Tiffin Project

Let’s Talk About Fat – Part 2 – Chemistry

by Todd Caldecott on February 10, 2011

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Also known as lipids, fats refer to a diverse range of chemicals including triglycerides, cholesterol, phospholipids, steroids, carotenoids, vitamins (A, D, E, K) and locally-acting hormones called eicosanoids. Like carbohydrates lipids are comprised of carbon, oxygen and hydrogen, but the amount of oxygen in a lipid is much less, making for fewer polar covalent bonds. This means that lipids are insoluble in polar solvents such as water, but soluble in non-polar solvents like alcohol. This makes fats quite different from proteins and carbohydrates, and is the reason why lipids such as cholesterol must be bound to proteins such as LDL or HDL, in order to be transported in the blood.

When we speak of dietary fat we are referring exclusively to triglycerides, comprised of a three-carbon glycerol molecule that forms the backbone, and three (tri) fatty acids attached to each carbon in the glycerol molecule. The fatty acids themselves are comprised of a chain of carbon atoms that have a variable size and shape, with hydrogen atoms attached on the side and at the end. When hydrogen atoms fill up every spot available on the chain it is called a ‘saturated’ fatty acid. When only one space is empty on the chain it is called ‘monounsaturated’, i.e. ‘mono’ referring to the one empty spot, and unsaturated referring to the fact that because this spot is empty, the fatty acid isn’t completely ‘saturated’ with hydrogen, and is therefore ‘unsaturated’. A third type of fatty acid is called ‘polyunsaturated’, meaning that two or more spots (i.e. ‘poly’) on the fatty acid chain are empty, and is therefore ‘polyunsaturated’.

A given triglyceride may be comprised of different types of fatty acids, and so rarely is a fat completely saturated or unsaturated (see link). Saturated fats tend to be solid at room temperature and are commonly found in animal fats such as lard and butter, but are also present in plant oils such as coconut and palm kernel. Saturated fats are relatively stable and are thus a better choice for cooking and especially frying. In comparison, dietary fats rich in monounsaturates are more unstable, but certainly more stable than oils rich in polyunsaturates, which oxidize quickly when exposed to heat, oxygen or light.

One of the key issues that relates to fats is their role in the production of compounds called eicosanoids, including prostaglandins, leukotrienes, prostacyclins and thromboxanes. Eicosanoids are derived from linoleic and linolenic acid, two types of polyunsaturated fatty acids that the body cannot synthesize, and hence are called essential fatty acids. Linoleic acid is an 18-carbon fatty acid also called the omega 6 fatty acid because the first double bond is six carbons from the end, whereas linolenic acid is an 18-carbon fatty acid called an the omega 3 fatty acid because the first double bond is three carbons from the end. Linoleic acid and its active metabolite arachidonic acid (AA) are converted into a series of eicosanoids that tend to have a pro-inflammatory effect, whereas linolenic acid and its metabolites including eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) tend to have an anti-inflammatory effect. During our human evolution the consumption of omega 3 and 6 fatty acids was about equal, but with our relatively recent dependence upon cereal grains and vegetable oils in our diet and in animal feed, the ratio of essential fatty acids we consume has become skewed in favor of omega 6 fatty acids. The net result of this imbalance is that we have a tendency to produce more pro-inflammatory eicosanoids, a feature that has been linked to chronic diseases including allergies, asthma, arthritis, cardiovascular disease and cancer. Although our daily requirement for both omega 3 and 6 fatty acids is rather small, given that the average person consumes upwards of 17 times the amount of omega 6 relative to omega 3, many practitioners recommend omega 3 fatty acid supplementation as a counter-measure. There are however issues with this recommendation, and a more sustainable measure is to eat a diet that is naturally rich in omega 3 fatty acids, emphasizing pasture-raised animal produce, wild meat and fish, leafy greens, sea vegetables, and select cereal grains such as flax and hemp seed.

Apart from the fats naturally found in the foods we eat, the fats we cook with must be extracted from their original source whether from animals or plants. The fat in milk called butterfat is extracted by first skimming off the cream, churning it into butter, and then rendering off the pure butterfat from the milk solids to make ghee . Animal fats including tallow, lard or fish fat are generally extracted by heat, such as boiling or roasting, and in industrial operations undergoes further processing and refinement. Home made animal fats such as goose, beef (tallow) or pork (lard) are made by carefully trimming the meat from the fat, and then slowly roasting the fat over an even heat.

The methods used to extract oil from plants depend on several factors. Oil-rich seeds such as sesame yield their fat with relatively simple means, whereas seeds that have a low oil content such grape seed require extreme pressure and solvents to separate out the oil. The simplest method of oil extraction is the expeller press, which applies a mechanical pressure to squeeze out the oil. This simple, low-tech method was used for many of the traditional vegetable oils we eat, all of which have a high percentage of fat and are thus relatively east to express, including olive (30% fat), sesame (50% fat), palm kernel (50% fat), almond (50-65% fat) and coconut (65% fat). The expeller process generally has a minimal impact on the quality of the oil, but under very high pressures temperatures can reach upwards of 120F (49C), high enough to cause some oils such as flax seed to oxidize. In some cases the nut or seed is boiled or roasted before expression to render the oils more conducive to extraction, or in more recent times, non-polar synthetic solvents such as hexane are used.

While traditional methods of extraction tend to yield an acceptable product, when intensive methods of extraction are used the quality and taste of an oil can be dramatically affected. As a result these oils require further processing, which may include degumming, neutralization, winterization, bleaching, deodorization (steam distillation) and hydrogenation. Some of these processes utilize chemicals and solvents to remove natural constituents such as waxes, gums, free fatty acids, vitamins and sterols. During deodorization in particular, the oil is heated to temperatures as high 446-500F (230-260C) for as long as several hours. Hydrogenation is a last step in which the oil is heated in a chamber with a metal catalyst and exposed to pressurized hydrogen gas. This is done to saturate oils rich in polyunsaturated fatty acids with hydrogen, turning it into a kind of saturated fat that allows it to be solid and shelf-stable at room temperature.

Hydrogenation is a process that fundamentally changes the physical structure of an unsaturated fatty acid, flipping hydrogen atoms across the double bond between two carbon atoms, changing them from cis (‘same side’) to trans (‘across’) configuration. Transfats have come under a lot of fire lately based on research that hydrogenated oils are associated with a higher risk of cardiovascular disease, diabetes and cancer. But ‘transfats’ are not the issue per se. In fact some transfats play a useful role in human health, like vaccenic and conjugated linoleic acid (CLA), both of which are naturally occurring in butterfat. More recently, researchers have linked trans-palmetoic acid, found in whole fat dairy products and meat, with a significant reduction in the risk of atherosclerosis and diabetes. The label of ‘transfat’ obscures the real issue, which is the way industrial processing dramatically alters the structure of the fats we eat, removing essential fatty acids, vitamins and antioxidants, leading to nutrient deficiencies and gross imbalances in the ratio of fatty acids in the diet. Significant damage also occurs when oils rich in polyunsaturated fatty acids are heated during processing, altering their intrinsic structure as well as generating disease-causing lipid peroxides.

~ Todd Caldecott


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