Freezing technology was advanced, but defrosting technology wasn’t. Because of this, frozen foods only became popular after 1950.
ご質問ありがとうございます。
・「Freezing technology was advanced, but defrosting technology wasn’t. Because of this, frozen foods only became popular after 1950.」
（意味) 冷凍技術は発達していたが、解凍技術は遅れていたので冷凍食品が普及したのは1950年以降だ
・technology 技術
・defrosting [解凍](https://eikaiwa.dmm.com/uknow/questions/89460/)
参考になれば幸いです。
Even though Japan was one of the leading countries around the world in regard to frozen food technology, they were behind when it came to thawing technologies, so frozen food didn't really start to permeate the market until after 1950.
これが日本のことを言っているなら、次のように言えます。
ーEven though Japan was one of the leading countries around the world in regard to frozen food technology, they were behind when it came to thawing technologies, so frozen food didn't really start to permeate the market until after 1950.
「日本は食品を冷凍する技術に関して世界でも主要国の１つだったが、解凍技術は遅れていたので、冷凍食品は1950年以降まで市場に普及し始めなかった」
to thaw「解凍する」
to permeate「普及する」
ご参考まで！
アメリカ合衆国 
カナダ 
