Heat treatment method for liquid food

Abstract

To provide a heat treatment method of a fluid food product which is mixed with a solid matter such as a puree of fruit or vegetable.SOLUTION: A high frequency pulse power source unit 9 is formed of a high pressure DC power source 20, and switches 21a to 21d for turning supply of current from the high pressure DC power source 20 ON / OFF. The switches 21a and 21b and the switches 21c and 21d are arranged in parallel with loads, respectively. Specifically, a switch 21a is provided on a wiring connecting a positive electrode of a high pressure DC power source 20 and one electrode across loads, a switch 21b is provided on a wiring connecting the positive electrode of a high pressure DC power source 20 and a non-contact electrode 15 across the loads, a switch 21c is provided on a wiring connecting the negative electrode of the high pressure DC power source 20 and the non-contact electrode 15 across the loads, and the switch 21d is provided on a wiring connecting the negative electrode of the high pressure DC power source 20 and the other contact electrode 16 across the loads.SELECTED DRAWING: Figure 3

Description

[Technical field]

[0001] The present invention relates to a method for continuously heat treating a liquid food product, such as a puree made by mashing raw fruits or vegetables, between electrodes. [Background technology]

[0002] Fruit and vegetable purees can be used without leaving any food ingredients, and can be used as raw materials for various processed foods, and also have many uses as foods for the elderly and infants who have difficulty chewing. As a method for sterilizing such purees to improve their shelf life, the following heat sterilization method has been conventionally known.

[0003] Patent Document 1 proposes a device for applying an alternating electric field to liquid food to heat and sterilize it, which comprises plate-like or cylindrical electrodes spaced apart from each other and an insulator placed between the electrodes, with the central electrode connected to a high-frequency power source and the electrodes placed outside the central electrode being grounded electrodes, and with a flow path for the liquid food formed in the insulator.

[0004] Patent Document 2 discloses a method for heat sterilization of liquid foods using an electric current unit sandwiched between one or more pairs of metal electrodes, in which the electric current unit is a closed system and microorganisms are sterilized by continuously passing filtered liquid food through the electric current unit to which a voltage is applied.

[0005] Patent Document 3 discloses that unheated fruit juice or unheated vegetable juice is continuously passed through an apparatus equipped with an electric current supply unit using an AC power source to inactivate the juice, and then cooled.

[0006] Patent Document 4 discloses a sterilization method in which liquid food is passed between a pair of electrodes to apply an electric field in which a pulse electric field with a field strength of 1.0 kV / cm or more is superimposed on an AC electric field that raises the temperature of the object to be sterilized up to 90°C.

[0007] Patent Document 5 proposes an apparatus for continuous sterilization of liquid food, which comprises a pumping means for continuously flowing liquid food material through a flow path, an AC high voltage application unit for applying an AC voltage between a pair of electrodes arranged at a predetermined distance within the flow path to perform AC high electric field sterilization, a continuous cooling unit for continuously cooling within the flow path downstream, and a flow rate detection means for detecting the flow rate of the liquid food material passing between the electrodes.

[0008] Patent Document 6 describes a method for producing vegetable puree by squeezing juice from shredded vegetables, in which the vegetables are heat-treated at a temperature of 65 to 95°C for five minutes after the temperature is reached, either after shredding or simultaneously with shredding.

[0009] Patent Document 7 proposes a fresh fruit paste and a preservative for fresh fruit paste, which are made by turning fruit directly into a paste, maintaining the color, flavor, and paste structure after processing, and are stable for a long period of time as a filling or topping for confectionery, bread, etc.

[0010] Patent Document 8 discloses a method in which apples with the skin are preheated at 80°C to 100°C for 5 to 15 minutes to destroy the skin and the tissues of the surrounding area, and juice is extracted in a state in which the pectin and pigments contained in large amounts in the skin and the surrounding area are dissolved.

[0011] Patent Document 9 discloses a method for producing apple juice in which a heat treatment that serves to inactivate enzymes and sterilize the juice is carried out in a later step, and the fruit is crushed, the juice is squeezed, the pulp content is controlled, and the juice receiving tank is treated all under a nitrogen gas atmosphere, and after the heat treatment, the juice is filled into containers.

