BLANCHING OF VEGETABLES

Name: T.L.V.Peiris Student Number: GS/Msc/Food/3630/08

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University of Sri Jayewardenepura

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1. DETERMINATION OF BLANCHING TIME OF VEGETABLES:

INTRODUCTION:

This experiment was carried out to determine the blanching time of vegetables. Blanching is an essential step of heat processing used for agricultural products. Fruits and vegetables are blanched prior to canning, freezing and dehydration. The aims of blanching are to inactivate the native enzymes and to destroy enzyme substrates such as peroxides to reduce the microbial load in the food and raw material, to soften and shrink the food facilitating the filling into containers expel cellular gases thereby reducing can corrosion and improve the texture particularly of hydrated foods. Over blanching damages the texture of the product.

Blanching is effected by heating the food rapidly to a selected temperature and holding the food at this temperature for a specific time to inactivate the enzymes followed by rapid cooling. Two methods are commonly used for blanching as hot water blanching and steam blanching.

Blanching is the scalding of vegetables in boiling water or steam. Blanching slows or stops the action of enzymes. Up until harvest time, enzymes cause vegetables to grow and mature. If vegetables are not blanched, or blanching is not long enough, the enzymes continue to be active during frozen storage causing off-colors, off-flavors and toughening

Blanching time is crucial and varies with the vegetable and size of the pieces to be frozen. Under blanching speeds up the activity of enzymes and is worse than no blanching. Over blanching causes loss of flavor, color, vitamins and minerals.

MATERIALS AND METHODS:

MATERIALS:

Petri dishesWire basketsKnife and a chopping boardCarrot

METHOD:

Six samples of sliced Carrot each containing about 10g of the sample of vegetable were dipped in hot water (750C – 1000C) using the wire baskets varying the time immersion time in hot water as 1min, , 2min, 2.5min, 3min, 4min, and 5min and followed by rapid cooling in cold water. Then the peroxide test was carried out to find the minimum blanching time.

Peroxide Test: The blanched sample was placed in a Petri dish and about 2 ml of 4% p-toludine solution was added to the sample and 2-3 drops of 20 volume hydrogen peroxide. And observed for development of brown colour on samples.

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Un-blanched Sample

Blanched Samples1 min. 1 ½ min. 2 min. 2 ½ min. 3 min.

Browning only outer edges

Sight brown colour was oberved

Sight brown colour was oberved

Sight brown colour was oberved

No brown colour

No brown colour

Conclusion

According to above results the best/minimum blanching time for carrot is 2.5 mins

DISCUSSION:

Blanching time depend on the type of vegetable, growing area and condition, size of the piece, enzyme concentration and distribution pattern, maturity stage. Blanching is a cooking technique involving boiling food (usually vegetables and fruits) in water for a very short time. Blanching is often followed by plunging the food into cold water to stop the cooking process. Though the word means to make something white or pale, blanching certain vegetables will make their colors more vivid. Blanching is commonly used to remove skins from tomatoes and almonds.

Vegetables are often blanched prior to freezing or canning. This helps preserve the food by slowing down or halting enzyme action that causes foods to break down, losing color, flavor, and nutritional value.

Blanching is similar to parboiling, which also involves boiling food briefly in water. Certain vegetables may benefit from being blanched or parboiled before being stir-fried..

2. DEHYDRATION OF VEGETABLES:

INTRODUCTION:

This experiment was carried out to dehydrate the vegetables. Food dehydration refers to the nearly complete removal of water from the food under controlled conditions that cause minimum or ideally no other changes in the food properties. A major criterion of the quality of dehydrated food is that when reconstituted the addition of water they are very close to or virtually indistinguishable from the original food material used in their preparation. During dehydration, food does not lose water at a constant rate. As drying progresses, the rate of water removal under any set of fixed conditions drops off.

Bacteria and micro-organisms within the food and from the air need the water in the food to grow. Drying effectively prevents them from surviving in the food. It also creates a hard outer-layer, helping to stop micro-organisms from entering the food.

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One of the oldest methods of food preservation is by drying, which reduces water activity sufficiently to delay or prevent bacterial growth. Most types of meat can be dried. This is especially valuable in the case of pork, since it is difficult to keep without preservation. Many fruits can also be dried; for example, the process is often applied to apples, pears, bananas, mangoes, papaya, apricot, and coconut. Zante currants, sultanas and raisins are all forms of dried grapes. Drying is also the normal means of preservation for cereal grains such as wheat, maize, oats, barley, rice, millet and rye.

Vacuum-packing stores food in a vacuum environment, usually in an air-tight bag or bottle. The vacuum environment strips bacteria of oxygen needed for survival, hence preventing the food from spoiling. Vacuum-packing is commonly used for storing nut

MATERIALS AND METHODS:

MATERIALS:

OvenCutting board and knifeBalancesDrying traysPansStrainersVegetable- CarrotSodium Meta bi-sulphite

METHOD:

Fresh vegetables↓

Selected, Graded↓

Washed, Cleaned↓

Sliced↓

Blanched, Sulphited (Dipped in 1% SMS cold water for 1-2 min)↓

Put in to cold water↓

Drained and put in to trays↓

Dried (600C)

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↓Stored for moisture equilibrium

↓ Weighed

↓Packed (Normal)

↓ Final product

The samples were prepared as the above flow diagram and kept for drying at 550C in the dry oven. Another sample of 10g was kept under the same conditions to determine moisture content by weighing at each ½ hour intervals to draw the drying curve. At the same time three more samples each containing 5g of above sample were kept in the oven at 1050C for 3 hours for the determination of moisture content. The moisture content was calculated using above and the drying curve was drawn (time vs moisture)

RESULTS AND CALCULATIONS:

RESULTS:Results of Oven dried samples (at 550C)

Weight of the dishes Dish A 29.845 g Dish B 42.251 g

Initial Weight of the Samples Sample A 10.050 g Sample B 10.066 g

Weight loss at ½ an hour time intervalsTime

30 min.

