Abstract:
Food material ( 12 ) is processed using a pulsed laser beam ( 16 ), wherein the wavelength of the laser beam is in the near-infrared (IR) range and the laser beam has a focussed laser spot ( 18 ). The method comprises the step of applying a laser pulse with a pulse duration in the range of 1 to 1000 fs to the food material, wherein the focussed laser spot lies on the surface of the food material or in the body of the food material and the laser pulse creates a cavity in the food material at the position of the focussed laser spot.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to a method of processing food material using a pulsed laser beam. 
       BACKGROUND ART 
       [0002]    It is known in the art to use continuous wave (CW) or pulsed laser beams with pulse durations in the ns range to cut or slice food material, such as cheese, meat or bakery products. Commonly, CO 2  lasers with wavelengths in the long-wavelength infrared (IR) range (8 to 15 μm) are used for this method. A cut or hole in the food material is usually created due to the melting and evaporation or sublimation of the material in the vicinity of the laser beam. One of the major problems associated with this approach is that a significant amount of heat is generated during the cutting process, leading for example to unclean cutting edges and thermal damage, such as burning, of the food material. This effect is particularly problematic when the laser cutting method is to be applied to thermally sensitive food material, e.g., with a low melting temperature, such as chocolate or confectionery. 
         [0003]    Recently, a method of slicing cheese using a pulsed laser beam with wavelengths (266 and 355 nm) in the ultraviolet (UV) wavelength range has been investigated (see “H. Choi and X. Li, Journal of Food Engineering 75, pages 90-95, 2006”). A pulse duration of 10 ns and a repetition rate of 20 Hz were used. In this approach, the food material is cut by photo-ablation, i.e., the uppermost layer of food material is consecutively vaporised (ablated) by the influence of the laser beam, thus creating a kerf and finally a cut line at the position of the beam. 
       SUMMARY 
       [0004]    Generally speaking, these techniques facilitate providing a reliable, precise and non-damaging method of processing food material. 
         [0005]    By one approach these teachings provide a method of processing food material using a pulsed laser beam, wherein the wavelength of the laser beam is in the near-infrared (IR) range and the laser beam has a focussed laser spot. The method comprises the step of applying a laser pulse with a pulse duration in the range of 1 to 1000 fs to the food material, wherein the focussed laser spot lies on the surface of the food material or in the body of the food material and the laser pulse creates a cavity in the food material at the position of the focussed laser spot. The term “near-infrared (IR) range” designates a wavelength range of about 750 to 1400 nm. The term “cavity” refers to a hollow space or recess that is formed in the surface or inside the body of the food material, depending on the position of the focussed laser spot. 
         [0006]    Since the area where the cavity is formed is essentially restricted to the position of the focussed laser spot, the size of the cavity created by the laser pulse is substantially determined by the size of the laser spot. In conventional optical techniques laser spot sizes of a few μms or even below 1 μm can be readily achieved, so that the cavity formation can be restricted to a very small area or volume. 
         [0007]    In addition, mainly due (it is thought) to the extremely short pulse duration but also due to the low photon energy of light in the near-IR range (as compared to UV light for example), only a relatively small amount of energy is deposited in the food material during the laser pulse and substantially no heat is generated outside the position of the laser spot. Thus, the cavity can be created in the food material with a high degree of precision and without causing any thermal damage to the material surrounding the cavity. These teachings can therefore also be applied to thermally sensitive food material, such as chocolate, confectionery or ice cream. 
         [0008]    In one embodiment, these teachings comprise the step of applying a sequence of laser pulses with a pulse duration in the range of 1 to 1000 fs to the food material, wherein the focussed laser spot lies on the surface of the food material or in the body of the food material and each laser pulse creates a cavity in the food material at the position of the focussed laser spot. 
         [0009]    By one approach, this can further comprise the step of moving the position of the focussed laser spot over the surface of the food material and/or through the body of the food material while applying the sequence of laser pulses, thereby creating a sequence of cavities in the food material. Herein, the movement of the laser spot can be effected, for example, by scanning the laser beam over the stationary food material or, alternatively, by keeping the laser beam stationary and moving the food material relative to the laser spot position by use of a positioning unit. A combination of these two techniques, i.e., moving both the laser beam and the food material simultaneously, would also be feasible. The method of this embodiment may for example be used to alter the texture and/or consistency (mouth feel) of food material by creating a plurality of cavities on its surface and/or inside its body without causing any damage to the material, e.g., by burning it. Furthermore, providing the surface of a food material with a number of cavities may be used to change the appearance and/or the grip feel of food material. 
