Vacuum frying technology faqs

1. Why fried crispy chips are popular?

Frying is an old process of manufacturing a food product worldwide. Oil content gives the deep fired snacks unique texture and desirable flavor. However, higher oil uptake, higher in calories, which is not good for health.

2. What is a vacuum frying theory?

Vacuum frying may be an alternative frying for production with low oil content and the desired texture and flavor characteristics. It is carried out under pressures well below atmospheric levels, preferably below 50 Torr (6.65 kPa).

3. Why the vacuum fried fruits and vegetables crispy chips are healthy?

Vacuum frying can reduce oil content in the fried product; preserve natural color and flavors of the product. More ever, the acrylamide, a potential carcinogen, will not be of risk in vacuum fried products due to the low frying temperature.

4. Why the vacuum fried fruits and vegetable chips are more popular?

Vacuum frying fruits and vegetable chips process should be thought of as dehydration rather than a traditional frying process. The final crispy chip has less than 5% moisture, more crunches, less oil uptake, no greasy and good mouthfeel. Another, frying under low temperature, less than 100℃, preserve natural color, flavors and nutrition of the product.

5. What is a key to operate vacuum frying process?

Keep a constant frying temperature, or a bit less fluctuation under a constant limited vacuum condition is one key point for a vacuum frying operation.

6. Why the frying temperature and vacuum condition will be changed in vacuum frying process?

The materials which is ready to be fried contain about 60-70% of moisture, in 5-10 minutes at the beginning of frying process, about 70% of the free water will be vaporization, it will quickly take away a lot of heat, thus, oil temperature will rapid fall, simultaneously, a great deal of secondary steam are generated, a rapid rising in vacuum degree happens if it cannot be removed.

7. Why to keep a constant frying temperature during vacuum frying operation is important?

If cannot rise the frying temperature with heat timely add, it is difficult to form enough oil and steam channel, the puffing of fruit and vegetable chips is not good with bending shape and hard mouthfeel. Also, a limiting vacuum is the best as it will help the free water vaporization volume to be large enough, breaking the cell wall and the longer fibers, thus forming a good oil vapor channel.

Only by providing heat quickly, the cell tissue can complete the dehydration, shrinkage, and vegetable protein gelatinization process in a short time to form a large number of unblocked gaps. The fibrous tissue that has been pulled-apart and lost the tension constitutes the framework of the above channel, oil comes in while vapor comes out, quickly complete the initial dehydration process (the initial expansion process). The good the expansion is, the smoother the final deoiling is. Low fat content is available.

Keep a constant frying temperature, or a bit less fluctuation is one key point for a vacuum frying operation.

8. Why building and keeping a limited vacuum condition in vacuum frying process is request?

At the beginning of frying, the higher the vacuum degree is, the larger the expansion volume is during water vaporization, and the easier it is to break the tissue fiber of fruits and vegetables. The better the expansion effect, the more unblocked the oil and vapor channels will be.

In order to ensure the most basic frying dehydration thermodynamic force，build and keep a limited vacuum condition, like 3KPa, is essential in a vacuum frying process. None but frying carried out at limited vacuum degree worthy less than 100 ℃ of frying oil temperature.

Oil temperatures, more than 120℃, will quickly be contaminated, causing dark colors, nutrient loss, poor flavor, and possibly acrylamide, also peroxides is easily exceeding，fat acidification deterioration, service life reduced.

9. What is the feature of vacuum pump applied in vacuum frying process?

The special fluid jet vacuum pump that establishes a high vacuum at fast speed has strong ability of secondary steam extraction and is maintenance-free.

Tests indicate it only takes 2 minutes 30 seconds to 3 minutes from normal pressure to -0.098MPa under the cooling water temperature of 25-30 °C of 1M3, and the vacuum recovered quickly in the early stage of frying, generally 1-3 minutes. 

Either water ring vacuum pumps or combination with Roots pump also has fast suction speed and ultimate a high vacuum degree. However, the temperature is requested to lower than 40 °C in extraction of vapor tubes, cooling tower or other chilling equipment should be considered if the surrounding temperature is high, following with an extra capital.

10.How to keep constant frying oil temperature during vacuum frying process?

Keep constant frying oil temperature during all the process, especially avoiding the temperature drop and recover shortly in the first 5 to 10 minutes, heat transfer plays an important role.

External circulation instantaneous steam dynamic heat transfer has several times heat exchange area than the interlayer steam static heat exchanger with a heat transfer 6-8 times efficiency.

Contrast a heating experiment, when heating equal amount of oil, the oil temperature is raised from 25 °C to 100 °C, the average time for each 1 °C increase is 38-42 seconds for the jacketed heat exchange (steam pressure 0.4-0.5 MPa); the average time for each 1°C increase is 5-7 seconds for circulating heat transfer (steam pressure 0.1-0.2MPa), so low-pressure high-efficiency heat transfer and constant low-temperature frying (80-100°C) are realized. Thereby, the caramelization reaction of the starch and sugar from the material in the frying and the fat deteriorated is avoided.

11.What is the best of the ratio of frying oil to raw material?

The ratio of frying oil to raw material is related to the frying temperature changing. The larger the ratio is, the smaller the oil temperature drop. It is not less than 10, generally 10-15. The dynamic cycle heat transfer oil ratio is greater than 35, but the actual amount of oil is very small.

12.What type vacuum fryers are practical applied now?

The practical applied vacuum fryers include intermittent vacuum fryer and automatic continues vacuum frying machine now.

13.What are the obstacles that affect the application of vacuum frying machine?

Both vacuum frying technology and vacuum fryer manufacturing are mature, only the initial investment is a large impact on the industrial application. Considering that vacuum Fried foods will be really popular healthy foods in the future, the market is huge and the profits are considerable, compared with this, the return on investment is worthwhile.

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