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First published in 1945, Bailey's has become the standard reference on the food chemistry and processing technology related to edible oils and the nonedible byproducts derived from oils. This sixth edition features new coverage of edible fats and oils and is enhanced by a second volume on oils and oilseeds. This sixth edition consists of six volumes: five volumes on edible oils and fats, with still one volume (as in the fifth edition) devoted to nonedible products from oils and fats. Some brand new topics in the sixth edition include: fungal and algal oils, conjugated linoleic acid, coco butter, phytosterols, and plant biotechnology as related to oil production. Now with 75 accessible chapters, each volume contains a self-contained index for that particular volume.
Monoj K. Gupta
MG Edible Oil Consulting International Richardson, Texas
Fried foods have provided culinary delight to people worldwide for centuries. It is difficult to determine when and where frying was first used by mankind. However, there is evidence that man used fried products long before the modern civilization reinvented fried products. Modern day frying involves sophisticated equipment, techniques, ingredients, and packaging. This is because the industrial fried products require long shelf life for warehousing, distribution, and sale.
In the frying process, food, such as vegetables, meat, or seafood, is brought in direct contact with hot oil. The food surface becomes golden yellow to dark brown in color and develops a pleasant fried food flavor.
Frying is done in homes, restaurants (food services), and at large industrial operations. Pan frying or griddle frying is done mostly at homes or at the restaurants. In this process, a thin layer of oil is heated on a skillet or a griddle. The food is fried in a layer of oil and fried until completion.
Restaurants also use batch fryers, where the food is placed in a wire basket, which is lowered into a bed of hot oil. The basket is removedfrom the hot oil when the product is fully fried.
The restaurants follow their guidelines on the frying temperature and time of frying. Frying temperature and frying time vary with the products fried.
Large-scale production of snack food is done in deep fat fryers. These are either batch or continuous fryers. In a batch fryer, the food is added into a large pan of hot oil. The oil is heated either directly from under the fryer or in an external heater. In the latter case, the oil is continuously recirculated into the pan and it is stirred with a stirrer. Formerly, manual stirring was common but modern kettles are generally equipped with mechanical stirrers. Fried product is removed and spun through a centrifugal device to remove the excess oil from the surface. The product is seasoned and packaged. The recovered oil is reused.
In a continuous fryer, the food enters the fryer at one end, is fried and taken out from the other end. The product is submerged in a bed of hot oil for a specified time depending on the type of food being fried. The oil is heated directly or indirectly as described above for the batch fryers.
Products in the above procedures are fully fried and are ready for consumption. There is another industrial method of frying that is used quite extensively. This is known as the par-frying process. The food is partially dehydrated in an industrial fryer and flash frozen at -20°C. The packaged par-fried food is stored at -5°F (20.6°C) to -10°F (23.3°C) and distributed in freezer trucks. The product remains in a freezer at the destination. It is taken out of the freezer and fried immediately without thawing. Most common par-fried products are French fries, potato wedges, breaded chicken, coated or uncoated vegetables, cheese-filled vegetables, coated cheese sticks, etc. This reduces the manpower and preparation times at the restaurants and provides a great deal of convenience and cost savings to the restaurants and food services.
Advancement in the packaging materials and packing methods has enabled the industrial frying operations to extend the shelf life of the fried products so they can be stored, distributed, and marketed over several weeks to several months, without losing freshness in the product. This has provided a tremendous boost to the growth of the packaged fried food industry.
Oil plays a great role in determining the storage stability quality of the fried product. However, oil is also prone to oxidation, which leads to rancidity of the product in storage. Use of packaging material with high oxygen, nitrogen, and moisture barrier properties can significantly reduce oil degradation and increase the shelf life of packaged fried food.
Frying oil has been available to man in various parts of the world. Most of the time a specific oil has been selected for frying because it is locally available. Man also has moved from the crude expelled oils to refined oils as the oil technology advanced. In addition, the availability of most oils across the world has also increased due to improved transportation and storage systems developed over the years. Consumers have been exposed to the taste of products fried in different types of oil for quite sometime. Production of other than the indigenous oils has also become common where the local climate, soil conditions, and overall agronomy have been favorable to a particular type of oilseed or oil palm trees.
In spite of the widespread distribution of various types of oils across the world, it is found that there are regional preferences for particular oils in fried foods. For example, cottonseed oil is considered as the "gold standard" for potato chips in the United Sates. This is largely because cottonseed oil was the primary vegetable oil grown in the United States when potato chips were introduced 150 years ago at Saratoga Falls, New York.
