JP3570330B2

JP3570330B2 - Refrigeration method and apparatus

Info

Publication number: JP3570330B2
Application number: JP2000061599A
Authority: JP
Inventors: 哲英横山; 和彦川尻; 憲朗光田; 美喜男森; 正毅池内; 明小沢
Original assignee: Mitsubishi Electric Corp
Current assignee: Mitsubishi Electric Corp
Priority date: 2000-03-07
Filing date: 2000-03-07
Publication date: 2004-09-29
Anticipated expiration: 2020-03-07
Also published as: JP2001245645A

Description

【０００１】
【発明の属する技術分野】
本発明は、魚介類、畜産物、野菜類、果実類などの生鮮食品、菓子類などの加工食品、臓器、血液などの細胞組織体などの冷凍方法および装置に関するものである。
【０００２】
【従来の技術】
生鮮食品等を冷凍して、その解凍時に鮮度や味を維持するためには、組織体の細胞を破壊しないこと、および濃縮（細胞外に溶質が流出する）を抑制することが重要である。これらが起こると、解凍時に液汁の流出（いわゆるドリップ）が発生して品質低下を招くためである。通常、水分を有する生鮮食品、加工食品、生物の組織体などの細胞を破壊することなく冷凍するためには、最大氷結晶生成帯（被冷凍体によって異なるが、一般的には−１〜−５℃の氷結晶が最も成長する温度帯）を通過する時間（以下、有効凍結期間と称す）を短くすることが有効である。この有効凍結期間を短くすることにより、氷の結晶を小さくできるので細胞の破壊を防止できるとともに、濃縮を抑制することができる。
【０００３】
上記のような理由から、鮮度や味を維持する手段として、大型冷凍機や、液体窒素や液体二酸化炭素などの極低温液体が用いられている。これらを用いて冷凍することにより、有効凍結期間を短くでき、上記細胞破壊や濃縮を抑制することができる。
【０００４】
しかしながら、前者の大型冷凍機で急速冷凍すると、原理的に被冷凍体表面からの熱伝導により内部を冷却されるため、例えばマグロのように大きな食品になると冷凍が完結するまでに数分〜数時間要し、この間に被冷凍体の表面と内部との温度差を生じ、被冷凍体表面と内部との有効凍結期間差が大きくなり、特に被冷凍体表面の氷の結晶が大きくなって細胞が破壊されたり、濃縮がおこる場合があった。また、後者の極低温液体を用いる方法では、有効凍結期間を短くできるが、原料の供給が必要でコスト高になるという問題があった。
【０００５】
このような問題を解決する手段として、常温から氷結点付近まで比較的急速に冷却する急速冷却処理を行い、続いて、被冷凍体表面と内部との温度差を小さくするため、氷結点以下まで０．０１〜０．５℃／時間の緩慢な冷却速度で冷却するスロークーリング処理を行い、この後に急速冷凍を行う方法が、例えば特開平８−２５２０８２号公報に開示されている。
また、同公報に、破壊点（未凍結領域の下限点）以上の温度帯で、５００ＭＨｚ〜５ＧＨｚの周波数領域のマイクロ波を照射することも開示されている。
これらの方法では、上記スロークーリング処理の過程、およびマイクロ波照射の過程で、氷結点以下の過冷却状態（液相を保った状態）が維持でき、結果として被冷凍体表面と内部との温度差を小さくすることができるため、被冷凍体表面の氷の結晶が大きくなって細胞が破壊されたり、濃縮が起こることを防止できる。
【０００６】
【発明が解決しようとする課題】
しかしながら、もともと上記過冷却状態は不安定なため、振動や電場などの外乱が加わると容易に過冷却状態が破壊される。また、一旦被冷凍体表面に氷の結晶が生成すると、この状態で液相の被冷凍体内部にマイクロ波が吸収され、被冷凍体表面と内部の温度差はいっそう広がり、被冷凍体表面で氷の結晶成長が局部的に進み、細胞組織の破壊、濃縮を逆に増大させる危険があった。
また、冷却速度０．０１〜０．５℃／時間のスロークーリングでは、冷凍完了までに時間がかかりすぎるという問題もあった。
【０００７】
この発明は上記の問題を解決するためになされたもので、被冷凍体表面と内部との温度差を小さくした状態で、過冷却状態（およびこれに近い状態）を維持できる、また、氷が生成しても、結晶成長を小さくして、被冷凍体の細胞破壊およおび濃縮を抑制し、生鮮食品等の品質の低下を防止できる冷凍方法、および冷凍装置を提供することを目的とするものである。
【０００８】
【課題を解決するための手段】
この発明に係わる第１の冷凍方法は、中波、短波、超短波のいずれかの周波数の電磁波を用いて、被冷凍体を誘電加熱し、前記誘電加熱により前記被冷凍体に吸収されるエネルギーよりも大きなエネルギーで冷却して、被冷凍体を凍結させるものである。
【０００９】
この発明に係わる第２の冷凍方法は、前記第１の発明の誘電加熱を、氷の比誘電損率が水より大きくなる周波数の電磁波で行うものである。
【００１０】
この発明に係わる第３の冷凍方法は、前記第１の発明の誘電加熱を、氷の比誘電損率の特異領域である５００ｋＨｚ以上６ＭＨｚ以下、あるいは３０ＭＨｚ以上６０ＭＨｚ以下の周波数の電磁波で行うものである。
【００１１】
この発明に係わる第４の冷凍方法は、前記第１の発明の電磁波の照射手段を、アンテナにしたものである。