         [0010]    By one approach the sequence of cavities in the food material defines a cutting line or a cutting plane along which the food material is cut. In this case, no additional preparation of the food material prior to cutting, such as freezing, dehydration, embedding in resin or paraffin or decalcification, is required, unlike with conventional cutting or slicing techniques. Since the volume of the cavities is essentially limited by the size of the laser spot, which can be made very small, as detailed above, and since no thermal damage is caused in the material surrounding the cavities, well-defined and precise cutting lines and/or cutting planes can be achieved. Furthermore, the method of the invention may be used to drill holes or grooves with accurately defined shapes and dimensions into any given food material. 
         [0011]    By one approach these teachings will also accommodate the step of separating the cut food material at the cutting line or cutting plane. In some embodiments, an additional external force acting on the cut food material (apart from gravity) is required for its complete separation. Due to its high level of precision, the present method allows for a controlled separation (cutting, slicing etc.) of food material with accuracies in the μm range. Moreover, problems associated with conventional cutting, slicing or milling techniques, such as the generation of a large amount of frictional heat, causing thermal damage to the food material, are avoided since with the present method no thermal damage is induced outside the area of the cavity (or cutting line/plane). In this way, even small food particles, such as sugar, sugar alternatives or salt crystals, can be precisely cut and shaped without generating a fine particle fraction (residue, debris) as in conventional techniques. Food material particle sizes, shapes and geometries can be controlled, on the pm scale, all at the same time, enabling a variety of food processing possibilities. Due to its high level of accuracy and control, the present method may advantageously be employed to produce a plurality of evenly shaped food particles, e.g., sugar, sugar alternatives or salt crystals, with identical particle shapes and/or sizes. 
         [0012]    For example, the present method may be used to controllably cut or mill sugar particles (or sugar alternatives, such as artificial sweeteners) without inducing the formation of amorphous layers in the sugar since substantially no heat is generated outside the cutting area. Cutting or milling sugar particles in this way offers various advantages. First, the risk of generating any undesired flavours in the processed particles is extremely low. Second, sugar with well-defined particle shapes and sizes can for example, be used in high concentrated suspensions, e.g., in confectionery products, to reduce the caloric value of the material because less fat phase is required to get at least similar flow properties and sensorial perception, such as mouth feel and taste release during chewing. By exceeding a required minimum quantity to form molecular solvent layers on such cut or milled particles, the overall creaminess of a food product can be controlled while at the same time fat add-on levels are kept very low. 
         [0013]    By one approach these techniques may be used to produce single particles with textured surfaces so as to manipulate the interfacial surface tension in order to reduce the amount of fat required to form particle surfaces that are completely covered with a monolayer. Moreover, the overall sweetness perception of a product can be manipulated by texture design features. In addition, if hydroscopic sugar particles are cut or milled in a precisely controlled manner, their material properties, such as their melting temperature etc., can be controllably altered. Furthermore, sugar or salt crystals could be cut or milled so as to exhibit a desired geometrical shape, such as a cube. Such precisely cut or milled crystals may then be used as seed crystals for growing larger crystals with a crystalline structure that is significantly improved in terms of defects, imperfections, contaminants etc. 
         [0014]    On the other hand, the present method can also be advantageously applied to larger sized food materials, such as nuts, cocoa beans, fruits or vegetables. For example, the method may be used to make the surface of skinned nuts desolate, so as to inhibit the migration of nut oils from the inside of the nuts to their surfaces. In this way, the formation of fat bloom can be avoided and the nuts can be prevented from drying out, thus extending their storage life. Furthermore, the method can be employed for peeling or cutting fruits or vegetables, such as salad. If, for example, a leaf of salad is cut using the method of the invention, the salad tissue in the vicinity of the cutting area remains undamaged after the cutting process, thereby avoiding the formation of brown edges. 
