JP7627907B2 - Heat treatment method for liquid food - Google Patents

Description

The present invention relates to a method for continuously heat-treating liquid foods, such as purees made by mashing raw fruits or vegetables, between electrodes.

Fruit and vegetable purees allow the full use of food ingredients, and can be used as ingredients in a variety of processed foods. They also have many uses as food for the elderly and infants who have difficulty chewing. The following heat sterilization method has been known as a method for sterilizing such purees to improve their shelf life.

Patent Document 1 proposes a device for applying an AC electric field to liquid food to heat and sterilize the food, which is made up of 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 serving as ground electrodes, and with a flow path for the liquid food formed in the insulator.

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.

Patent Document 3 discloses that unheated fruit juice or unheated vegetable juice is deactivated by continuously passing it through a device equipped with a current-carrying unit using an AC power source, and then cooled.

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 an electric field strength of 1.0 kV/cm or more is superimposed with an alternating electric field that raises the temperature of the object to be sterilized to 90°C.

Patent Document 5 proposes an apparatus for continuously sterilizing liquid food, which has a pumping means for continuously flowing liquid food material through a flow path, an AC high voltage application section 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 section 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.

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 after shredding or simultaneously with shredding for within 5 minutes after the temperature is reached.

Patent Document 7 proposes a fresh fruit paste and a preservative for fresh fruit paste that are stable for a long period of time as a filling or topping for confectionery and bread by turning fruit directly into a paste and maintaining the color, flavor, and shape of the paste after processing.

Patent Document 8 discloses that apples with the skin are preheated at 80°C to 100°C for 5 to 15 minutes to destroy the skin and the tissues around it, and juice is extracted in a state where the pectin and pigments contained in large amounts in the skin and the surrounding tissue are dissolved.

Patent Document 9 discloses a method for producing apple juice in which a heat treatment that inactivates enzymes and sterilizes 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.

Patent Document 10 describes a method for producing processed fruit puree that has the flavor and storage stability of fresh fruit puree by crushing fruit in a mixer to produce a puree, and then sterilizing the puree by heating at 55°C for 60 to 90 minutes.

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.

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.

JP 2018-201460 A JP 2007-029014 A JP 2005-080562 A JP 2007-229319 A JP 2006-296368 A Japanese Patent Application Publication No. 10-000076 JP 2017-225365 A JP 2012-179034 A JP 2002-281949 A JP 2018-029569 A Japanese Patent Application Publication No. 7-255388 Special table 2018-536435 publication

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.

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.

In other words, fruit and vegetable purees contain a lot of solid matter such as fiber, and as shown in Figure 9, when 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. Because vegetable fibers are not metallic, they do not cause a short circuit, but the drop in impedance increases the flow of current, which could destroy the semiconductor switches that make up the switching unit, such as IGBTs (Insulated Gate Bipolar Transistors) and MOSFETs. To prevent this, 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.

To solve the above problems, the present invention is a method for heating a liquid food by passing the liquid food through a flow path formed between a grounded electrode and an ungrounded electrode and applying a voltage to the liquid food, in which the ungrounded electrode is connected to a high-voltage DC power supply unit, which is made up of a high-voltage DC power supply and a plurality of switches arranged in parallel with the load, and DC high-frequency pulses generated by the ON-OFF operation of these switches are applied to the liquid food via the electrodes.

The liquid food is typically a fruit or vegetable puree, but the present invention can also be suitably applied to other liquid foods that contain solid matter that can cause short circuits in the electrodes.

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.

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

Overall view of a system for carrying out the heat treatment method for liquid food according to the present invention. FIG. 2A is a perspective view of an electrode unit, and FIG. 2B is an exploded view of the electrode unit. Circuit diagram of high voltage DC power supply unit (all switches OFF) High voltage DC power supply unit circuit diagram (+ voltage) Circuit diagram of high voltage DC power supply unit (- voltage) (a) is a graph showing a waveform of a high-frequency pulse, and (b) is a graph showing a temperature history. Photographs comparing the method of the present invention with the conventional method Graph showing the enzyme activity of each experimental example shown in FIG. Diagram explaining the short circuit state in the flow path

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.

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

As shown in Figure 2, the electrode unit 3 is structured with three flat electrodes arranged in parallel, with the central non-grounded electrode 15 connected to the high-voltage DC power supply unit 9, and the left and right grounded electrodes 16, 16 grounded, with Teflon (registered trademark) insulators 17, 17 densely arranged between the grounded electrodes 16, 16 and the non-grounded electrodes 15. The grounded electrodes 16, 16 are also insulated from the terminals of the high-voltage DC power supply unit 9.

The non-grounded electrode 15 and the grounded electrodes 16, 16 have holes 18 through which liquid food can pass, and the insulators 17, 17 have long holes 19 that serve as flow paths.

As shown in FIG. 3, the high-voltage DC power supply unit 9 is composed of 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.

Switches 21a, 21b and switches 21c, 21d are each arranged in parallel with the load. Specifically, switch 21a is provided on the wiring connecting the positive electrode of high-voltage DC power supply 20 and one electrode sandwiching the load, switch 21b is provided on the wiring connecting the positive electrode of high-voltage DC power supply 20 and the non-grounded electrode 15 sandwiching the load, switch 21c is provided on the wiring connecting the negative electrode of high-voltage DC power supply 20 and the non-grounded electrode 15 sandwiching the load, and switch 21d is provided on the wiring connecting the negative electrode of high-voltage DC power supply 20 and the other grounded electrode 16 sandwiching the load.

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

In conventional AC high electric field power supplies, AC is generated by opening and closing a switch from a DC power supply of about 200V, which is then boosted to 1000V by a step-up transformer, and an AC electric field is applied to the load via a protective resistor of about 1Ω. In contrast, in the present invention, the current from the 1000V high voltage DC power supply is applied directly to the load without being converted to AC via a transformer. Therefore, even if the 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.

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-ripening Fuji apples from Yamagata Prefecture, which were peeled with a peeler, cored with a cork borer, cut into eight equal pieces, and crushed in a mixer while adding 0.2% ascorbic acid solution for use in the test.

In high frequency pulsed electric field treatment (HFP), apple puree was transported 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).

On the other hand, in the hot bath treatment (CH) used 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.

Figure 7 is a graph showing the measurement of polyphenol polyoxidase (PPO) activity in the samples subjected to the above heat treatment, and Figure 8 is a photograph of each sample.
7 and 8 show that by performing high-frequency pulsed electric field treatment (HFP) or conventional hot bath treatment (CH), polyphenol polyoxidase is inactivated and browning after 24 hours at 25° C. is suppressed.

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 thermal denaturation of the puree can be suppressed.

The heat treatment method for liquid foods according to the present invention can be applied to foods for an aging society and foods for infants.

1...tank, 2...pump, 3...electrode unit, 4...cooler, 5...back pressure 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...long hole, 20...high voltage DC power supply, 21...switch.

Claims (2)

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. 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 .

Images