Interesterified fats are processed lipids developed as alternatives to traditional fats. These fats are fundamentally triglycerides, the main form of fat found in nature, that have been structurally modified by a chemical process. The modification involves rearranging the fatty acids attached to the glycerol backbone of the fat molecule. This restructuring achieves specific functional properties desired by manufacturers. Interesterified fats are common in the global food supply because they provide necessary texture and stability without artificial trans fatty acids.
How Interesterified Fats are Created
Fats and oils are composed of triglycerides, which feature three fatty acid chains connected to a central glycerol molecule. Interesterification targets the bonds connecting these fatty acids to the glycerol backbone, known as ester bonds. By breaking and reforming these bonds, the process shuffles the fatty acid chains both within a single triglyceride molecule and between different triglyceride molecules in a blend. The final product retains the same fatty acids—such as saturated, monounsaturated, and polyunsaturated—but their arrangement on the glycerol molecule is altered.
Manufacturers primarily use two distinct techniques to achieve this molecular rearrangement. Chemical interesterification involves the use of high heat and a strong base catalyst, such as sodium methoxide, to initiate the reaction. This method causes a random exchange of fatty acids, resulting in a wide variety of new triglyceride structures. While this approach is often cost-effective, it requires post-processing steps to remove the chemical catalyst and by-products.
The alternative, enzymatic interesterification, utilizes specific enzymes called lipases to facilitate the exchange of fatty acids. This enzymatic method operates at lower temperatures, typically around 70°C, and is considered a cleaner process because it produces fewer by-products that require removal. Certain lipases are also stereospecific, allowing for more directed control over the final fat structure. Regardless of the method used, the objective is to change the physical behavior of the fat without altering the chemical nature or proportion of the original fatty acids.
Why Manufacturers Use Interesterified Fats
The fundamental reason manufacturers modify fats is to control the physical properties of the final product, especially its melting behavior and texture. Interesterification allows food scientists to tailor the solid fat content (SFC) of an oil blend, which dictates how solid or liquid a fat is at various temperatures. The rearranged structure of the triglycerides forms different crystal patterns, which is what gives a fat its desired firmness and plasticity.
This modification results in a wider plasticity range for the fat, meaning it maintains a consistent physical state across a broader temperature span without separating. For products like margarine and shortening, this is a significant advantage, as it ensures the spreadable or pliable texture remains stable both in refrigeration and at room temperature. Interesterified fats are also frequently used in baked goods, such as cookies, biscuits, and pie crusts, to provide structure and a desirable mouthfeel.
Beyond texture, these fats are valued for their role in improving the stability and shelf life of processed foods. The altered crystal structure helps prevent the separation of the oil and solid components within a product over time, a process known as oiling out. This increased stability makes them suitable for products like icings, confectionery, and dairy fat replacers. Interesterification allows manufacturers to transform inexpensive liquid oils and solid fats into a functional blend that mimics the properties of more costly traditional fats like butter or lard.
Health and Nutritional Considerations
The primary driver for the widespread adoption of interesterified fats was the global effort to remove artificial trans fats from the food supply. Artificial trans fats are created through partial hydrogenation, a process known to generate trans fatty acids that are significantly detrimental to cardiovascular health. Interesterification serves as a direct alternative because it modifies the physical properties of fat without creating trans fatty acids. This distinction is why interesterified fats are often marketed as trans-fat free replacements.
However, the nutritional profile of interesterified fats is complex, and research into their metabolic effects has yielded mixed results. Compared to diets high in trans fats, interesterified fats are consistently considered nutritionally superior. Yet, when compared to natural fats or blends with an equivalent amount of saturated fat, the outcomes are not always clear. Some human studies have shown that interesterified fats have a neutral effect on fasting blood lipids, such as LDL and HDL cholesterol, compared to non-interesterified fats with the same overall fatty acid composition.
Other research, particularly studies involving high levels of interesterified fats, has raised questions about potential effects on metabolic markers. A few studies have suggested that high consumption of specific interesterified saturated fats, such as those rich in stearic or palmitic acid, might negatively affect blood glucose control and certain lipoproteins. Critics of these findings often point out that the test diets sometimes contained unrealistically high quantities of interesterified fat or used types of fat not common in commercial products.
Interesterified fats often contain a high proportion of saturated fatty acids, even though they are trans-fat free. While the process can sometimes reduce the saturated fat content compared to other solid fats, the final product remains a source of saturated fat. Public health guidance advises limiting total saturated fat intake to support cardiovascular health. The rearranged structure may also affect digestion, as placing a saturated fatty acid at the middle position of the glycerol backbone may increase its absorption, though this effect is not consistently demonstrated. The long-term health consequences and ideal consumption levels of interesterified fats are still an active area of scientific investigation.