         [0015]    By one approach, at least a portion to be processed of the food material is optically transparent at the wavelength of the laser beam. 
         [0016]    In this case, the focussed laser spot may be positioned such that it lies in the body of the food material, i.e., inside the food material, underneath its surface. With such an approach, the food material to be processed may be purely cut inside its body without the need to cut its surface. For example, a plurality of cavities may be formed inside the food material in order to alter its texture and/or consistency (e.g., mouth feel), leaving its surface unchanged. Furthermore, an “invisible” (i.e., not visible from the outside) breaking line or plane can be created within a food material, such as a chocolate tablet, acting as a predetermined breaking area. Such lines may be used to guide the consumer, for example, to use a portion associated with a certain caloric value. 
         [0017]    For many application settings the pulse duration can advantageously be in the range of 1 to 800 fs, more preferably in the range of 1 to 400 fs. All else being equal, the shorter the duration of the applied laser pulse or pulses is, the smaller is the amount of energy deposited in the food material per laser pulse. Thus, a decrease in pulse duration yields a further increase in the precision with which a cavity can be formed in the food material. This is particularly beneficial for the case that food material which is extremely susceptible to thermal damage is processed. 
         [0018]    By one approach the repetition rate of the sequence of laser pulses is preferably in the range of 1 to 1000 MHz. A repetition rate of this order allows for the fast processing of food materials, in particular when used in combination with a fast laser scanner and/or positioning unit. 
         [0019]    The teachings will accommodate when the cavity (cavities) in the food material is (are) created by photodisruption. The term “photodisruption” designates the process of creating a cavity (hollow space) in a material by inducing an optical breakdown in the area of the material where the cavity is to be formed. Specifically, the high light intensity within the focussed laser spot causes ionisation of the atoms of the material within the spot region through non-linear effects, such as multiphoton or cascade ionisation, thus creating a plasma at the spot position. If the density of the thus generated free electrons exceeds a given threshold value, an optical breakdown occurs. The locally created plasma gives the energy stored therein off to the material in the region of the laser spot, whereby said material is disrupted and a cavity is formed. The photodisruption process is a very localised process that is essentially limited to the region of the focussed laser spot. Therefore, cavity (or cutting line/plane, drill hole) formation by photodisruption allows for a high degree of positional precision without causing any thermal damage to the material surrounding the cavity (or cutting line/plane, drill hole). 
         [0020]    By one approach at least a portion to be processed of the food material has a plane and even surface. Herein, the term “even” designates a plane surface with a low surface roughness, such as a peak to valley value (distance between the highest and the deepest surface irregularity) of no more than 4 μm and an RMS value (Root Mean Square; mean square deviation related to the surface) of no more than 2 μm (e.g., for a sugar cube with an edge length of 20±2 μm). Such a geometry of the food material allows for a precise positioning of the focussed laser spot, whether on the surface or in the body of the food material, and an accurate control of its exact size. In this way, complications, such as deterioration of the focus due to inhomogeneous light absorption, reflection, diffraction or scattering, that may arise in the case of an uneven or rough food material surface can be avoided. Moreover, a liquid, such as an immersion oil, may be applied to the surface of the food material portion to be processed, in order to fill surface valleys or troughs and thereby further smoothen the surface. Such a liquid may further have good index matching properties so as to match the refractive index of the food material to be processed, thus minimising losses due to light scattering and reflection. 
         [0021]    For many application settings it may be helpful that at least a portion to be processed of the food material exhibits substantially no pin holes in the material and/or has a surface that is substantially free of defects and/or imperfections. Such a configuration of the food material allows for a further improvement of the controllability and precision of the processing step. 