[0012] Patent document 10 describes a method for producing a processed fruit puree having the flavor and storage stability of fresh fruit puree, in which fruit is crushed in a mixer to make a puree, and this puree is then heat-sterilized at a heating temperature of 55°C for a heating time of 60 to 90 minutes.

[0013] Patent Document 11 describes a method for heating food using microwaves, which employs at least two types of microwaves with different frequencies and irradiates the microwaves with different frequencies through a waveguide and a waveguide to heat the food.

[0014] Patent Document 12 describes a sterilization method in which a liquid product is rapidly and uniformly heated to a predetermined temperature by resistive heating, specifically, a minimum of one pulse is applied to each fluid element during passage with a pulse duration of at least 10 microseconds, and the electric field strength is 0.1 to 5 kV / cm. [Prior art documents] [Patent documents]

[0015] [Patent Document 1] JP 2018-201460 A [Patent Document 2] JP 2007-029014 A [Patent Document 3] JP 2005-080562 A [Patent Document 4] JP 2007-229319 A [Patent Document 5] JP 2006-296368 A [Patent Document 6] Japanese Patent Application Publication No. 10-000076 [Patent Document 7] JP 2017-225365 A [Patent Document 8] JP 2012-179034 A [Patent Document 9] JP 2002-281949 A [Patent Document 10] JP 2018-029569 A [Patent Document 11] Japanese Patent Application Publication No. 7-255388 [Patent Document 12] Special Publication No. 2018-536435 Summary of the Invention [Problem to be solved by the invention]

[0016] As disclosed in the above-mentioned patent documents, methods of thermal sterilization of food currently in use include boiling in water, using a heat exchanger, applying an AC electric field, and applying a pulsed electric field. Among these, the method of heat sterilization using a water bath takes time to raise the temperature of the center of the food, and there is a risk of thermal denaturation when applied to purees. The method of heat sterilization using a heat exchanger cannot process highly viscous liquids such as purees.

[0017] When liquid food is passed between electrodes and heated by applying an AC electric field or a pulsed electric field, the temperature can be raised to the center of the food in a short time, which solves the problem of thermal denaturation. However, AC electric fields, which have a narrow electrode gap, are not suitable as a means of heat sterilization of purees. When using an AC power supply as a pulse power supply, a step-up transformer must be inserted at the final stage to obtain a high voltage, which means that the pulse waveform is no longer a desirable square wave. Also, it cannot handle sudden impedance fluctuations in the load.

[0018] That is, fruit and vegetable purees contain a lot of solid matter such as fiber, and as the puree flows through the flow path between the electrodes, the solid matter in the puree bridges the electrodes, causing an extreme drop in impedance, as shown in Figure 9. Vegetable fibers are not metallic and do not cause a short circuit, but the drop in impedance increases the flow of current, which can destroy semiconductor switches such as IGBTs (Insulated Gate Bipolar Transistors) and MOSFETs that make up the switching unit. Therefore, it is possible to insert a protective resistor, but the voltage applied to the load would be reduced by the amount of the protective resistor, reducing heating efficiency. [Means for solving the problem]

[0019] In order to solve the above problems, the present invention provides a method for heating a liquid food by flowing the liquid food through a flow path formed between a grounded electrode and a non-grounded electrode and applying a voltage to the liquid food, wherein the non-grounded electrode is connected to a high-voltage DC power supply unit, and this high-voltage DC power supply unit consists of a high-voltage DC power supply and a plurality of switches arranged in parallel with the load, and a DC high-frequency pulse generated by the ON-OFF operation of these switches is applied to the liquid food via the electrode.

