No manufacturer wants to hear a complaint about their product smelling off. So, testing is crucial to know how long a product lasts and the quality of its ingredients. Food products containing fats with long carbon chains and varying levels of unsaturation are susceptible to decay, thus limiting their shelf life.
Rancidity refers to the oxidative or hydrolytic degradation of fats and oils, which can render food unfit for consumption. This chemical reaction, triggered by exposure to air, light, moisture, and bacterial activity, is a common culprit behind the spoilage of food items.
The science behind Rancidity:
Fats and fatty acids in food are the key players in rancidity. Fatty acids can be saturated (single carbon linkage) or unsaturated (multiple carbon linkages). Rancidity happens in three steps:
- Initiation: Triggered by external factors such as heat and air, this step involves the formation of radicals (highly chemically reactive atom/molecule/ion with an unpaired electron) on the food substance. The reaction equation: RH → R* + H*
- Propagation: Oxygen gives rise to peroxides, which react with more unsaturated fatty acids to produce new radicals. The step equation: R* + O2 → ROO** (peroxide)
- Termination: Two radicals combine, forming a new single bond. Step equation: ROO* + ROO* → end products
Upon termination, fats, oils, and other lipids are broken down into short chain fatty acids, which are not only volatile but also highly reactive. These are the culprits behind the unpleasant smell and taste characteristic of rancid food. In some instances, rancidification can also result in the loss of vitamins.
Types of Rancidity:
- Oxidative Rancidity: Oxygen damages the natural structure of oils, changing color, odor, and taste. It can produce toxic compounds that destroy vitamins A and E.
- Hydrolytic Rancidity: This causes an unpleasant odor by liberation of free fatty acids from glycerides. When these react with water, they may undergo oxidative rancidification, forming toxic compounds.
Factors affecting Rancidity:
Several factors can influence rancidity, including oxygen exposure, microorganisms, and physical factors like temperature and light.
- Oxygen: More soluble in fats, leading to oxidation and formation of free radicals.
- Microorganisms: Some microorganisms release an enzyme, lipase, causing the hydrolysis of lipids.
- Physical Factors: Heat and light play a significant role in rancidification, speeding up the oxidation process.
Methods of testing for Rancidity:
Not all rancidity tests are equal. Let's break down a few:
- Peroxide Value (PV) Testing: This test measures the amount of peroxides in a sample, indicating early signs of rancidity (peroxides are the initial indicators of lipid oxidation, which react further to produce secondary products e.g. aldehydes). Pairing it with p-Anisidine Value gives a fuller picture of product quality since peroxide production diminishes over time.
- TBA Rancidity (TBAR): Primarily for low-fat samples, this test measures aldehydes (primarily malondialdhyde). Useful when you want to analyze the entire sample, not just extracted lipids.
- p-Anisidine (p-AV) Testing: This checks for reactive aldehydes and ketones in a sample, which can produce bad smells and flavors. Dark samples may not work well with this test as the calorimeter may not correctly measure the wavelength required.
- Oxidative Stability Index (OSI): Shows how well a sample resists oxidation. Samples are heated in the presence of air to accelerate oxidation and the amount of time taken to reach the inflection point is recorded. Helpful for testing the effectiveness of antioxidants in a product, especially under stressful conditions, since antioxidants should deter propagation of radicals.
- Free Fatty Acids (FFA) Testing: Determines the amount of liberated fatty acids by titration method. The type of fat being tested must be considered for accurate results because the method does not discriminate the type of fat. Different conversion factors should be used for different fats depending on the proportion of palmitic/lauric acid content.
How to prevent Rancidity?
Preventing rancidity is crucial to preserving the desirable qualities of food products. This can be achieved by:
- Storing food items in airtight containers, away from direct exposure to light and air.
- Addition of antioxidants, which can inhibit auto-oxidation in foods rich in fats and oils. These antioxidants can be either natural (e.g., vitamin C, vitamin E, flavonoids, and polyphenols) or synthetic. Sequestering agents like EDTA can also be used to slow down oxidation, thereby effectively preventing rancidity.
