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Lipid Oxidation and Oil Migration

 

Lipid Oxidation and Oil Migration are undesirable chemical reactions of the oils in almonds. Through proper storage and processing, they can be reduced or avoided.

Oxidation and rancidity.

Lipid oxidation is a complex series of undesirable reactions that cause the breakdown of fats and oils. In oil-containing foods like almonds, the oxidation reactions lead to a loss of quality as the nuts develop “rancid” flavours and odors. During lipid oxidation, oxygen reacts spontaneously with the fatty acids in fats to form primary breakdown products (i.e. peroxides, conjugated dienes) and, as oxidation progresses, secondary products (i.e. volatile aldehydes, ketones) are formed that give rise to off-flavours and off-odours. Oxidation can be measured by testing for the presence or accumulation of one or more of these primary and secondary products. For example, almonds can be tested for peroxide value (PV) and free fatty acids (FFA).

High storage temperatures, increased moisture, light and some metals (i.e. iron) may promote lipid oxidation in almonds and reduce shelf life. Processing also makes almonds more susceptible to oxidation; blanching and cutting increase the surface area exposed to oxygen, and roasting changes the almond microstructure, which allows more oil within the cells to be exposed to oxygen.

Water activity (aw) level affects lipid oxidation rates; lipid oxidation is typically lowest when almond aw is ~0.25 to 0.35 (~3–4% moisture content), and increases above or below that aw range.

 

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Oil migration and chocolate bloom.

Roasted whole or cut almonds are used in chocolate as a centre or inclusion. Almonds have a high oil content, and this characteristic is often assumed to be a potential concern for chocolate bloom, the quality defects that occur over time in chocolate products. The bloom may cause:

  • Softening of chocolate;

  • “Fat bloom” on chocolate surfaces; and/or

  • Hardening of fillings.

These defects are generally the result of complex interactions involved in oil or fat migration. Oil migration has a critical impact on the quality and shelf life of chocolate products.Oil migration cannot be completely prevented but can be minimised through technical considerations in ingredient selection and chocolate-processing steps, such as proper tempering, as well as suitable handling and low-temperature (<20°C or <68°F ) storage.

Recent studies of almond microstructure after roasting and almond oil migration in model confectionery systems by researchers at the University of California, Davis, are providing valuable insight into the processing requirements and behaviour of almonds as ingredients in confectionery. In well-tempered chocolate products, light-roasted almonds didn’t show oil migration into chocolate media within a storage duration equivalent to a shelf life for commercial chocolate products. In a two-phase system with fillings made from ground almond products (butter, paste, crème), the researchers did not observe oil migration in the samples stored at or below 25°C (77°F); this indicates that the storage temperature is more critical than the filling composition for prevention of oil migration.

 

Additional resources:

 

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USDA Standards Almonds In-Shell
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USDA Standards Shelled Almonds