[0020] The liquid food is typically a fruit or vegetable puree, but the present invention is also suitable for use with other liquid foods that contain solid matter that may cause short circuits in the electrodes. Effect of the Invention

[0021] According to the heat treatment method of the present invention, the temperature of the liquid food to be treated, such as fruit or vegetable puree, can be raised to the temperature required for sterilization in a short period of time, and thermal denaturation of the fruit or vegetable puree, etc. can be prevented. In addition, the oxidase can also be inactivated, allowing the product to be stored for a long period of time without discoloration.

[0022] In addition, instead of using a high-frequency AC power supply, a high-voltage DC power supply is used, and high-frequency pulses are generated by switching, resulting in high heating efficiency and stable heating and sterilization without a drop in voltage even if impedance between the electrodes decreases due to solid matter contained in fruit and vegetable purees, etc. [Brief description of the drawings]

[0023] [Figure 1] Overall view of a system for carrying out the heat treatment method for liquid food according to the present invention. [Diagram 2] (a) Perspective view of the electrode unit, (b) Exploded view of the electrode unit [Diagram 3] Circuit diagram of high voltage DC power supply unit (all switches are OFF) [Figure 4] Circuit diagram of high voltage DC power supply unit (+ voltage) [Diagram 5] Circuit diagram of high voltage DC power supply unit (- voltage) [Figure 6] (a) is the waveform of a high-frequency pulse, and (b) is a graph showing the temperature history. [Figure 7] Photographs comparing the method of the present invention with the conventional method [Figure 8] Graph showing the enzyme activity of each experimental example in FIG. 5 [Figure 9] Diagram explaining the short circuit state in the flow path DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0024] Hereinafter, an embodiment of the present invention will be described with reference to the accompanying drawings. The heat treatment system sends liquid food such as fruit or vegetable puree stored in tank 1 to electrode unit 3 using pump 2, and the liquid food that has been heat-treated in electrode unit 3 is cooled in cooler 4 and then sent to outlet 6 via exhaust pump 5.

[0025] A thermometer 8 is disposed between the pump 2 and the electrode unit 3, a high-voltage DC power supply unit 9 is connected to the ungrounded electrode of the electrode unit 3, a thermometer 10 and a temperature holding tube 11 are disposed between the electrode unit 3 and the cooler 4, a pressure gauge 12 is disposed between the cooler 4 and the back-pressure pump 5, and a thermometer 13 is disposed downstream of the back-pressure pump 5. The measured values ​​of the thermometers 8, 10, and 13 are sent to a data logger 14.

[0026] 2, the electrode unit 3 has a structure in which three flat electrodes are arranged in parallel, with the central ungrounded electrode 15 connected to the high-voltage DC power supply unit 9, the left and right grounded electrodes 16, 16 being grounded, and Teflon (registered trademark) insulators 17, 17 being densely arranged between the grounded electrodes 16, 16 and the ungrounded electrode 15. The grounded electrodes 16, 16 are also insulated from the terminals of the high-voltage DC power supply unit 9.

[0027] The non-grounded electrode 15 and the grounded electrodes 16, 16 are formed with holes 18 for the liquid food, and the insulators 17, 17 are formed with elongated holes 19 serving as flow paths.

[0028] 3, the high voltage DC power supply unit 9 includes a high voltage DC power supply 20 and switches 21a, 21b, 21c, and 21d that turn on and off the supply of current from the high voltage DC power supply 20. Semiconductor switches such as IGBT (Insulated Gate Bipolar Transistor) elements are used as the switches 21a to 21d.

[0029] The switches 21a and 21b and the switches 21c and 21d are arranged in parallel with the load. Specifically, the switch 21a is provided on a wiring that connects the positive electrode of the high voltage DC power supply 20 and one electrode across the load, the switch 21b is provided on a wiring that connects the positive electrode of the high voltage DC power supply 20 and the non-grounded electrode 15 across the load, the switch 21c is provided on a wiring that connects the negative electrode of the high voltage DC power supply 20 and the non-grounded electrode 15 across the load, and the switch 21d is provided on a wiring that connects the negative electrode of the high voltage DC power supply 20 and the other grounded electrode 16 across the load.

