Science & Technology: Maintaining Flavor with Long-Term Food Preservation

Small Ice Crystals—The Secret to Maintaining Flavor

Frozen food kept at temperatures below -18˚C can be preserved for years, as bacteria and mold cannot grow at such low temperatures. Freezing technology has enabled us to eat meat, fish and other foods imported from abroad, as well as ready-prepared frozen meals.

Rapid-frozen tuna from a ship arrives at port (Photo: JTB Photo Communications, Inc.)

However, there has been a downside to conventional freezing. Once food is frozen, it does not taste as good as freshly picked or freshly cooked food. The main reason is that when food is frozen, ice crystals inside the food expand, causing the cells to burst. When they burst, the nutrients turn to liquid (called "drip") and seep out as the food defrosts. The texture of the food also changes.

Tuna sushi—familiar to everyone now, thanks to progress in freezing technology (Photo: AFLO)

In an attempt to solve this problem, a method called "rapid freezing" was developed. When the water inside food freezes, ice crystals develop at temperatures between -1˚C and -5˚C. If this temperature range is passed through quickly (ideally within 30 minutes), most of the ice crystals do not grow too large, and the cells are not damaged. This process is the rapid freezing technique.

In Japan, rapid freezing technology was immediately put to use around 1960 on fishing boats to preserve tuna and other catches. The flavor of the rapid-frozen tuna also lasts much longer when preserved at a temperature below -50˚C. This has greatly helped to make Japanese dishes using raw fish—such as sashimi and sushi—popular around the world.

The Search for Freezing Technology That Doesn't Damage Cells

However, freezing technology was not perfect. Even when using rapid freezing, some cell damage still occurred—it was not possible to completely avoid changes in color or quality in foods that had been preserved at very low temperatures.

Then along came the development of "instant freezing." Water and food do not necessarily begin to freeze at 0˚C. If the water molecules or other substances that form the core of ice crystals are not present, then water and food do not begin to freeze even at -10˚C, a state known as "supercooling." When a supercooled liquid is agitated, everything freezes instantaneously. This is the process known as "instant freezing."

Left: Acerolas, so fresh you would not believe they were frozen.
Right: Rice that has been frozen for 10 years shows no sign of cracking or yellowing. Both have been kept in a CAS freezer.

With normal freezing methods, including rapid freezing, the food freezes from the outside to the inside. However, with instant freezing, the surface and the inside freeze at the same time. Therefore, the ice crystals freeze before they can clump together with other ice crystals to expand, so they remain small. This means that the cells are hardly damaged at all. Still, there remains an obstacle: the temperature range in which food supercools depends on the type of food and the specific part, so instant freezing cannot be used for every food.

In recent years, a groundbreaking instant freezing technology developed in Japan known as "Cells Alive System" (CAS) has been gaining attention as a method to address this problem. With CAS technology, the food is placed in a magnetic field that inverts repeatedly, so the food undergoes rapid freezing while being vibrated. According to scientists, the process of CAS freezing occurs instantaneously—just as with supercooling—with almost no time difference between the freezing of the surface and the inside. The ice crystals do not become too large, and the cells are hardly damaged, which means the original flavor, aroma, texture and color are maintained. In fact, fruit and vegetables that are difficult to freeze using conventional methods have been successfully preserved for long periods, maintaining their original fresh qualities. The mechanism behind CAS has not yet been fully clarified, but experts are considering various medical uses for the process, including the preservation of transplant organs and so forth.

From rapid freezing to instant freezing to CAS—nowadays, Japanese freezing technology is pushing the boundaries, making a substantial contribution to the improvement of the global food supply and the development of food culture across the globe.

Conventional freezing

(1) Water molecules in the material before freezing.
(2) Ice crystals near the surface expand before the whole item freezes, destroying the cell membrane as well as the nutrient and aroma elements inside the cell.
(3) Once defrosted, the phenomenon called "drip" occurs, meaning that the water, nutrient and aroma elements seep out of the cell.

CAS and other instant freezing

(1) Water molecules in the material before freezing.
(2) The materials all freeze instantaneously.
(3) After defrosting, the molecules return to their original state, and the material retains its freshness.