Similarly, the Mexican consumers prefer sesame seed oil or safflower oil in fried snack foods. Consumers in the Indian sub-continent prefer peanut (groundnut) oil in fried snacks. Bias towards the original indigenous oil can be found in every oil-producing country. Availability and the necessity for sufficient supply of the oil have played a great role in local selection of oil for frying products. For example, the Mexican consumers have accepted palmolein for frying snack foods because the fried food has good flavor and taste, although they prefer safflower oil or sesame seed oil. Acceptance of palmolein in Mexico has been influenced by the fact that sesame seed and safflower oils are in short supply and more expensive and palmolein produces good fried food at reduced cost.
2. ROLE OF OIL OR FAT IN FRYING
Oil provides several important attributes to the fried product that makes the fried food palatable and desirable to the consumers, these include:
Fried food flavor
Fortunately, oil has also been an excellent heat transfer medium for dehydration of the food during frying. Some mechanical engineers in the frying industry tend to treat the oil as a true heat transfer medium. Subsequent discussions in this chapter will show that oil plays a much greater role than just being a heat transfer medium in frying.
3. APPLICATIONS OF FRYING OIL
As previously mentioned, frying oil is used in homes, restaurants (food services), and industrial frying operations. Home fried food is consumed almost immediately after preparation. At restaurants, the fried food is generally made to order and consumed within minutes of its preparation. Frying oil is always considered acceptable at homes or restaurants when it produces good flavor and texture in the food. There is little or no concern regarding the shelf life of the fried product at either of these locations.
Industrial products, on the other hand, are packaged and distributed for sale. Some of these products may require weeks or months for their distribution and sale. Therefore, these products must maintain good flavor and texture in order to be acceptable to consumers when they are purchased. The oils (fats) used for industrial frying must have good oxidative and flavor stability in order to achieve good shelf life for the products. In this chapter, one will be able to understand the requirements that are critical for industrial frying oil. All subsequent discussions on oils in this chapter will be pertaining to industrial frying, although, the same criteria apply in restaurant frying.
4. SELECTION OF FRYING OIL
The following criteria are applied for the selection of oil for industrial frying:
1. Product flavor
2. Product texture
3. Product appearance
6. Shelf life of the product
7. Availability of the oil
9. Nutritional requirements
Flavor, aroma, and appearance are generally the first three attributes that the consumer looks for in the fried food. Subsequently, the consumer judges the fried food for texture, mouthfeel, and aftertaste. Thus, the first five items from the above list are important for consumer acceptance of the product.
Product shelf life is important for quality and economic reasons. All products require a certain number of weeks or even months for distribution and sales. The product flavor and texture must be acceptable to the consumer at the time it is used. The texture of the product (staleness) is caused by moisture pickup during storage. This can be corrected through proper initial moisture control and the use of appropriate packaging with a good moisture barrier property.
Oil quality and oil flavor stability greatly influence the flavor stability of the product in storage.
Availability and cost of oil are important economic factors. Even the best performing frying oil is not beneficial to the business if it is not available in sufficient quantities. The cost of oil is extremely critical for the industry. Most fried snack foods contain 20-40% oil. Therefore, the snack food company has to minimize the delivered cost of oil at the plant. Sometimes, the procurement department purchases oil from a supplier that does not have good control over their operation. This ends up costing money and goodwill for the snack food company in the long run.
Nutritional value of oil in the snack food has become important. To meet today's consumer desire's, the frying oil must have the following attributes:
1. Low in saturated fat
2. Low in linolenic acid
3. High oxidative and flavor stability
4. Not hydrogenated (trans-fat free)
This is a difficult challenge for the snack food industry to meet because the modified composition oils are in very limited supply. Palmolein has no trans-fat but it is high in saturated fat. Soybean oil and canola oil must be hydrogenated for industrial frying. Thus, they will have trans-fat. Moreover, it is important to recognize the fact that the joint supply of palm oil and soybean oil constitutes almost 80% of the world's oil consumption. There is not enough of either of these two oils to supply the world's total oil needs. Corn oil, cottonseed, modified sunflower, and modified canola oils are in limited supply. They are grown in limited geographic areas where they are facing stiff competition against other cash crops. Therefore, nutritional needs in fried snack foods can be met in limited geographic areas and at a significantly higher cost.