【００１２】
この発明に係わる第５の冷凍方法は、前記第１の発明の電磁波の照射手段を、ＴＥＭ（Transverse Electromagnetic）セルにしたものである。
【００１３】
この発明に係わる第１の冷凍装置は、前記第１ないし第５のいずれかの発明の電磁波を発生する手段と、前記電磁波を被冷凍体に照射して誘電加熱する手段と、前記誘電加熱により前記被冷凍体に吸収されるエネルギーよりも大きなエネルギーで冷却して被冷凍体を凍結させる手段とを備えたものである。
【００１４】
【発明の実施の形態】
実施の形態１．
以下、本発明の実施の形態を図に基づいて説明する。図１は、本発明の実施の形態１における生鮮食品等の冷凍方法を温度と時間の関係で表した説明図である。この冷凍方法においては、まず、被冷凍体を冷却し、被冷凍体表面の温度が最大氷結晶生成帯に近づいた時点で高周波誘電加熱を開始する。この時高周波誘電加熱により被冷凍体が吸収するエネルギーより大きなエネルギーで冷却することにより、冷却エネルギーは被冷凍体に奪われ、被冷凍体表面と被冷凍体内部の温度は、最大氷結晶生成帯内で、ほぼ同じ温度となる。その後冷却エネルギーは被冷凍体の温度を更に下降させるために消費され、被冷凍体表面温度が最大氷結晶生成帯下限を超える。この時点で、誘電加熱を終了し、被冷凍体の温度をさらに下降させ、生鮮食品等の冷凍を行う。
上記で説明した最大氷結晶生成帯の範囲は、被冷凍体が変われば変わるが原理的には同じである。
【００１５】
上記誘電加熱とは、高周波電界中の誘電体（絶縁体）の誘電損失現象を利用して加熱する方法であり、周波数３００ｋＨｚ〜３ＭＨｚの中波、あるいは３〜３０ＭＨｚの短波、あるいは３０〜３００ＭＨｚの超短波を用いた誘電加熱を高周波誘電加熱という。上記電磁波を被冷凍体に照射すると、比較的温度の高い被冷凍体内部に吸収される特性があり、さらに、マイクロ波（３００ＭＨｚ〜３０ＧＨｚ）を用いた誘電加熱に比べ、電力半減深度（電磁波エネルギーが物体内部に吸収され半分になる深さ）が大きく内部まで均一にエネルギーを与えることができ、被冷凍体全体として温度分布を均一化できる。
【００１６】
さらに、上記誘電加熱によって、誘電体に吸収されるエネルギーＰ［Ｗ／ｍ^３］を、数式で表すと、次式のようになる。
Ｐ＝（１／１．８）×ｆ×ｖ^２×ε_ｒ”×１０^−１０（１）
ε_ｒ”＝ε_ｒ’×ｔａｎδ （２）
ここで、ｆ［Ｈｚ］は周波数、ｖ［Ｖ／ｍ］は電界の大きさ、ε_ｒ”は比誘電損率、ε_ｒ’は比誘電率、ｔａｎδは誘電体損失角である。したがって、上記数式（１）より、誘電体（被冷凍体）に吸収されるエネルギーＰは、物質の比誘電損率ε_ｒ”に比例することがわかる。
【００１７】
また従来から、高周波誘電加熱の周波数領域の比誘電損率は、大部分の周波数領域で、水は氷に比べて桁違いに大きいことが知られていたが、本発明者らは、詳細に氷の誘電損失の周波数依存性を調べた。そして、Ｅｌｌｉｓｏｎらの文献（ＥｌｌｉｓｏｎＷＪ，ＪＭｏｒｅａｕ：Ｊ．Ｍｏｌ．Ｌｉｑ．Ｖｏｌ．６８，Ｎｏ．２／３，ｐ１７１−２７９，１９９６）、および真鍋らの文献（真鍋武嗣、Ｈ．ＬＩＥＢＥ、Ｇ．Ｈｕｆｆｏｒｄ：電情通学会技報、ｖｏｌ．８７，Ｎｏ．３６７，ｐ１−６，１９８８）から水（約２５℃）の比誘電率、比誘電損率を、Ｈｕｆｆｏｒｄらの文献（ＧＨｕｆｆｏｒｄ：Ｉｎｔ．Ｊ．ｏｆＩｎｆｒａｒｅｄａｎｄＭｉｌｌｉｍｅｔｅｒＷａｖｅｓ，Ｖｏｌ．１２，Ｎｏ．７，ｐ．６７７−６８２，１９９１）から氷（約−５℃）の比誘電率、比誘電損率を、白樫らの文献（白樫了ら：日本機械学会熱工学講演会論文集、ｐ４３〜４４、１９９９）から特異領域における氷の比誘電損率を鑑み、図２に示す水と氷の比誘電率ε_ｒ’、および比誘電損率ε_ｒ”と周波数の関係を得た。そしてこれより、
（１）５００ｋＨｚ以上６ＭＨｚ以下の周波数領域において、氷の比誘電損率の特異領域があり、水と氷との比誘電損率差が極めて小さくなる
（２）３０ＭＨｚ以上６０ＭＨｚ以下の周波数領域において、氷の比誘電損率の特異領域があり、水と氷との比誘電損率が逆転する
ことを把握し、最大氷結晶生成帯においても被冷凍体の氷部分に多くのエネルギーを吸収させることを見出した。つまり、氷を加熱することで氷の結晶が大きく成長することを抑制し、氷結晶の微細化と均一分布化を達成でき、被冷凍体の細胞破壊や濃縮を抑え、生鮮食品等の品質の低下を著しく防止できることがわかった。
【００１８】
実施の形態２．
図３は、本発明の実施の形態２における生鮮食品等の冷凍方法を実現できる冷凍装置を示す構成図であり、１は高周波誘電加熱機、２は三極管やトランジスタなどで構成される高周波発生電源、３はインダクタンス、４はキャパシタンス、５は印加電極、６は発振用回路、７は被冷凍体、８は冷凍機、９は冷却用熱交換器、１０は圧縮機、１１は電磁波遮断シールド、１２は断熱シールド、１３は冷凍庫、３７は絶縁台、４２は温度検出部である。
この生鮮食品等の冷凍装置においては、高周波誘電加熱機１は、高周波発生電源２、インダクタンス３、キャパシタンス４、平行板電極５を発振用回路６で接続して構成し、キャパシタンス４と例えば平行板電極からなる印加電極５とを制御して発振周波数を制御する。被冷凍体７は例えば二枚の平行板電極間に挟む。冷凍庫１３内は冷凍機８の冷却用熱交換器９で冷却され、低温雰囲気をつくることができる。
【００１９】
本実施の形態において、印加電極５として平行平板電極を示したが、電極形状は、不平行平板型、格子状、コイル状、円筒状、ローラ状などでもよい。また、移動式のものであってもよい。
【００２０】
