Download PDF The Influence of Blanching Time on Frozen Potatoes

Frozen vegetable technology is a vegetable processing preservation technology, first appeared in America in the 1930s. Compared with other processing
and storage methods, it can maintain the original luster of vegetables, flavor, and nutritional value. With the improvement of international cold
chain technology and the shift of people’s diet structure from subsistence to nutrition and health-based, the international market demand of frozen
vegetables is increasing.
Before freezing the fruit and vegetable, the processes contain selecting, cleaning, slicing, blanching, cooling, de-watering, etc. Blanching is an important
pre-treatment of freezing vegetables and fruit, mainly to destroy the oxidation system and restrain enzymes, which can prevent further oxidation
of vitamin C and produce browning in order to retain the beautiful luster of the vegetable. In the processing of potato frozen, if the blanching is
inadequate and not all enzyme is inactive, the quality of blanched potatoes may decrease faster than those without blanching during the frozen storage,
and easier to brown. If blanching is overdone, it will make poor quality potatoes and increase the cost due to heating longer time and more fuel
consumption. So, it is necessary to master the appropriate degree of blanching to avoid inadequate or overdoing blanching. This experiment is
about the appropriate time of blanching in the process of frozen potato.


Material and Method

1. Material

Test potatoes are purchased in the local market
The main equipment: Kang Bavaria refrigerator, hand-held hardness tester, ultraviolet spectrophotometer, thermostatic water bath

2. Method

Processing Flow of Frozen Potatoes
Potatoes–cleaning–peeling–slicing–blanching–cooling–dehydration–transferring to a plate–pre-freezing–freezing–packaging–frozen
Testing and Evaluation
Hardness Measurement: use the hand-held hardness tester to test
teat of the extent of the enzyme activity: use a knife to crosscut the blanching potatoes, and put them into guaiacol solution right now.
Pick them out after a moment, drip a few drops of 0.3 % hydrogen peroxide solution on the cut surface. Observed discoloration in a few minutes
.If blanching is inadequate, the unspoiled enzyme will react with and become red-brown. If the color does not change, the enzyme has been
VC Determination: Determined by an iodometric method.
Determination Drowning: Mix the sample with distilled water at the ratio of 1: 10, homogenize at a low temperature for 2 minutes,
extract supernatant after in centrifugal device for 15 minutes (4000 r.min-1). use the ultraviolet spectrophotometer at a 416nm wavelength to
measure absorbance. That is the browning value.

3. Experimental Design

After cleaning and peeling, the potatoes are cut into potato slices whose thickness is 0.5cm. Then put the potato slices into cool
water to protect their original color, waiting for blanching. Put the slices into the 100℃boiling water for 0s, the 20s, 40s, 60s, 80s, 100s
and 120s respectively. Then cool the boiled potato slices with ice water to 0~5 ℃ quickly, and detect on each index, mark the blanching time
to 0s as a blank control. The blanching time is determined by the hardness, VC content, the degree of enzyme inactivation, and browning




Results and Analysis

The Influence of Blanching Time on Hardness

The experiment result shows that as the blanching time increases, the hardness of potato chips decreases. After the 60s, the hardness of potato chips
changed little and gradually tend to stabilize. If the hardness is too small, it will increase the breakage rate during the process of cooling and
transfer to a plate.

The Influence of Blanching Time on VC Content

With the influence of blanching, the VC content of potato chips decreases greatly. But the VC content will change little if the blanching time is the 20s,
40s, 60s, 80s, 100s, 120s. That means blanching time has little influence on VC content. The reason may be that the high content of starch gelatinates
at high temperature and forms a protective film which hinders the dissolution of VC and has a certain protective effect on VC.

The Influence of Blanching Time on Enzyme Inactivation

As the blanching time increases, the degree of discoloration potato chip section decreases. That means the enzyme inactivation increases.
As peroxidase has the strongest heat resistance, all other enzymes will be destroyed at the same temperature which can destroy peroxidase.
When the blanching time is the 60s and 80s, the color of potato chips changes little.
Most enzyme has been destroyed. When the blanching time is 100s, no color reaction occurs. It means all the enzyme has been inactive.
Bottcher said, if peroxidase is completely passivated, it means excessive blanching. So it is better to choose 60s and 80s as the blanching time.

The Influence of Blanching Time on Browning

Blanching time plays a great role in browning. As blanching time increases, the browning value of potato chips decreases. the browning value decreases
from 0.08 at 0s to 0.02. at 60s. It tends to be stable after 60s. The browning of potato chips has a close relationship with polyphenol oxidase.
From the former study of enzyme inactivation, we can see after the blanching reaches to 60s, most enzyme is destroyed. Polyphenol oxidase becomes
inactive. So the browning decreases. It turns out that 60s and 80s blanching can meet the demands of enzyme inactivation.



(1) The hardness of potato slices decreases as the blanching time increases,
(2) It has no significant relationship with the blanching time.
(3) As the blanching time increases, the degree of discoloration decreases, and the extent of enzyme inactivation increases.
(4) As the blanching time increases, the browning degree of potato chips decreases and tends to be stable after blanched 60s.
In consideration of the influences of potato chips blanching time on the hardness, VC content, enzyme inactivation, browning degree of potatoes and fuel cost, etc.,

the best blanching time is 60s for blanching the potato chips with the thickness of 0.5m in 100℃ boiling water and this blanching time
can inactive enzyme and decrease the browning value and maintain a certain degree of hardness, which is beneficial for the following
processing operations.

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