Research method

CT Observation of State of Fine Ice Crystals inFrozen Foods

Imaging

Quantitative evaluation of freezing method that keeps texture of foods

Role of SPring-8

Achievements

Fine ice crystals in foods frozen by a home refrigerator with an instantaneous
freezing function based on supercooling* were successfully observed byhigh-brilliance X-ray computed tomography (CT ).
Conventionally, frozen foods have mainly been evaluated after thawing, but aquantitative evaluation method using high-brilliance X-rays have beendeveloped for frozen foods before thawing. The effects of freezing technology on
the growth of ice crystals in foods have been confirmed.
R&D institution: Mitsubishi Electric Corporation*Supercooling: The unstable state in which substances such as water exist as liquids even at temperatures lower than its freezing point. For
example, supercooled water exists as a liquid even below 0 °C.
**SUNBEAM: Contract beamlines at SPring-8 managed by the SUNBEAM Consortium consisting of 13 companies. There are two beamlines: aninsertion device (ID) beamline (BL16XU) and a bending magnet (BM) beamline (BL16B2).

BackgroundResults

When frozen foods such as meats, fishes, andvegetables thaw, losses of moisture, called drips,
from foods often occur because ice crystals grownin foods break the cell tissue of the foods. Drips isconsidered to deteriorate tastes of foods afterthawing. To solve this problem, an instantaneous
freezing technique called Supercool Freezing,
which is based on supercooling, was developed. It
was found that ice crystals in foods frozen by
Supercool Freezing are smaller than those in foods
frozen by conventional freezing. Smaller ice
crystals have a significant effect on suppressing
drips and maintain the texture of foods. Methods
to measure quantitatively the size of ice crystals infrozen foods are required. The conventional
methods involve indirectly observing pieces ofstained or frozen/dried foods with limited
applications for food samples. Therefore, direct
nondestructive methods for observing the state ofice crystals in entire food samples have beenrequired.

X-ray CT is effective method to observe the internal
state of substances nondestructively. However,conventional X-ray CT equipment using a wide energy
band of X-ray are unable to distinguish substances withsmall differences of X-ray absorption coefficients, such as
foods and ice crystals.
High-brilliance monochromatic X-rays at SPring-8
enables detailed imaging of the internal state ofsubstances such as foods and ice crystals. We carried out
a preliminary experiment at BL19B2 (Engineering ScienceResearch I), which is well equipped with sample freezing
systems. Afterwards, we introduced a frozen imaging
system at BL16B2 of SUNBEAM**. Using these systems,
beef, tuna, and boiled potato, which were frozen with
Supercool Freezing and conventional freezing, werephotographed while being continuously rotated by 180°
to obtain a few hundred tomographic images. The
cross-sectional images of the ice crystals were comparedaccordingly to the freezing methods. Supercool Freezing
leads to smaller size of ice crystals and less distort tissuesof foods than conventional freezing methods.
CT image obtainedat BL16B2

Frozen imaging system at BL16B2
CT imaging at BL19B2

When water starts to freeze, ice nucleus forms. Then ice crystals growaround the ice nucleus. Conversely, water will not freeze even at atemperature lower than its freezing point (0 °C) unless an ice nucleus forms.
When a fluctuation in temperature or an impact is applied to supercooled
water, an ice nucleus abruptly forms and the freezing process occurs
throughout the water upon the formation of ice nucleus. The ice crystals
that form in supercooled water are smaller than those frozen
conventionally.

Beef (upper) and boiled potato (lower) frozen by conventional
freezing (left) and Supercool Freezing (right) were compared.
The regions with bright contrast are food fibers or proteins,
whereas the regions with dark contrast are ice crystals. The icecrystals are distributed more widely in the samples frozen by
Supercool Freezing than those frozen by conventional freezing.
It was demonstrated that smaller ice crystals have less effect on
the tissue structure of frozen foods.

Temperature

Principle of freezing based on supercooling(1) Supercooling(2) Freezing aftersupercooling

Conventional
freezing

CameraSlit

SamplecoverSystem forintroducing
low-temperature
nitrogen gas

OpticalhutchCooling
stage

BeammonitorScintillator2θ–θ goniometer (rotary stage)

N2Gas

XY tomography

The CT image of afrozen beef indicatesthat the sample can beclearly photographedwith negligible thawing
and little frost duringobservation.

Liquidnitrogen

SupercoolFreezing
BeefFood temperature

0 °C
TimeFreezing
point
End of supercoolingTemperature in freezer

Diagram of transition of temperature insupercooling and subsequent freezing
14(a) Cooling stage

Boiled potato(b) Cooling stage withsample cover

A holder containing a frozen food was placed on a rotary sample
stage set in the experimental hutch and three-dimensionallyphotographed by X-ray CT system. A equipment blowing
low-temperature nitrogen gas to samples was set at the upper
cover of the holder to prevent thawing of the ice and frost on theholder during photography. This system enabled us tocontinuously obtain images at intervals of 250 ms while keeping
the temperature of samples at approximately −30 °C and rotating
the stage by 180° at a rate of 2.2°/s.

Refrigerator with
Supercool Freezing function
This series of refrigerators first appearedon the market in 2007, and such
refrigerators with improved functions
are still commercially available.

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