C _w of the food. In one embodiment, the corresponding relationship may be expressed as a moisture desorption isotherm curve dependent on the type of food, heating temperature and air pressure. Figure 4 shows an example of the water desorption isotherm curve of a given type of food at a pre-determined temperature.

Figure 5 is a flowchart of another method for apparatus 100 to determine water content of food. The operations at blocks 506-510 are substantially similar to the operations performed by apparatus 200 at blocks 302-306 in Figure 3. In various embodiments, at block 502 apparatus 100 may be further configured to pre-heat container 102 with the lid 108 open to decrease the water in the air in container 102. By doing so, the initial relative humidity may be reduced to a negligible level so that the water in the air of the container may be assumed to be dominated by non-chemically bound "free" water released from the food, so that the calculation of the water activity may be more accurate. Another reason for performing such a step is to reduce the water pressure in the air of the container to be significantly below the water content of the food so that a desorption process may be established in the container within a short period of time, for example in terms of seconds. Otherwise, the desorption process may be too slow rendering an increase of the measurement time, and an adsorption isothermal process may be establi shed therefore a different corresponding relationship would be needed for determining of the water content. Alternatively, food 101 may fill up most of container 102, for example 2/3 of container 102 to decrease the initial relative humidity of the air in container 102.

At block 504, apparatus 100 may be configured to maintain container 102 at a pre-determined temperature. In various embodiments, to achieve an accurate calculation of the water activity of food 101, the heating of container 102 may be an isothermal one and may be a mild heating, for example from 25 °C to 70°C, so that no chemically bound water would receive enough energy to be released into the air of container 102.

Figure 6 is an illustration of an ultrasound wave hygrometer configured to measure relative humidity values as mentioned above. As illustrated in Figure 6, ultrasound wave hygrometer 600 may comprise a measurement space 602 which may be a closed space. In one embodiment, space 602 may be posited in an isothermal environment 606 In various embodiments, measurement space 602 may share the space with container 102 of apparatus 100.

In various embodiments, ultrasound wave hygrometer 600 may comprise one or more transducers 604 posited in space 602 configured to transmit and receive ultrasound waves. In one embodiment, ultrasound wave hygrometer 600 may comprise two transducers 604, a transmitting transducer and a receiving transducer, which may be positioned facing each other with a distance in between. In one embodiment, the distance between the transducers may be for example 30 cm. In other embodiments, hygrometer 600 may comprise only one transducer working as the transmitting as well as the receiving transducer. In various embodiments, transducer 604 may be air coupled and may be made of materials endurable under the temperature and humidity during measurement.

Ultrasound wave hygrometer 600 may also comprise processor 610 configured to generate electric pulse signals and provide the signals to transducers 604. In various embodiments, ultrasound waves with a frequency of 20 kHz to 10 M Hz may be used for the purpose of the measurement.

In one embodiment, electric pulse signals may be converted into mechanic pulse on a transmitting transducer interface and in turn a bursting wave in the air. After traveling through the air in space 602, the mechanic wave may reach the receiving transducer and may be converted back to electric signal s. In one embodiment, processor 610 may also be configured to receive signals from the receiving transducer and to calculate the traveling time of the ultrasound wave.

In one embodiment, ultrasound wave hygrometer 600 may also comprise a thermal meter 608 coupled with processor 610 configured to measure temperature in space 602 and provide the measured temperature to processor 610. In one embodiment, thermal meter 608 may be accurate to at least 0.05°C.

In various embodiments, processor 610 may be configured to calculate the transmission velocity of the ultrasound wave based on the traveling time and the traveling distance of ultrasound wave. Therefore, processor 610 may be further configured to calculate the relative humidity values based on the temperature measured by thermometer 608 and the transmission velocity of the ultrasound wave, using the following Equation 5 :

v = a ₀ (T) + a ₁ (T)RH + a ₂ (T)RH ² (5) wherein ao, &i, a ₂ are temperature dependent factors obtained through a prior curve-fitting step, T is the temperature in space 602 during the measurement of the relative humidity values.

In one embodiment, processor 610 may be configured to output the relative humidity values calculated to processor 1 10 of apparatus 100 for further calculation of the water activity.

It should be noted that the above described embodiments are given for describing rather than limiting the invention, and it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention as those skilled in the art readily understand. Such modifications and variations are considered to be within the scope of the invention and the appended claims. The protection scope of the invention is defined by the accompanying claims. In addition, any of the reference numerals in the claims should not be interpreted as a limitation to the claims. Use of the verb "comprise" and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. The indefinite article "a" or "an" preceding an element or step does not exclude the presence of a plurality of such elements or steps.