5. THE FRYING PROCESS
Frying is a complex process where simultaneous heat and mass transfer as well as chemical reactions take place. In this process, the hot oil supplies the heat to the product being fried. Heat turns the internal moisture of the food product into water vapor. The water vapor comes out of the product through the outer surface (see Figure 1). This is why one can always see bubbles around the food being fried. Bubbling is vigorous at the beginning when the food is added into the hot oil and stops when the moisture in the product drops to a low level.
The food product undergoes dehydration. At the same time, several physical changes and chemical reactions occur in the food as well as the frying oil, as described below:
5.1. The Changes Occurring in the Food
The food loses moisture
The food surface develops a darker color (sometimes, hard crust)
The fried food develops a firmer texture (or crust)
The food also develops fried flavor and aroma
5.2. The Changes Occurring in the Oil
The fresh oil passes through a breaking-in period during which the fried food appears quite bland
Fried food flavor develops as the frying process continues
Along with flavor development, the oil undergoes the following chemical reactions:
1) Hydrolysis 2) Autoxidation 3) Oxidative Polymerization, and 4) Thermal Polymerization
The oil in the fryer becomes darker
The oil quality and the fried food flavor go through an optimum stage. Thereafter, both oil quality and product flavor decline. All of the above chemical reactions alter the chemical structure of the oil molecules. The unsaturated fatty acids are mostly affected. Some desirable as well as undesirable chemical compounds are formed in the oil during frying. Oil in the freshly fried foods contains the same compounds that are present in the fryer oil. The desirable compounds help provide good flavor to the freshly fried product. Sometimes, the undesirable oil components can affect the fresh product flavor. In many instances, a fried product with good initial flavor may develop oxidized or rancid flavor during storage. This is because the products of oil oxidation are strong catalysts and cause further degradation of the oil (contained in the product) during storage. This phenomenon is quite pronounced when the oil is abused in the frying process. This is even more evident in products fried in oil with poor fresh oil quality. Therefore, oxidative stability of the oil in packaged fried foods is critical for achieving the desired shelf life for the product.
Darkening of the product surface, also called the browning reaction, is produced by the chemical reaction between the frying oil or oil present in the food (lipids in general) and proteins, and saccharides present in the food. This reaction is known as Maillard reaction, which is responsible for the following:
1. Brown or dark brown surface appearance of the fried product
2. Fried flavor of the product
Browning reaction also provides some protection against photooxidation, which will be discussed later.
6. CHEMICAL REACTIONS OCCURRING IN OIL DURING FRYING
It has been mentioned earlier that several chemical reactions take place in the oil during frying. These include hydrolysis, autoxidation, oxidative polymerization, and thermal polymerization, as explained below.
In this process, an oil (triacylglycerol, also known as triglyceride) molecule reacts with a molecule of water, releasing a molecule of fatty acid, commonly known as free fatty acid (FFA), and a molecule of diacylglycerol (DG, also called diglyceride). The reaction scheme is shown below:
Triacylglycerol + [H.sub.2]0 = Free Fatty Acid(FFA) + Diacylglycerol
Although, it is common for the oil to undergo this reaction during frying, presence of a surfactant is required for hydrolysis to occur. Hydrolysis cannot occur unless oil and water form a solution. Oil and water do not mix except at very high temperatures under high pressure at 500°F (260°C) or higher, water boils at 212°F (100°C), at sea level. Therefore, one can expect that very little oil and water solution should result at frying temperatures (300-415°F or 149-213°C), unless there is a small amount of surfactant present in the fryer. A surfactant can facilitate the formation of an oil/water solution during frying. This is primarily responsible for generating the FFA in the fryer oil. Several sources of surfactants are listed below.
6.1.1. Fresh Oil. Fresh frying oil is obtained by refining palm oil or seed oils. It would be appropriate to briefly discuss the vegetable oil refining process for the readers to understand how various processing steps impact the quality of freshly refined oil. Vegetable oils are refined principally by:
1. Physical refining method
2. Chemical refining method
Palm oil and coconut oil are refined by the physical refining method. The crude oil is bleached with acid-activated clay and citric acid at elevated temperatures under vacuum. The objective is to remove phosphorus (phospholipids), trace metals, oil decomposition products, and some of the color bodies from the crude oil. The volatile impurities in the bleached oil are then removed via steam distillation under very low absolute pressure and high temperature in a deodorizer.
Excerpted from Bailey's Industrial Oil and Fat Products, Edible Oil and Fat Products Copyright © 2005 by John Wiley & Sons, Inc.. Excerpted by permission.
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