また、本実施の形態では、発振周波数の変更によって電波障害を起こす恐れを防止するために、電磁波遮断シールド１１を設けているが、不要な場合は除いてもよい。
【００２１】
実施の形態３．
次に、上記実施の形態２の冷凍装置を用いて、被冷凍体７としてマグロの切り身を用い、前記実施の形態１の概念を示唆するために、被冷凍体７表面と内部の温度を測定した。図４は、本発明の実施の形態３による冷凍方法において被冷凍体を冷却したときの冷凍曲線である。被冷凍体７であるマグロの切り身は約２００ｇのものを用いた。発振周波数は、インダクタンス３とキャパシタンス４を制御して４７ＭＨｚ（氷の比誘電損率が水より大きくなる特異領域）にセットし、出力レベル約２．５Ｗで、被冷凍体７の表面温度が５℃に降下した時点で照射を開始し、被冷凍体７の表面温度が最大氷結晶生成帯の下限（−５℃）を超えた時点で、照射を終了することにより高周波誘電加熱を行った。この間出力レベルは約２．５Ｗ一定とした。本実施の形態の効果を示唆するために、図５に、誘電加熱を用いない従来の冷凍方法による被冷凍体の冷凍曲線を示している。
【００２２】
図４に示した本実施の形態の冷凍方法においては、氷の比誘電損率が水より大きくなる周波数の電磁波を用いて、被冷凍体を誘電加熱し、前記誘電加熱による吸収エネルギーよりも大きなエネルギーで冷却して凍結させたので、氷の結晶成長を抑制して冷凍することができ、氷結晶の微細化と均一分布化を達成できた。図５に示した従来の冷凍方法に比し、被冷凍体７表面と、被冷凍体７内部の有効凍結期間のずれ２０ａ、２０ｂが小さくなったことでも確認できる。
【００２３】
実施の形態４．
発振周波数を変えた他の実施の形態として、上記実施の形態２の冷凍装置を用いて、発振周波数２７．１２ＭＨｚ（特異領域外ではあるが氷と水の比誘電損率差が小さい）の場合の冷凍曲線を得た。図６は本発明の実施の形態４による被冷凍体の冷凍曲線である。本実施の形態では、特異領域外の高周波誘電加熱により、一旦被冷凍体に氷の生成が起こり、均一化と逆の効果を生じることを避けるために、被冷凍体表面の温度が最大氷結晶生成帯に近づいた時点で誘電加熱を行い、最大氷結晶生成帯近傍で誘電加熱を終了した。
本実施の形態では、氷の比誘電損率が水より大きい周波数４７ＭＨｚを用いた前記実施の形態３ほどの効果は、得られなかったが、従来の電磁波を照射しない冷凍方法に比べ、最大氷結晶生成帯上限の通過において、被冷凍体７表面と内部の有効凍結期間のずれ２０ａが約７分から５分とやや改善された。
【００２４】
実施の形態５．
発振周波数を変えた他の実施の形態として、上記実施の形態２の冷凍装置を用いて、発振周波数２ＭＨｚ（特異領域外ではあるが氷と水の比誘電損率差が小さい）の場合の冷凍曲線を得た。図７は本発明の実施の形態５による被冷凍体の冷凍曲線である。
本実施の形態では、上記実施の形態４と同様に、特異領域外の高周波誘電加熱により、一旦被冷凍体に氷の生成が起こり、均一化と逆の効果を生じることを避けるために、被冷凍体表面の温度が最大氷結晶生成帯に近づいた時点で誘電加熱を行い、最大氷結晶生成帯を越えしばらく後に誘電加熱を終了した。本実施の形態においても、氷の比誘電損率が水より大きい周波数４７ＭＨｚを用いた前記実施の形態３ほどの効果は、得られなかったが、従来の電磁波を照射しない冷凍方法に比べ、最大氷結晶生成帯上限の通過において、被冷凍体７表面と内部の有効凍結期間のずれ２０ａが約７分から４分とやや改善された。
【００２５】
実施の形態６．
上記実施の形態では、本発明の実施の形態における被冷凍体７の表面と内部との最大氷結晶生成帯上限および下限を超える時間差、すなわち有効凍結期間のずれ２０ａ、２０ｂを比較したが、鮮度および食感に関し、定量的な評価を行うため、氷の結晶が大きいほど多くなるドリップ量の比較を行った。ドリップ量は、本発明の実施の形態および従来の冷凍方法により冷凍したマグロの切り身を、冷凍開始から２４ｈｒ経過後、冷蔵庫（約１０℃の暗所）で自然解凍して測定した。表１は、本実施の形態の冷凍方法によるドリップ量と、型崩れ、食感を示したものである。
本発明による冷凍方法では、いずれもドリップ量を、従来の冷凍方法より少なくすることができ、型崩れもなく、食感も改善できることが確認できた。
【００２６】
【表１】

【００２７】
上記実施の形態１ないし６では、被冷凍体７として、いずれもマグロの切り身を用いたが、マグロ本体のような大きなものでは、さらに顕著な差が期待できる。また、マグロ以外の豚肉、豆腐などの食品、血液などの組織体でも同様の効果が確認できた。
【００２８】
また、上記実施の形態１ないし６では、氷の比誘電損率が水より大きくなる周波数、あるいは氷と水の比誘電損率差が小さい周波数という観点で周波数を選択したが、適正な周波数は、温度や被冷凍体に含まれる塩分、油、タンパク質、糖質などの不純物によって変化するので、被冷凍体に合わせて最適値を選ぶことが望ましい。
【００２９】
実施の形態７．
図８は、本発明の実施の形態７による生鮮食品等の冷凍装置を示す構成図であり、図において、１は高周波誘電加熱機、７は被冷凍体、８は冷凍機、９は冷却用熱交換器、１０は圧縮機、１１は電磁波遮断シールド、１２は断熱シールド、１３は冷凍庫、３１は高周波発振器、３２は高周波用アンプ、３３はアンテナ、３４は電磁波、３７は絶縁台、４２は温度検出部である。
この冷凍装置においては、例えば４７ＭＨｚの高周波を高周波発振器３１で発生させ、高周波用アンプ３２で増幅し、アンテナ３３から冷凍庫１３に、電磁波を照射するので、１００Ｗ程度以下の低出力の電磁波を広範囲に照射することが可能である。したがって、マグロや肉類などの大物が積み重ねられた冷凍庫においても、被冷凍体７を均一に加熱することができる。