         [0022]    By one approach the food material to be processed by the method of the invention is sugar or salt or a nut or a cocoa bean or a fruit or chocolate or milk powder or salad or ice cream. In this case, numerous beneficial effects and various possible applications of the present method have already been explained above. On the other hand, the method of the invention is not restricted to these materials but may in general be advantageously applied to any kind of food material, such as cocoa husks, meat, cheese, fish or frozen foods. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]    Hereinafter non-limiting examples and experimental results of the method of the invention are explained with reference to the drawings, in which: 
           [0024]      FIG. 1  shows a schematical cross sectional representation of the set-up used for applying the method according to these teachings; 
           [0025]      FIG. 2  shows an OCT (Optical Coherence Tomography) image of a food material sample (rock sugar) prior to cutting; 
           [0026]      FIG. 3  shows an OCT image of the food material sample of  FIG. 2  after being cut using the method according to the embodiment of  FIG. 1 ; 
           [0027]      FIG. 4  shows an OCT image of another food material sample (rock sugar) after being cut using the method according to the embodiment of  FIG. 1 ; 
           [0028]      FIG. 5  shows an SEM (Scanning Electron Microscopy) image of another food material sample (rock sugar) after being cut using the method according to the embodiment of  FIG. 1 ; 
           [0029]      FIG. 6  shows an SEM image with larger magnification of the food material sample of  FIG. 5 ; 
           [0030]      FIG. 7  shows an SEM image of yet another food material sample (rock sugar) after being cut using the method according to the embodiment of  FIG. 1 ; and 
           [0031]      FIG. 8  shows an SEM image with larger magnification of the food material sample of  FIG. 7 . 
       
    
    
     DETAILED DESCRIPTION 
       [0032]      FIG. 1  shows a schematical cross sectional representation of an illustrative set-up used for applying a method in accord with these teachings. The set-up includes a commercially available laser microtome  10  (Laser Microtome LMT F14 by Rowiak GmbH) and a sample holder  14 . A food material sample  12 , which, in this embodiment, is a piece of rock sugar, is placed on the sample holder  14  with a layer of immersion oil applied between sample  12  and holder  14  for optical adaptation. 
         [0033]    A conventional OCT (Optical Coherence Tomography) device (“Spectral Radar” by Thorlabs HL), which is not shown in  FIG. 1 , is used to image the rock sugar sample  12  from the side, i.e., in a direction perpendicular to the x-z plane (see  FIG. 1 ) prior to and after performing the cutting. The parameters of the OCT device used when taking the images were a wavelength of about 930 nm, an image rate of 1 Hz, an axial and lateral resolution (i.e., in z- and x-direction, see  FIGS. 1 ) of 4 to 6 μm and an image size of 1024×512 pixels. 
         [0034]    Furthermore, a conventional scanning electron microscope (not shown in  FIG. 1 ) is employed to image the rock sugar sample surface parallel to the plane of the sample holder  14 . The laser microtome  10  produces a pulsed laser beam  16  with a wavelength of about 1030 nm and a focussed laser spot  18 . The rock sugar sample  12  is optically transparent at this wavelength of the laser. Prior to cutting, the rock sugar sample  12  may be ground, e.g., by using a fine abrasive paper, so as to create a plane and even sample surface, allowing for a precise positioning of the focussed laser spot and an accurate control of its size during the cutting step. 
         [0035]    The rock sugar samples shown in  FIGS. 3 and 4  are continuously cut along the x-direction with the focussed laser spot  18  positioned in the body of the sample  12  (see  FIG. 1 ). During the cutting process, the laser spot  18  is moved across the sample  12  by use of a laser scanner that is part of the laser microtome  10  and not explicitly shown in  FIG. 1 , resulting in a “planar” cutting line  20  that lies entirely in a sample plane parallel to the plane of the sample holder  14  ( FIG. 1 ). 
         [0036]    In principle, the present teachings can be used to create all kinds of different cutting line or plane geometries with a high degree of precision. An example of such a different geometry, namely a “tunnel” cutting line or plane ( 20 ′, see  FIG. 1 ), will be explained below with reference to  FIGS. 5 to 8 . The cavity formation and hence also the formation of the cutting lines (planes)  20 ,  20 ′ in the rock sugar samples  12  is based on the physical process of photodisruption which is explained in detail above. For cutting the rock sugar samples  12  shown in  FIGS. 3 and 4 , the laser pulse duration was about 350 fs and the repetition rate was 10 MHz. The beam power during cutting was about 1 W and the cutting speed was about 1.5 mm/s. The thickness of the cut line  20  in the z-direction was chosen to be 75 μm ( FIGS. 3 ) and 50 μm ( FIG. 4 ), respectively. During the cutting process, a yellow glow was observed in the sample  12 , which is attributed to the generation of a plasma, owing to the fact that the food material  12  is cut due to photodisruption. 