[0030] To generate a high-frequency pulse, a state in which switches 21a and 21d are ON and switches 21b and 21c are OFF (FIG. 4) and a state in which switches 21a and 21d are OFF and switches 21b and 21c are ON (FIG. 5) are alternately repeated. In this manner, the waveform shown in FIG. 6 is obtained.

[0031] In conventional AC high electric field power supplies, DC from a DC power supply of about 200V is turned into AC by opening and closing a switch, which is then boosted to 1000V by a step-up transformer, and an AC electric field is applied to a load via a protective resistor of about 1Ω. In contrast, in the present invention, the current from a 1000V high voltage DC power supply is applied directly to the load without being converted to AC via a transformer. Therefore, even if solid matter in the puree bridges the electrodes and the impedance drops, the allowable current of the pulse power supply is large (80A), so no voltage drop occurs until the load current exceeds the allowable current, and stable heat sterilization can be performed.

[0032] In order to verify the effect of the present invention, the high frequency pulsed electric field treatment (HFP) of the present invention was compared with the conventional hot bath treatment (CH) and with no treatment. In order to confirm whether browning of the puree can be suppressed by browning, apples, which are prone to browning, were used as ingredients. The apples used were early Fuji apples from Yamagata Prefecture, which were peeled with a peeler, cored with a cork borer, cut into eight equal pieces, and then crushed in a mixer while adding 0.2% ascorbic acid solution for the tests.

[0033] In the high frequency pulsed electric field treatment (HFP), apple puree was conveyed at a constant flow rate (25 ml / s) in the system shown in Figure 1, and high frequency pulses with the waveform shown in Figure 6(a) were applied. The temperature history is shown in Figure 6(b). Note that the temperature history up to 90°C is omitted in Figure 6(b).

[0034] On the other hand, in the hot bath treatment (CH) as a comparative example, 50 g of the same sample was vacuum-packaged and heated by immersing it in thermostatic water baths at 60°C, 70°C, and 80°C for 30 minutes.

[0035] Figure 7 is a graph showing the polyphenol polyoxidase (PPO) activity of the samples subjected to the above heat treatment, and Figure 8 shows photographs of each sample. 7 and 8 show that high-frequency pulsed electric field treatment (HFP) or conventional hot bath treatment (CH) inactivates polyphenol polyoxidase and inhibits browning after 24 hours at 25°C.

[0036] Both high frequency pulsed electric field treatment (HFP) and conventional hot bath treatment (CH) inactivate enzymes, but the method of the present invention has the advantage that the treatment can be completed in a short time and that thermal denaturation of the puree can be suppressed. [Industrial Applicability]

[0037] The heat treatment method for liquid foods according to the present invention can be applied to foods suitable for an aging society, foods for infants, and the like. [Explanation of symbols]

[0038] DESCRIPTION OF SYMBOLS: 1...tank, 2...pump, 3...electrode unit, 4...cooler, 5...exhaust pump, 6...outlet, 8, 10, 13...thermometer, 9...high voltage DC power supply unit, 11...temperature holding tube, 12...pressure gauge, 14...data logger, 15...ungrounded electrode, 16...grounded electrode, 17...Teflon (registered trademark) insulator, 18...hole for liquid food, 19...slot, 20...high voltage DC power supply, 21...switch.

Claims

1. A method for heating a liquid food by flowing the liquid food through a flow path formed between a grounded electrode and a non-grounded electrode and applying a voltage to the liquid food, wherein one of the grounded electrode and the non-grounded electrode is arranged in parallel on both sides of the other electrode, the grounded electrode is connected to one of the positive or negative electrodes of a high-voltage DC power supply unit via a plurality of switches arranged in parallel, the non-grounded electrode is connected to the other of the positive or negative electrode of the high-voltage DC power supply unit via a plurality of switches arranged in parallel, the high-voltage DC power supply unit does not include a step-up transformer, and a DC high-frequency pulse generated by the ON-OFF operation of the switch is applied directly from the electrode to the liquid food.

2. 2. The method for heat-treating a liquid food according to claim 1, wherein the voltage of the high-voltage DC power supply unit is 1000V.

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