【００３０】
上記アンテナ３３の長さは一般的に１／４波長以上必要であるため、例えば５０ＭＨｚの電磁波の波長６ｍに対し１．５ｍとなりスペースを要するため、ヘリカルアンテナ方式など小型化可能なものを用いることが好ましい。バイコニカルアンテナ、対数らせんアンテナ、対数周期アンテナ、ダイポールアンテナ、ループアンテナ、パラボラアンテナ、長導線アンテナなどであってもよい。また、マイクロストリップアレイ方式、特に高誘電体セラミックにアンテナを平面積層した通称セラミックアンテナは小型化、平面化に適しているため好ましい。
【００３１】
実施の形態８．
図９は、本発明の実施の形態８による生鮮食品等の冷凍装置を示す構成図であり、図において、７は被冷凍体、８は冷凍機、９は冷却用熱交換器、１０は圧縮機、１２は断熱シールド、１３は冷凍庫、３１は高周波発振器、３２は高周波用アンプ、３４は電磁波、３６はＴＥＭセル、３７は絶縁台、３８は外部方形導体、３９は中心導体板、４０は同軸コネクタ、４１は同軸終端負荷である。
この冷凍装置においては、例えば外部方形導体３８、中心導体板３９、同軸終端負荷４１からなるＴＥＭセル３６が設けられているので、高周波電気信号を強電界の電磁波に変換して、被冷凍体７に照射でき、強電界の電磁波を外部に漏らさず発生させることが可能となり、電磁波シールド１１が省略できる。したがって装置構成を簡略にすることができる。
【００３２】
【発明の効果】
この発明は、以上説明したように構成されているので、以下に示すような効果を奏する。中波、短波、超短波のいずれかの周波数の電磁波を用いて、生鮮食品を誘電加熱し、前記誘電加熱による吸収エネルギーよりも大きなエネルギーで冷却して、凍結させたので、電磁波エネルギーは、比較的温度の高い被冷凍体内部に吸収され、被冷凍体全体として均一な温度分布を得ることができる。
【００３３】
また、氷の比誘電損率が水より大きくなる周波数の電磁波を用いて、生鮮食品を誘電加熱し、前記誘電加熱による吸収エネルギーよりも大きなエネルギーで冷却して、凍結させたので、氷の結晶成長を抑制して冷凍することができ、ドリップ量の低減、型崩れを防止でき、被冷凍体の鮮度を維持することができる。
【００３４】
また、氷の比誘電損率の特異領域である５００ｋＨｚ以上６ＭＨｚ以下、あるいは３０ＭＨｚ以上６０ＭＨｚ以下の周波数の電磁波を用いて、生鮮食品を誘電加熱し、前記誘電加熱による吸収エネルギーよりも大きなエネルギーで冷却して凍結させたので、一旦被冷凍体表面に氷結晶が生成しても、液相の被冷凍体内部に電磁波は吸収されにくく、被冷凍体表面と内部の温度差を広げることなく、氷の局部成長を抑制できる。
【００３５】
また、電磁波の照射手段をアンテナにしたので、電磁波を広範囲に照射することができ、被冷凍体が大物であったり、積み重ねられた状態でも、被冷凍体を均一に加熱することができる。
【００３６】
また、電磁波の照射手段をＴＥＭセルにしたので、強電界の電磁波を外部に漏らさず、安全に誘電加熱することができる。
【００３７】
また、中波、短波、超短波のいずれかの周波数、あるいは氷の比誘電損率が水より大きくなる周波数を発生する手段と、前記電磁波を生鮮食品に照射して誘電加熱する手段と、前記誘電加熱による吸収エネルギーよりも大きなエネルギーで冷却して凍結させる手段とを備えたので、氷の結晶成長を抑制して冷凍することができ、ドリップ量の低減、型崩れを防止でき、被冷凍体の鮮度を維持することができるという効果がある。
【図面の簡単な説明】
【図１】この発明の実施の形態１による生鮮食品等の冷凍方法を示す説明図である。
【図２】この発明の実施の形態１による水と氷の比誘電率および比誘電損率と周波数の関係を示す特性図である。
【図３】この発明の実施の形態２による生鮮食品等の冷凍装置を示す構成図である。
【図４】この発明の実施の形態３による温度と経過時間の関係を示す特性図である。
【図５】従来の生鮮食品等の冷凍方法による温度と経過時間の関係を示す特性図である。
【図６】この発明の実施の形態４による温度と経過時間の関係を示す特性図である。
【図７】この発明の実施の形態５による温度と経過時間の関係を示す特性図である。
【図８】この発明の実施の形態７による生鮮食品等の冷凍装置を示す構成図である。
【図９】この発明の実施の形態８による生鮮食品等の冷凍装置を示す構成図である。
【符号の説明】
１高周波誘電加熱機、２高周波発生電源、３インダクタンス、４キャパシタンス、５印加電極、６発振回路、７被冷凍体、８冷凍機、９冷却用熱交換器、１０圧縮機、１１電磁波遮断シールド、１２断熱シールド、１３冷凍庫、２０ａ、２０ｂ有効凍結期間のずれ、３１高周波発振器、３２高周波用アンプ、３３アンテナ、３４電磁波、３６ＴＥＭセル、３７絶縁台、３８外部方形導体、３９中心導体板、４０同軸コネクタ、４１同軸終端負荷。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method and an apparatus for freezing fresh foods such as fish and shellfish, livestock products, vegetables and fruits, processed foods such as confectionery, and cell tissues such as organs and blood.
