**2. Determination of freezing point**

The freezing point is the storage critical point of all the cryogenic stored products and plays a decisive role in corresponding storage process. Therefore, the research of the freezing point has not only formed the basis of refrigeration field, but also been a hot researching topic. The moisture in food is not pure water, but liquor including organic and inorganic substances. So the ice crystals won't produce until the temperature of food reduce to subzero. According to LaWuEr second law, the reduction of liquor freezing point is proportional to the solute concentration. Freezing point will decrease 1.86°C with increasing 1 molar concentration. Due to the difference of food types, the stored conditions after harvest and the concentration of muscle plasma etc, different food has different freezing point. The freezing point of general food is from -0.5°C to -2.5°C.

There are two common ways to measure the freezing point, one is the traditional freezing method, and the other is DSC testing method. In this study the first method was adopted, for that we can clearly see the cooling process, the corresponding supercooling points and phase change during the measuring time in the method. Figure 1 shows the test setup.

1. alternating thermostatic box, 2. tested items 3. Thermocouple, 4. Data collector, 5. computer

Fig. 1. Diagram of the setup testing the freezing point

Based on these advantages of the ice temperature storage, the study of the ice temperature technology on fruits and vegetables was carried out in our lab. The obtained result shows that the key for the ice temperature preservation is to realize a hibernation state of the product. The "hibernation" process is a cooling process during which the product can reduce its activity ability and energy consumption through the self adaption, starting from the change of components within cells, and at the same time ensuring their own living life characteristics. It is a typical phenomenon of natural adaption. However, the traditional

precooling process ignores that and the storage effect couldn't reach to the best level.

processing and storage environment were carried out.

point. The freezing point of general food is from -0.5°C to -2.5°C.

Fig. 1. Diagram of the setup testing the freezing point

2 2 2

3

3

**2. Determination of freezing point** 

1

Consequently, for the ice temperature storage technology, the prime technical key points are determination of the freezing point, the cooling process and the stability of storage environment. According to these key points, studies of the influence of freezing point,

The freezing point is the storage critical point of all the cryogenic stored products and plays a decisive role in corresponding storage process. Therefore, the research of the freezing point has not only formed the basis of refrigeration field, but also been a hot researching topic. The moisture in food is not pure water, but liquor including organic and inorganic substances. So the ice crystals won't produce until the temperature of food reduce to subzero. According to LaWuEr second law, the reduction of liquor freezing point is proportional to the solute concentration. Freezing point will decrease 1.86°C with increasing 1 molar concentration. Due to the difference of food types, the stored conditions after harvest and the concentration of muscle plasma etc, different food has different freezing

There are two common ways to measure the freezing point, one is the traditional freezing method, and the other is DSC testing method. In this study the first method was adopted, for that we can clearly see the cooling process, the corresponding supercooling points and phase change during the measuring time in the method. Figure 1 shows the test setup.

4

5

1. alternating thermostatic box, 2. tested items 3. Thermocouple, 4. Data collector, 5. computer

3

Fig. 2. Curve of temperature change during freezing

From the Figure 2, the temperature profile can be divided into three stages:

The first stage: the food's temperature is reduced from the initial temperature to the freezing point, the heat releasing from food is sensible heat. Compared with the total heat, this value is small, so the cooling speed is fast and the freezing curve is relatively steep.

The second stage: the food's temperature is almost kept at a constant temperature, which value is from 0°C to -5°C. At this stage, most of the water in food is frozen and release a lot of latent heat which is about 50~60 times as much as sensible heat. It is in the second stage that most of the heat in food freezing process is released. So there is a flat segment in the curve.

The third stage: the food's temperature reduce from the phase change temperature to the final temperature, the heat released at this time is partly due to ice cooling ,and partly because of the residual small amount of water getting frozen. The freezing curve in this stage is also relatively steep.

#### **2.1 Influence of reducing sugar on freezing point**

The freezing point shown in Fig. 3 has an increasing trend with the increase of mass fraction of reducing sugar, which is not apparent. But it doesn't match traditional recognition. Usually the freezing point is believed to drop with the increase of mass fraction of reducing sugar. The tendency of pear's freezing point just reflects the relationship in Fig. 4.The reason maybe that kiwi has a large amount of acid which influence the effect of sugar on freezing point. This is just reflected in figure 7 and figure 8.

#### **2.2 Influence of total sugar on freezing point**

From the trend line of relationship between freezing point and total sugar in Fig. 5 and Fig. 6, we can find that freezing point increases with the increase of mass fraction of total sugar, but this relationship is weak.

