Reaction of FFO with Moisture (Water)

The most usual reaction of FFO with water is hydrolysis. It occurs in the triglyceride fraction resulting in formation of free fatty acids:

For users of FFO, the moisture content of purchased products is essentially nil, barring any introduction of water during storage or transport after deodorization. A typical maximum moisture specification for fully processed fats and oils is < 0.10%. The solubility of water in oil is about 0.1% at 20°C (68°F) that will increase with temperature and decrease as the average chain length of the component fatty acids increase. For FFO with fatty acids having chain lengths of C16, or greater, flavor deterioration due to hydrolysis is of little concern since these fatty acids are essentially flavorless. Fats and oils with component fatty acids < C16 will develop off flavors often described as “hydrolytic rancidity.” For very short chains, such as those found in butter, the resulting off flavors are quite pronounced. Hydrolysis of the longer chains such as the C12 and C14 found in coconut and palm kernel oils lead to “soapy” flavors.

In addition to the potential for soapy flavors due to fatty acids < C16 there is another important factor to consider in the use of FFO. “Lauric” acid oils such as coconut and palm kernel oils, for reasons not well understood, but nevertheless practically evident, will foam excessively during frying when these oils are mixed with oils of longer chain length.

Contact of FFO with water is, of course, unavoidable when frying moisture-containing products. This may contribute to FFO breakdown due to some hydrolysis at the high temperatures of frying and is often said to occur because of the inevitable increase in FFA over time in a fryer. It should be noted that the determination of FFA is a simple titration to an endpoint of pH 8.3 with sodium hydroxide and the results expressed as %FFA (AOCS Method Ca5a-40).

As purchased, moisture in FFO is usually of little concern. Subsequent handling practices should be monitored to insure there are no opportunities to introduce water into the FFO. Most often this happens during cleaning operations and sometimes occurs by reflux, or “drip back” from steam exhausting systems, with the latter being especially detrimental to fry life from both hydrolysis and oxidation. This will be discussed later in the chapter.

Introduction of excess or free water to hot FFO decreases fry life. It is also extremely dangerous to personnel and equipment due to explosive spattering. It also requires unnecessary and costly expenditure of heat energy to evaporate and dissipate the excess moisture. It is recommended that every precaution be taken to remove free moisture from products before being placed into hot FFO. This applies to both small retail frying operations as well as in larger commercial operations. Excess moisture may also contribute unnecessarily to the reflux mentioned above.

As shown in Fig. 2.1, oxidation also leads to the formation of acidic products. As a result, what is expressed as FFA may include both actual FFA and acidic products produced by oxidation.

This is simply a point of clarification and does not lessen the usefulness of using FFA as a measure of FFO deterioration in a frying vessel. In fact, FFA is a simple and practical method of following and/or estimating such deterioration.

Reaction of Frying Fats with Oxygen

Detailed mechanisms will be addressed in subsequent chapters. For purposes of this chapter the simplified diagram in Fig. 2.1 illustrates the oxidative reactions of FFO.

Practically speaking, the propensity for FFO to oxidize is related to their fatty acid composition (FAC), and more specifically to the amount and type of unsaturation. There may be some effect due to position of the fatty acid on the glycerol, but this is probably of minor consequence.

Relative rates of reaction of the three most common unsaturated fatty acids with oxygen are approximated and shown in Table 1.11. The relative rates of reaction for the type and amount of individual fatty acids can be useful for assessing the suitability of common fats and oils when formulating FFO. For any fat or oil, this is done by multiplying the decimal fraction of each unsaturated present by its relative rate of reaction and then summing to arrive at a relative measure of Inherent Stability (IS). The higher the IS, the more susceptible the fat or oil is to oxidation. IS is a convenient tool to calculate and assess the basic oxidative stability of a FFO when the fatty acid composition (FAC) is known. Using the FAC listed in USDA Handbook 8-4 (1) the inherent stability of common fats and oils is shown in Table 1.13.

Inherent Stability also supports the need for better handling of those fats and oils with higher numbers. Use of unhydrogenated refined, bleached, and deodorized (RBD) cottonseed and peanut oils for frying is possible because of their lower IS number. Greater use of the more unsaturated oils such as soybean oil with higher IS required the industry to develop better handling practices. This included use of stainless steel equipment, reconsideration of “standard” refining practices, and improved finished product handling. It also invited the use of hydrogenation to improve the oxidative stability of finished products, especially for use as FFO. The inherent instability of any FFO can be easily and simply calculated from the fatty acid composition and provides a basis for comparing FFO in relation to their expected susceptibility to oxidation.

Catalytic factors, as shown in Fig. 2.1, also affect oxidative stability. Avoidance of copper or iron in finished product storage and in frying equipment is mandatory. Exposure to light is usually not of concern during the frying process but may be in packaged products such as fried snacks where large surface areas are exposed. Both opaque packaging and inert gas packing are particularly effective in preventing both light and oxidative induced off flavor development.

Solid FFO

Solid FFO are received by smaller fryers as cubes in boxes with distinctively colored plastic liners, which are usually blue. This is to assure that such plastic liners are easily distinguished from the normally white shortening, thereby reducing the chance of the plastic being introduced into the frying equipment.

Handling of such cubed product is straightforward as to maintaining quality. Storage should be at or near normal room temperature (20°C). Depending on the product, it may be necessary to warm the cube to a higher temperature to facilitate easier scooping or cutting portions for addition to frying equipment. This should be done for each cube as needed rather than the whole stock on hand, however. Use of cubes should be on a first-in, first-used basis and product storage on site should be kept at a minimum.

Larger commercial fryers may use solid shortenings that are shipped in tank cars or trucks and are shipped as melted product in insulated vessels or as solid product requiring re-melting at the plant site for off-loading. Product specifications for such bulk shipments may include inert gas (nitrogen) purging and blanketing during loading. Gas (nitrogen) blanketing should be...