[0002]
[Prior art]
In order to freeze fresh food or the like and maintain freshness and taste during thawing, it is important not to destroy cells of the tissue and to suppress concentration (solute outflow from cells). If this occurs, the juice will flow out (so-called drip) at the time of thawing, and this will cause quality deterioration. Usually, in order to freeze without destroying cells such as fresh foods, processed foods, and biological tissues having moisture, the maximum ice crystal formation zone (depending on the frozen body, generally -1 to- It is effective to shorten the time (hereinafter, referred to as the effective freezing period) during which the ice crystal at 5 ° C. passes through the temperature zone in which it grows most. By shortening the effective freezing period, ice crystals can be reduced, so that destruction of cells can be prevented and concentration can be suppressed.
[0003]
For the reasons described above, large-sized refrigerators and cryogenic liquids such as liquid nitrogen and liquid carbon dioxide have been used as means for maintaining freshness and taste. By freezing using these, the effective freezing period can be shortened, and the cell destruction and concentration can be suppressed.
[0004]
However, if the former is rapidly frozen in a large refrigerator, the inside is cooled by heat conduction from the surface of the body to be frozen in principle, so that when the food becomes large such as tuna, it takes several minutes to several It takes time, and during this time, a temperature difference occurs between the surface and the inside of the frozen object, and the difference in the effective freezing period between the surface and the inside of the frozen object increases. Was sometimes destroyed or concentrated. In the latter method using a cryogenic liquid, the effective freezing period can be shortened, but there is a problem that the supply of raw materials is required and the cost is increased.
[0005]
As a means to solve such a problem, a rapid cooling process of relatively rapidly cooling from room temperature to around the freezing point is performed, and then, in order to reduce the temperature difference between the surface of the frozen object and the inside, to a temperature below the freezing point. A method of performing a slow cooling process of cooling at a slow cooling rate of 0.01 to 0.5 ° C./hour and thereafter performing a rapid freezing is disclosed in, for example, JP-A-8-252082.
Further, the publication also discloses that a microwave in a frequency range of 500 MHz to 5 GHz is irradiated in a temperature zone equal to or higher than a destruction point (lower limit of an unfrozen region).
In these methods, a supercooled state (a state in which a liquid phase is maintained) below the freezing point can be maintained in the process of the slow cooling process and the process of microwave irradiation, and as a result, the temperature between the surface of the frozen object and the inside thereof can be maintained. Since the difference can be reduced, it is possible to prevent the ice crystals on the surface of the frozen object from becoming large and destroying the cells or preventing concentration.
[0006]
[Problems to be solved by the invention]
However, since the supercooled state is originally unstable, the supercooled state is easily destroyed by disturbance such as vibration or electric field. Also, once ice crystals are formed on the surface of the frozen object, microwaves are absorbed into the liquid-phase frozen object in this state, and the temperature difference between the surface of the frozen object and the inside further expands, and the surface of the frozen object is exposed to the microwave. There was a danger that ice crystal growth proceeded locally and the destruction and enrichment of cell tissues would be increased.
In addition, slow cooling at a cooling rate of 0.01 to 0.5 ° C./hour has a problem that it takes too much time to complete freezing.
[0007]
The present invention has been made in order to solve the above-mentioned problem, and can maintain a supercooled state (and a state close to this) while reducing the temperature difference between the surface of the object to be frozen and the inside thereof. It is an object of the present invention to provide a refrigeration method and a refrigeration apparatus that can reduce crystal growth even when they are generated, suppress cell destruction and concentration of a frozen body, and prevent deterioration in quality of fresh foods and the like. Things.
[0008]
[Means for Solving the Problems]
The first refrigeration method according to the present invention, medium wave, short wave, by using electromagnetic waves of any frequency of the VHF, dielectrically heating the frozen body, the energy that is absorbed the to be frozen body by the dielectric heating It is to cool the object to be frozen by cooling with greater energy.
[0009]
The second refrigeration method according to the invention, the dielectric heating of the first invention, the dielectric loss factor of the ice is performed with electromagnetic waves having a frequency greater than that of water.
[0010]
The third refrigeration method according to the invention, the dielectric heating of the first invention, 500 kHz or more 6MHz or less specific region of the dielectric loss factor of the ice, or to perform an electromagnetic wave of 60MHz frequencies below 30MHz or higher It is.
[0011]
Fourth refrigeration method according to the invention, the electromagnetic wave irradiation means of the first invention is obtained by the antenna.
[0012]
Fifth refrigeration method according to the invention, the electromagnetic wave irradiation means of the first invention is obtained by the TEM (Transverse Electromagnetic) cells.
[0013]
The first refrigeration apparatus according to the present invention comprises a means for generating an electromagnetic wave of any one of the first to fifth means for dielectric heating by irradiating the electromagnetic wave to be frozen body, the dielectric heating And means for freezing the object to be cooled by cooling with energy larger than the energy absorbed by the object to be frozen.
[0014]
BEST MODE FOR CARRYING OUT THE INVENTION
Embodiment 1 FIG.
Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is an explanatory diagram showing a method of freezing fresh food and the like according to Embodiment 1 of the present invention in relation to temperature and time. In this refrigeration method, first, the object to be frozen is cooled, and high-frequency dielectric heating is started when the temperature of the surface of the object to be frozen approaches the maximum ice crystal formation zone. At this time, by cooling with higher energy than the energy absorbed by the frozen body by high-frequency dielectric heating, the cooling energy is deprived of the frozen body, and the temperature of the surface of the frozen body and the inside of the frozen body are set to the maximum ice crystal formation zone. Within, the temperature is almost the same. Thereafter, the cooling energy is consumed to further lower the temperature of the frozen object, and the surface temperature of the frozen object exceeds the lower limit of the maximum ice crystal formation band. At this point, the dielectric heating is terminated, the temperature of the frozen object is further lowered, and fresh food or the like is frozen.
The range of the maximum ice crystal formation zone described above changes when the object to be frozen changes, but is in principle the same.