Fig. 6. freezing point of pear vs. total sugar

**2.3 Influence of acid on freezing point** 

Fig. 7. freezing point of kiwi vs. acid

Fig. 8. freezing point of pear vs. acid

the case of smaller acidity.

From Fig.7 and Fig.8 it can be found that the relationship between freezing point and acid is not apparent, but the freezing point has a downward tendency with the increase of acid in

399

Fig. 3. freezing point of Kiwi vs. reducing sugar

Fig. 4. freezing point of pear vs. reducing sugar

Fig. 5. freezing point of kiwi vs. total sugar

Fig. 3. freezing point of Kiwi vs. reducing sugar

Fig. 4. freezing point of pear vs. reducing sugar

Fig. 5. freezing point of kiwi vs. total sugar

Fig. 6. freezing point of pear vs. total sugar

#### **2.3 Influence of acid on freezing point**

From Fig.7 and Fig.8 it can be found that the relationship between freezing point and acid is not apparent, but the freezing point has a downward tendency with the increase of acid in the case of smaller acidity.

Fig. 7. freezing point of kiwi vs. acid

Fig. 8. freezing point of pear vs. acid

Kiwi:

Pear:

experimental ones.

**3. Influence of precooling process** 

compared with the traditional one.

**3.1.1 Precooling process of kiwi** 

24 hours.

humidity is 85%.

in 48 hours.

humidity is 85%.

relative humidity is 85%.

**3.1.2 Precooling process of pear** 

**3.1 Material preparation and test method** 

was carried out, including kiwi, pear and peach.

117198.0 055817.0 095504.0 07142.0 97913.2 *t x* sugar reducing *x* sugar total *x*acidity *x* solid soluble (1)

401

39643.0 213383.0 746773.5 14007.0 669404.0 *t x* sugar reducing *x* sugar total *x*acidity *x* solid soluble (2) By the checking, calculation, the freezing point of pear by Eq.(1) is calculated, the maximum error is 15%and the minimum error is 1.7%,while calculated by Eq.(2),the maximum error of pear is 25% and the minimum error is 1%. The predicted results coincide better with

As living organizations, the fruit and vegetable are affected by severe environmental changes in precooling process. During the precooling process, there are physiological changes in the inside of the fruit and vegetable to adapt to the environment, which can be reflected from two aspects. One is the change of physical structure, such as the body stress, epidermal pore and so on; another one is biochemistry changes in vivo like respiratory intensity and nutrition. In order to investigate the effect of the precooling process, the preceooling experiments of three kinds of fruits were carried out, and the results were

In this experiment, the experimental storage of different precooling processes on three fruits

The kiwi was produced in Shengxi Province, after harvested, they was transported to lab in

Traditional storage process of kiwi: kiwi is precooled rapidly from 29°C to 3°C in 3 hours and be stored in the storehouse of which the ambient temperature is 3°C and the relative

The variable cooling rate precooling process of kiwi: the kiwi was precooled rapidly from 29°C to 5°C in 2.5 hours, then change the precooling rate, the kiwi was cooled slowly to 3°C in 24 hours ,then to 0°C in 10 hours ,then to -0.8°C in 10 hours, and at last to the storage temperature -1.3°C in 10 hours, stored in the ice-temperature storehouse at -1.3°C and its

2.5 <sup>24</sup> <sup>10</sup> <sup>10</sup> <sup>10</sup> 29C 5C 3C 0C 0.8C 1.3 C *hours hours hours hours hours*

The pear was produced in Xingjian Province, and after harvest they were transported to lab

Traditional storage process of pear: pear was precooled rapidly from 29ºC to 3ºC in 3 hours and was stored in the storehouse of which the ambient temperature is 3ºC and the relative

#### **2.4 Influence of soluble solid on freezing point**

Seen from Fig.9 and Fid.10, the freezing point has a downward tendency with the increase of mass fraction of soluble solid, but the decline rate of kiwi's freezing point is smaller than that of the pear.

Fig. 9. freezing point of kiwi vs. soluble solid

Fig. 10. freezing point of pear vs. soluble solid

From the curve relationship between above four kinds of nutrition and freezing point, it can be found that there is a strong relationship between freezing point and the nutritional proportions for a defined fruit. The experimental results show that freezing point of fruits is related with multiple nutritional proportions, among which the interaction effect may influence the changes in freezing point. For example, the acid has a reverse effect on sugar. In general, the freezing point will drop accordingly as sugar content increases. However, this effect becomes weak in the role of acid, which can be verified by the relationship between the freezing point and reducing sugar and total sugar shown in Fig. 3 and Fig.5.
