Peeling of Apple
Peeling operation is the removal of outer layer of a fruit or vegetable. There are different methods for peeling such as;
1. Peeling by hand
2. With steam or boiling water
3. With lye or alkalies (NaOH, KOH)
4. By fry caustic peeling with infrared heat
5. By flame
6. By mechanical knife peeling
7. By freezing
8. Abrasion peeling
Choosing a peeling method for apples (and the other fruits) depend on different properties. No method is completely perfect and the amount of usable food may be the difference between profit and loss for manufacturer. There is a direct relation between peel loss of given fruit and the size of it. The small sized items, due to the surface area, may lose 4 or 5 time loss from the larger item within same category and as a second factor the shape of given item has a great importance too. For example, the roughness or contours of a given item may make it almost impossible to peel completely for many different methods. The rate of injury caused by peeling method should be taken into consideration when defining the system. As a fresh fruit, apple has a high aw and this situation causes easy injured of under layer tissues. When all of these characteristic properties are considered, the most suitable method for peeling of apple is mechanical knife peeling. In addition a different apple preparation system has been developed by FMC (1988) that automatically peels, cores and slices apples in a high-speed continuous operation.
The process of getting a peeled apple begins with harvesting. After harvesting, apples are transported to the processing plant. The second step of the process is pre-processing operations. At first apples are cleaned. After cleaning, the sorting, peeling and trimming operations are applied to apple. Sizing of the apples are very important for mechanical peeling. Generally, a screen removes apples less then 6.35 cm in diameter. In peeling and coring operations, 30-35% losses of the raw material occur.
The following operation is dipping the apples into sodium sulfite solution and slicing pieces that are 9.5-12.5 mm in thickness. To obtain dried apple rings, cored apples must be sliced at right angles to the core or cut into three, four, six or eight approximately equal units. When apples are cut into more then eight equal units, the product is referred to as sliced.
To provide dehydrated apples, the moisture of apples must be less than %3. So apples dehydrated by using of a forced-air drier such as the continuous belt dryer.
The product is then compressed and packaged. Both dried and dehydrated apple products are marketed on a large scale and their utilization particularly of low moisture products has been expanding in recent years.
Apples are either dried immediately after harvest or after being held in cold and/or controlled-atmosphere storage until a convenient processing. Only artificial driers are used by commercial apple drying plants.
Water is removed relatively easily from apples to the level usually attained in dried fruit products (%10-25 moisture).
Simple driers, such as kiln or tunnel drier are commonly used to produce dried apples. Secondary processing of the dried apples to attain products with less than %3 moisture requires the use of a forced-air drier such as the continuous belt drier.
Air-drying is sufficient for products intended for bakery utilization; if faster dehydration, greater bulk density or better texture in the dried stage is required, the use of vacuum equipment is necessary.
The processing of dried apples starts with sorting, peeling and trimming fruit. Sizing of the fruit is very important for mechanical peeling. Peeling and coring losses average 30-53 % of raw material. After peeling and trimming apples are dipped into a sodium solution and then sliced to pieces 9.5-12.5 mm in thickness. (Repeated information)
Quality Control Parameters
· Water used should be free from heavy metals and other chemical pollutants. Drinking water is suitable.
· Must not remove the underlying food.
· The container in which peeling is carried out and equipments used must be cleaned regularly.
· The peeling process must not reduce the nutritional value of apple.
· Must not cause changes in color and flavor since these two properties are the major points in evaluation of fruit products.
· Apple slices are protected from browning by immersing them in water or brine for up to 30 min at 40°C. An alternative method is to draw a vacuum. Maillard reactions are the most important quality problem in dried fruits.
Quality control measurements that are unique for dried fruit products are
1. Moisture content
2. Sulfur dioxide content
3. Screen analysis
4. Physical characteristics of dried fruit
5. Reconstitution ratio
6. Bacterial count
7. Oxygen content of gas packed products.
Oxidative browning is a very important problem in handling most fruit including apples. Browning may take place before peeling due to bruising but is accelerated once the skin is broken or the tissue cells are ruptured. The ways of controlling browning are;
1. Sulfurous acid
2. High acid
3. Ascorbic acid
4. Sugars addition
5. Vacuum packaging
SO2 is the only chemical additive widely added to dried fruits for its antioxidant and preservative effects. FDA recognizes several sulfite salts and SO2 gas as safe for use in foods. Apples often treated with solutions of sulfite before dehydration.
Amount of sulfuring should not be so large that it adversely affects the flavor and,
Causes corrosion of equipment
Destroys some important nutrients such as Vitamin B1
Peeled and cored apple pieces are exposed to 2-3% bisulfite solution for a few minutes while in the flume water, after the cutting operation. This is followed by exposure to SO2 fumes in the ‘kiln’ during the first 3-5 hour of drying, which is designed to yield a product containing approximately 24% moisture.
In order to control the content of SO2 in the finished product, it is common practice today to dry apples in kiln below the 24% moisture level to about 16-18% moisture, then determine the SO2 content and remoisturize the apple to 24% moisture with a solution containing a sufficient amount of sulfite.
Before dehydration, fruits generally carry microorganisms. Removing water lower the availability of water to microorganisms that cause aw to decrease. The reduction in water activity causes molds to fall in the rate of spore germination. Yeasts need more water than molds. So decrease in water activity inhibits fermentation of by yeasts. Growth of the vast majority of bacteria, yeasts and molds is prohibited at water activity values below 0.9-0.85 and 0.80 respectively. Each organism has its characteristic growth range of water activity. Molds are the most troublesome group of microorganisms in dried fruits. Aspergilus glaucus will grow at water activity value below 0.7 certain osmophilic yeasts and certain xerofhilic molds and fungi are responsible for the spoilage of dried fruit.
Washing is the simplest and most commonly used method of removing contaminating organisms, however washing can cause m.o. spread from an infected area to an uninfected one and the film of water left on the surface of the fruit may be a source of contamination. Chlorinated water is preferred for this purpose.
Fruit processors are more concerned with microbial spoilage during the harvesting, transportation and storage of fruits than during the growing of these products. Both infection and detectable spoilage by m.o. can occur at any point from preharvest to the consumption. Bacteria do not play an important role in fresh fruit spoilage according to the inherent acidity with most fruits. Yeasts are responsible for much of the fermentation of fruit products and may eventually cause the food material to become distasteful.
Ø L. P. Somogy & B. S. Luh, ‘Dehydration of Fruits’.
Ø Robert C. Wiley, ‘Minimally Processed Refrigerated Fruits & Vegetables’, Chapman & Hall; New York, London, 1994.
Ø Wilbur A. Gould, ‘Unit Operations for Food Industries’, CTI Publications, Maryland, USA, 1996.