[0015]
The dielectric heating is a method of heating using a dielectric loss phenomenon of a dielectric (insulator) in a high-frequency electric field, and has a medium frequency of 300 kHz to 3 MHz, a short wave of 3 to 30 MHz, or a short wave of 30 to 300 MHz. Dielectric heating using ultrashort waves is called high-frequency dielectric heating. When the object to be frozen is irradiated with the electromagnetic wave, the object has a characteristic of being absorbed inside the object to be heated at a relatively high temperature, and has a power half-depth (electromagnetic wave energy) as compared with dielectric heating using microwaves (300 MHz to 30 GHz). Is absorbed into the inside of the object, and the energy can be uniformly applied to the inside of the object, and the temperature distribution can be made uniform throughout the object to be frozen.
[0016]
Further, the energy P [W / m ³ ] absorbed by the dielectric by the above-described dielectric heating is expressed by the following equation when expressed by a mathematical expression.
P = (1 / 1.8) × f × v ² × ε _r ″ × 10 ⁻¹⁰ (1)
ε _r ″ = ε _r ′ × tan δ (2)
Here, f [Hz] is the frequency, v [V / m] is the magnitude of the electric field, ε _r ″ is the relative dielectric loss factor, ε _r ′ is the relative dielectric constant, and tan δ is the dielectric loss angle. From the above equation (1), it can be seen that the energy P absorbed by the dielectric (the object to be frozen) is proportional to the relative dielectric loss factor ε _r ″ of the substance.
[0017]
Conventionally, it has been known that the relative dielectric loss factor in the frequency region of high-frequency dielectric heating is, in most frequency regions, that water is orders of magnitude greater than ice. The frequency dependence of the dielectric loss of ice was investigated. And the literature of Ellison et al. (Ellison WJ, J Moreau: J. Mol. Liq. Vol. 68, No. 2/3, p171-279, 1996) and the literature of Manabe et al. (Takeshi Manabe, H. LIBE, G. Hufford: The relative permittivity and the relative dielectric loss factor of water (about 25 ° C.) from the IEICE Technical Report, vol. 87, No. 367, p1-6, 1988), are described by Huffford et al. (G Huffford). : Int. J. of Infrared and Millimeter Waves, Vol. 12, No. 7, p. 677-682, 1991), the relative permittivity and relative permittivity of ice (about −5 ° C.) are described by Shiragashi et al. (Ryo Shiragashi et al .: Proceedings of the Japan Society of Mechanical Engineers Thermal Engineering Conference, pp. 43-44, 1999) Water and the dielectric constant of ice epsilon _r ', and the relative dielectric loss factor epsilon _{r "and} give the relation between the frequency. Then this,
(1) In the frequency region of 500 kHz or more and 6 MHz or less, there is a specific region of the relative dielectric loss factor of ice, and the difference in the relative dielectric loss ratio of water and ice becomes extremely small. (2) In the frequency region of 30 MHz or more and 60 MHz or less, Understand that there is a specific region of the relative dielectric loss rate of ice, and that the relative dielectric loss rate of water and ice is reversed, so that the ice portion of the frozen object absorbs a lot of energy even in the maximum ice crystal formation zone. Was found. In other words, by heating the ice, it is possible to suppress the large growth of ice crystals, to achieve the miniaturization and uniform distribution of the ice crystals, suppress the cell destruction and concentration of the frozen object, and improve the quality of fresh foods and the like. It was found that the drop could be significantly prevented.
[0018]
Embodiment 2 FIG.
FIG. 3 is a configuration diagram showing a refrigerating apparatus capable of realizing a method for freezing fresh food or the like according to the second embodiment of the present invention. Reference numeral 3 denotes inductance, 4 denotes capacitance, 5 denotes an applied electrode, 6 denotes an oscillation circuit, 7 denotes an object to be frozen, 8 denotes a refrigerator, 9 denotes a cooling heat exchanger, 10 denotes a compressor, 11 denotes an electromagnetic wave shielding shield, Reference numeral 12 denotes a heat insulating shield, 13 denotes a freezer, 37 denotes an insulating table, and 42 denotes a temperature detecting unit.
In the apparatus for refrigerating fresh foods and the like, the high-frequency dielectric heater 1 is configured by connecting a high-frequency generation power supply 2, an inductance 3, a capacitance 4, and a parallel plate electrode 5 by an oscillation circuit 6, and the capacitance 4 and the parallel plate, for example. The oscillation frequency is controlled by controlling the application electrode 5 composed of an electrode. The frozen object 7 is sandwiched between, for example, two parallel plate electrodes. The inside of the freezer 13 is cooled by the cooling heat exchanger 9 of the refrigerator 8 to create a low-temperature atmosphere.
[0019]
In the present embodiment, a parallel plate electrode is shown as the application electrode 5, but the shape of the electrode may be a non-parallel plate type, a lattice shape, a coil shape, a cylindrical shape, a roller shape, or the like. Further, it may be a mobile type.
[0020]
Further, in the present embodiment, the electromagnetic wave shielding shield 11 is provided in order to prevent the possibility of causing radio interference due to the change of the oscillation frequency, but may be omitted when unnecessary.
[0021]
Embodiment 3 FIG.
Next, using the refrigeration apparatus of the second embodiment, a tuna cut is used as the body 7 to be frozen, and the temperature of the surface and the inside of the body 7 is measured in order to suggest the concept of the first embodiment. did. FIG. 4 is a refrigeration curve when the object to be frozen is cooled in the refrigeration method according to Embodiment 3 of the present invention. About 200 g of tuna fillets to be frozen 7 were used. The oscillation frequency is set to 47 MHz (a specific region where the relative dielectric loss factor of ice is larger than that of water) by controlling the inductance 3 and the capacitance 4, and at an output level of about 2.5 W, the surface temperature of the frozen object 7 is 5 The irradiation was started when the temperature dropped to ° C., and the high-frequency dielectric heating was performed by terminating the irradiation when the surface temperature of the frozen object 7 exceeded the lower limit (−5 ° C.) of the maximum ice crystal formation zone. During this time, the output level was kept constant at about 2.5 W. In order to suggest the effect of the present embodiment, FIG. 5 shows a freezing curve of an object to be frozen by a conventional freezing method without using dielectric heating.