60 min.

90 min.

120 min.

150 min.

180 min.

210 min.

240 min.

270 min

300 min

330min

360min

A 9.89

5

8.38

2

7.12

2

6.17

2

5.21

0

4.38

5

3.80

2

3.46

5

2.94

6

2.56

3

2.11

6

1.84

4

B 9.81

6

8.25

1

6.97

2

6.02

8

5.13

1

4.45

0

3.78

1

3.32

9

2.89

3

2.38

5

2.11

1

1.79

4

% Moisture content = Weight after drying X 100 Weight of the sample

% moisture content with time

Time 30 min.

60 min.

90 min.

120 min.

150 min.

180 min.

210 min.

240 min.

270 min

300 min

330min

360min

A – % moisture

98.46

83.40

70.87

61.41

51.84

43.63

37.81

34.48

29.31

25.50

21.05

18.35

B – % moisture

97.52

82.00

69.26

59.88

50.97

44.21

37.56

33.07

28.74

23.69

20.97

17.82

Averag97.9 82.7 70.0 60.6 51.4 43.9 37.6 33.7 29.0 24.6 21.0 18.3

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e % Moisture

9 0 6 4 0 2 8 8 2 0 1 4

Moisture loss at 105 0C after 3 hrs

Sample A Sample B Sample C

Weight of Dishes 30.980 g 40.100 g 34.681 g

Initial wgt. of Sample 5.010 g 5.015 g 5.021 g

Final weight 0.398 g 0.400 g 0.410 g

CALCULATIONS

% Moisture = Weight loss × 100 Weight of the sample

Sample A Sample B Sample C % Moisture (5.010 - 0.398) g * 100

5.010 g(5.015 - 0.400) g * 100 5.015 g

(5.021 - 0.410) g * 100 5.021 g

92.06 % 92.02 % 91.83%

Average of % Moisture = 92.06 + 92.02 + 91.83 3= 91.97%

Therefore, weight of the sample after moisture removed totally = 100 – 91.97% = 8.03 %Results of Oven dried samples (at 55 0C)

DISCUSSION:

Preservation of foods by drying is one of the earliest and simplest techniques used for centuries. Dehydration is the process by which surplus water is removed with out drastically reducing the taste and nutritive value of the foods. Foods such as meat, fish, figs, apples, raisins, apricots, gooseberries, herbs, and vegetables are generally preserved by this method. They keep well because of the combination of the physical changes.

Drying removes the moisture from the vegetables so that bacteria, yeasts and molds cannot grow and spoil the vegetables. It also slows down the action of enzymes, but does not inactivate them. Because drying removes moisture, the vegetable becomes smaller and lighter in weight. The optimum temperature for drying vegetables is 105° C. If higher temperatures are used the vegetables may "case harden", that is cook and harden on the outside while trapping moisture on the inside. The vegetables will eventually mold when

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the moisture equilibrates in the item. Thus, the drying process should never be hurried by raising the drying temperature. 

For vegetables, drying time is critical to tenderness. The longer the drying time, the less flavorful and poorer the product. Drying time can be hastened by drying small, uniformly cut pieces. 

CONCLUTION:

Average moisture percentage of carrot is 91.97%.

3. RECONSTITUTION TEST FOR THE DRIED VEGETABLE SAMPLES

INTRODUCTION:

This experiment was carried out to test the reconstitution of the dried vegetable sample. In reconstitution water is added to the product which is restored to a condition similar to that when it was fresh. This enables the food product to be cooked as if the person using fresh fruit or vegetables. It can be used as a measure of the quality of the product.

MATERIALS AND METHODS:

MATERIALS:

Previously dried sampleBeakerCold water

METHOD:

5g of previously dried sample was weighed and put in to a small beaker. Cold water was added 10 times of the weight of the dried product. The container was covered and boiled and simmered gently until the product was tendered. The cooking time was 30 minutes after the boiling point had been reached. The sample was tested for palatability, toughness, flavour and presence or absence of bad flavour. Re-hydration ratio is evaluated.

RESULTS AND CALCULATIONS:

RESULTS

Weight of the dried sample = 0.398gWeight of the re-hydrated sample = 4.696g

CALCULATIONS

Weight of the dried sample = 0.398g

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Weight of the re-hydrated sample = 4.696g

Re-hydration ratio = Wr/WdWr = Weight of the sample after re-hydrationWd = Weight of the dried sample

Re-hydration ratio = 4.696/0.398

CONCLUTION

According to the experiment re-hydration ratio is 4.696/0.398

DISCUSSION:

A significant advantage of this process is the short drying time in so far as it is not necessary to go beyond the inflexion point. The finished products after rehydration/reconstitution are of a better quality compared with products obtained by dehydration alone.

Most vegetables are soaked or rehydrated in cold water prior to use. However, there are 2 other acceptable rehydration methods: add the dried product to boiling water or add the dried vegetable to a product with lots of liquid, such as soup. Whichever rehydration method is chosen, the vegetables return to their original shape. Vegetables can be soaked in either water or, for additional flavor. Using boiling liquid speeds up the soaking time. Save and use the soaking liquid in cooking. Adding dried vegetables directly to soups and stews is the simplest way to rehydrate vegetables. Add sufficient water to keep them covered and simmer until tender

REFERENCES AND LITERATURE CITED:

http:// chinesefood.about.com/od/cookingfaqs/f/blanchvegetable.htmhttp://en.wikipedia.org/wiki/Blanching