         [0037]    The OCT images shown in  FIGS. 2 to 4  are turned upside down as compared to the representation of the set-up geometry shown in  FIG. 1 , so that the bottom side of  FIGS. 2 to 4  is the side where the pulsed laser beam  16  enters the sample  12 . 
         [0038]    An OCT image of the rock sugar sample  12  prior to cutting is shown in  FIG. 2 . The surface  22  of the sample holder  14  and the surface  24  of the rock sugar sample  12  can be clearly identified. 
         [0039]      FIG. 3  shows an OCT image of the rock sugar sample  12  of  FIG. 2  after the cutting was performed with the set-up geometry depicted in  FIG. 1 , using the method and parameters detailed above. A cutting line  20  (thickness 75 μm) is formed within the body of the rock sugar sample  12  just underneath its surface  24 , as evidenced by a bright line  20  in the OCT image that is substantially parallel to the surface  22  of the sample holder  14 . A comparison of  FIG. 3  with  FIG. 2  shows that the sugar material underneath the cutting line  20 , i.e., the material through which the pulsed laser beam  16  had to pass for cutting the line  20 , is substantially unchanged, that is, no damage was done to this material during the cutting process. 
         [0040]      FIG. 4  shows an OCT image of another rock sugar sample after the cutting was performed using the same geometry, method and parameters as those of  FIG. 3 , apart from the thickness of the cutting line (here 50 μm). As in the case of  FIG. 3 , a cutting line  20  that is formed within the body of the rock sugar sample just underneath its surface  24  can be clearly identified (bright line  20  in  FIG. 4 ). 
         [0041]      FIGS. 5 to 8  show SEM (Scanning Electron Microscopy) images of two further rock sugar samples after being cut using the method according to the embodiment of  FIG. 1  with a laser pulse duration of about 400 fs and a pulse repetition rate of 10 MHz. The set-up geometry used was substantially that of  FIG. 1  with the only difference that the laser beam  16  was shone onto the sample from underneath, through the sample holder  14 . As a sample holder  14 , a glass slide was employed that is transparent to laser light at the wavelength used for the cutting process (1030 nm). 
         [0042]    As has been indicated above, the sample shown in  FIGS. 5 and 6  and the sample shown in  FIGS. 7 and 8  were cut differently from the samples of  FIGS. 3 and 4 , namely with a “tunnel” cutting line or plane  20 ′. As is schematically shown in  FIG. 1 , such a tunnel cutting plane  20 ′ comprises a horizontal portion substantially parallel to the plane of the sample holder  14  and two vertical portions substantially perpendicular to the horizontal portion and connected thereto. By applying such a cut geometry, well-defined structures can be cut out and lifted off from the sample. In this way, a plurality of evenly shaped food particles with identical particle sizes and/or shapes, such as cubes or bars, can be quickly and efficiently produced.  FIGS. 5 to 8  show arrays of the vertical portions of such cutting planes  20 ′, wherein these portions have a depth (along the z-direction, see  FIG. 1 ) of 30 μm extending from the sample surface into the body of the sample and are arranged in parallel to one another. 
         [0043]      FIGS. 6 and 8 , which have a larger magnification than  FIGS. 5 and 7 , show the presence of protruding or “overhanging” sample portions  26  adjacent to the vertical cut portions, demonstrating that, in these areas, material was removed from underneath the sample surface during the cutting process without damaging the overlying sample layers and thus indicating the presence of a horizontal tunnel cut portion. 
         [0044]    As is evident from  FIGS. 2 to 8 , these teachings can be used for cutting a transparent food material sample  12  inside its body with a high degree of precision and without damaging the material surrounding the cutting line (plane)  20 ,  20 ′ or the surface  24  of the sample  12 .  FIGS. 5 to 8  further demonstrate that the present method is capable of creating, in a sample, arrays of cutting lines and/or planes  20 ′ having a well-defined geometry with a high degree of precision. The method may thus, for example, be advantageously employed to produce, in an efficient and quick manner, a plurality of evenly shaped food particles with identical particle sizes and/or shapes.