[0022]
In the refrigeration method of the present embodiment shown in FIG. 4, the object to be frozen is dielectrically heated by using an electromagnetic wave having a frequency at which the relative dielectric loss factor of ice is higher than that of water, and the energy to be absorbed is larger than the energy absorbed by the dielectric heating. Since it was cooled by energy and frozen, it was possible to freeze while suppressing ice crystal growth, and to achieve the miniaturization and uniform distribution of ice crystals. Compared with the conventional freezing method shown in FIG. 5, it can also be confirmed that the difference 20a, 20b between the effective freezing period of the surface of the frozen object 7 and the inside of the frozen object 7 is reduced.
[0023]
Embodiment 4 FIG.
As another embodiment in which the oscillation frequency is changed, the case where the oscillation frequency is 27.12 MHz (the difference between the relative dielectric loss factors of ice and water is small outside the singular region) using the refrigerating apparatus of the second embodiment is described. Was obtained. FIG. 6 is a refrigeration curve of the object to be frozen according to the fourth embodiment of the present invention. In the present embodiment, the temperature of the surface of the object to be frozen is set to the maximum ice crystal temperature in order to avoid the generation of ice on the object to be frozen once due to the high-frequency dielectric heating outside the singular region, and to prevent an effect opposite to the homogenization. Dielectric heating was performed when approaching the formation zone, and the dielectric heating was terminated near the maximum ice crystal formation zone.
In this embodiment, the effect of the third embodiment using the frequency of 47 MHz, which has a higher relative dielectric loss factor of ice than water, was not obtained. In the passage through the upper limit of the crystal formation zone, the difference 20a in the effective freezing period between the surface of the frozen object 7 and the inside thereof was slightly improved from about 7 minutes to 5 minutes.
[0024]
Embodiment 5 FIG.
As another embodiment in which the oscillating frequency is changed, refrigeration in the case of an oscillating frequency of 2 MHz (a difference in the relative dielectric loss factor of ice and water outside the singular region is small) using the refrigeration apparatus of the second embodiment. A curve was obtained. FIG. 7 is a refrigeration curve of the object to be frozen according to the fifth embodiment of the present invention.
In the present embodiment, as in the above-described Embodiment 4, in order to avoid the generation of ice on the object to be frozen once due to the high-frequency dielectric heating outside the singular region and the effect opposite to the homogenization to occur, Dielectric heating was performed when the temperature of the frozen body surface approached the maximum ice crystal formation zone, and the dielectric heating was terminated some time after the temperature exceeded the maximum ice crystal formation zone. Also in the present embodiment, the effect of the third embodiment using the frequency of 47 MHz in which the relative dielectric loss factor of ice is higher than that of water was not obtained, but the maximum was smaller than the conventional refrigeration method without irradiation with electromagnetic waves. In the passage through the upper limit of the ice crystal formation zone, the difference 20a in the effective freezing period between the surface and the inside of the frozen object 7 was slightly improved from about 7 minutes to 4 minutes.
[0025]
Embodiment 6 FIG.
In the above embodiment, the time difference exceeding the upper and lower limits of the maximum ice crystal formation band between the surface and the inside of the frozen object 7 in the embodiment of the present invention, that is, the difference 20a, 20b of the effective freezing period was compared. In order to quantitatively evaluate the texture and texture, a comparison was made between the amounts of drip that increased as the size of the ice crystals increased. The amount of drip was measured by naturally thawing a tuna fillet frozen by the embodiment of the present invention and the conventional freezing method in a refrigerator (about 10 ° C. in a dark place) 24 hours after the start of freezing. Table 1 shows the amount of drip, shape loss, and texture by the freezing method of the present embodiment.
In any of the freezing methods according to the present invention, it was confirmed that the amount of drip could be reduced as compared with the conventional freezing method, the shape did not collapse, and the texture could be improved.
[0026]
[Table 1]

[0027]
In each of the first to sixth embodiments, a tuna cut is used as the frozen object 7, but a large difference such as a tuna body can be expected to have a more remarkable difference. Similar effects were confirmed in tissues other than tuna, such as pork and tofu, and in tissues such as blood.
[0028]
Further, in the above first to sixth embodiments, the frequency is selected in terms of the frequency at which the relative dielectric loss factor of ice is higher than that of water or the frequency at which the difference between the relative dielectric loss factors of ice and water is small. Since the temperature varies depending on the temperature and impurities such as salt, oil, protein, and sugar contained in the frozen object, it is desirable to select an optimum value according to the frozen object.
[0029]
Embodiment 7 FIG.
FIG. 8 is a configuration diagram showing a refrigerating apparatus for fresh food or the like according to a seventh embodiment of the present invention. In the drawing, 1 is a high-frequency dielectric heater, 7 is a frozen object, 8 is a refrigerator, and 9 is a cooling machine. Heat exchanger, 10 is a compressor, 11 is an electromagnetic wave shielding shield, 12 is an adiabatic shield, 13 is a freezer, 31 is a high-frequency oscillator, 32 is a high-frequency amplifier, 33 is an antenna, 34 is an electromagnetic wave, 37 is an insulating table, 42 is It is a temperature detector.
In this refrigerating apparatus, for example, a high frequency of 47 MHz is generated by the high frequency oscillator 31, amplified by the high frequency amplifier 32, and irradiated with electromagnetic waves from the antenna 33 to the freezer 13. Irradiation is possible. Therefore, even in a freezer in which large items such as tuna and meat are stacked, the frozen object 7 can be uniformly heated.
[0030]
Since the length of the antenna 33 is generally required to be 1/4 wavelength or more, for example, a space of 1.5 m is required for a wavelength of 6 MHz of 50 MHz electromagnetic wave. Is preferred. It may be a biconical antenna, a logarithmic spiral antenna, a logarithmic periodic antenna, a dipole antenna, a loop antenna, a parabolic antenna, a long conducting wire antenna, or the like. Further, a microstrip array system, particularly a so-called ceramic antenna in which an antenna is planarly stacked on a high dielectric ceramic is preferable because it is suitable for miniaturization and planarization.
[0031]
Embodiment 8 FIG.
FIG. 9 is a configuration diagram showing a refrigerating apparatus for fresh food or the like according to an eighth embodiment of the present invention. In the figure, 7 is a frozen object, 8 is a refrigerator, 9 is a cooling heat exchanger, and 10 is a compression unit. , 12 is a heat shield, 13 is a freezer, 31 is a high-frequency oscillator, 32 is a high-frequency amplifier, 34 is an electromagnetic wave, 36 is a TEM cell, 37 is an insulating table, 38 is an outer rectangular conductor, 39 is a central conductor plate, and 40 is The coaxial connector 41 is a coaxial termination load.
In this refrigerating apparatus, for example, since the TEM cell 36 including the outer rectangular conductor 38, the center conductor plate 39, and the coaxial terminal load 41 is provided, the refrigeration apparatus converts a high-frequency electric signal into an electromagnetic wave of a strong electric field, and And the electromagnetic wave of the strong electric field can be generated without leaking to the outside, and the electromagnetic wave shield 11 can be omitted. Therefore, the device configuration can be simplified.
[0032]
【The invention's effect】
Since the present invention is configured as described above, it has the following effects. MF, shortwave, using electromagnetic waves of any frequency of the VHF, fresh foods and dielectric heating, the cooled even greater energy than the absorbed energy by dielectric heating, so frozen, the electromagnetic energy is compared As a result, a uniform temperature distribution can be obtained as a whole of the object to be frozen.
[0033]
The dielectric loss factor of the ice using electromagnetic waves of larger frequency than that of water, fresh foods and dielectric heating, the cooled even greater energy than the absorbed energy by dielectric heating, so frozen, ice Can be frozen while suppressing crystal growth, reducing the amount of drip, preventing shape loss, and maintaining the freshness of the frozen object.
[0034]
Further, 500 kHz or more 6MHz or less specific region of the dielectric loss factor of the ice, or by using an electromagnetic wave of 60MHz frequencies below or 30 MHz, fresh foods and dielectric heating, with greater energy than the absorbed energy by the dielectric heating Since it is cooled and frozen, even if ice crystals are once generated on the surface of the frozen object, electromagnetic waves are hardly absorbed inside the frozen object in the liquid phase, and the temperature difference between the surface of the frozen object and the inside is not expanded, Local growth of ice can be suppressed.
[0035]
Further, since the electromagnetic wave irradiating means is an antenna, it is possible to irradiate the electromagnetic wave over a wide range, and even if the frozen objects are large or stacked, the frozen objects can be uniformly heated.
[0036]
Further, since the electromagnetic wave irradiation means is a TEM cell, the electromagnetic wave of the strong electric field does not leak to the outside, and the dielectric heating can be performed safely.
[0037]
Further, MF, shortwave, and means any frequency VHF or ice dielectric loss factor of generating a frequency larger than that of water, and means for dielectric heating by irradiating the electromagnetic wave in fresh foods, Means for cooling and freezing with energy greater than the energy absorbed by the dielectric heating can be used to suppress freezing while suppressing ice crystal growth, reduce the amount of drip, prevent shape loss, and There is an effect that freshness of the body can be maintained.
[Brief description of the drawings]
FIG. 1 is an explanatory diagram showing a method for freezing fresh food and the like according to Embodiment 1 of the present invention.
FIG. 2 is a characteristic diagram showing the relative permittivity of water and ice and the relationship between the relative permittivity and the frequency according to the first embodiment of the present invention;
FIG. 3 is a configuration diagram showing a refrigerator for fresh food or the like according to a second embodiment of the present invention.
FIG. 4 is a characteristic diagram showing a relationship between temperature and elapsed time according to a third embodiment of the present invention.
FIG. 5 is a characteristic diagram showing a relationship between temperature and elapsed time by a conventional method for freezing fresh foods and the like.
FIG. 6 is a characteristic diagram showing a relationship between temperature and elapsed time according to a fourth embodiment of the present invention.
FIG. 7 is a characteristic diagram showing a relationship between temperature and elapsed time according to a fifth embodiment of the present invention.
FIG. 8 is a configuration diagram showing an apparatus for freezing fresh food or the like according to Embodiment 7 of the present invention.
FIG. 9 is a configuration diagram illustrating a freezing apparatus for fresh food or the like according to an eighth embodiment of the present invention.
[Explanation of symbols]
REFERENCE SIGNS LIST 1 high-frequency dielectric heater, 2 high-frequency generation power supply, 3 inductance, 4 capacitance, 5 applied electrode, 6 oscillation circuit, 7 frozen object, 8 refrigerator, 9 cooling heat exchanger, 10 compressor, 11 electromagnetic wave shielding shield, Reference Signs List 12 heat insulation shield, 13 freezer, 20a, 20b shift of effective freezing period, 31 high frequency oscillator, 32 high frequency amplifier, 33 antenna, 34 electromagnetic wave, 36 TEM cell, 37 insulating stand, 38 outer rectangular conductor, 39 center conductor plate, 40 Coaxial connector, 41 coaxial termination load.

Claims

MF, shortwave, using electromagnetic waves of any frequency of the VHF, dielectrically heating the frozen body, said cooling with energy greater than the energy that is absorbed into the freezing material by the dielectric heating, the frozen body refrigerating how to characterized in that freezing the.

Dielectric heating, refrigeration method of claim 1, the relative dielectric loss factor of the ice and performing electromagnetic waves of larger frequency than that of water.

Dielectric heating, refrigeration method according to claim 1, characterized in that electromagnetic waves of 500kHz or more 6MHz less, or 30MHz or 60MHz frequencies below.

Irradiation means of the electromagnetic wave, refrigeration method according to claim 1, characterized in that an antenna.

Irradiation means of the electromagnetic wave, refrigeration method according to claim 1, characterized in that the TEM cell.

A means for generating an electromagnetic wave having a frequency according to any one of claims 1 to 3, a means for irradiating the object to be subjected to dielectric heating by irradiating the electromagnetic wave with the electromagnetic wave, and an energy absorbed by the object to be frozen by the dielectric heating. refrigerating device you characterized by comprising also be cooled with large energy and means for freezing the object frozen body.