Analysis of Dairy Products, using SIFT-MS
The sensory appeal of dairy products is in part due to the very desirable aromas that they exhibit. These aromas are most often due to volatile organic compounds (VOCs). In this post, we demonstrate some benefits of SIFT-MS to the analysis of a range of dairy products.
The world production of cow’s milk was 480 million tonnes in 1999, with over 43% of this produced in Europe and nearly 20% in North and Central America.1 Many different consumer products are produced from milk, such as cheese, yogurt, and butter, or use milk as an important ingredient, such as milk chocolate.
Good quality raw or low-temperature pasteurized fresh milk has a rather bland flavor.2 The slightly sweet and salty taste arises from lactose and milk salts, and the weak aroma from a complex mixture of aroma compounds, most of which occur at concentrations less than the flavor threshold. The rather weak and bland flavor of milk means that it is very susceptible to undesirable modification. Cheese manufacture, for example, typically requires the use of the highest sensory quality milk to avoid undesirable flavors in the end product.
Volatile organic compounds (VOCs) are very important contributors to the characteristic aromas and aroma defects perceived in milk and its products.3
Volatile Aroma Compounds in Milk and Its Products
As mentioned above, milk has a rather bland flavor when fresh and of good quality. Due to the very low levels of compounds contributing to the aroma, it has proved difficult to definitively identify the most important constituents. It appears that some of the more important ones in low-temperature pasteurized milk are dimethylsulfide, diacetyl (2,3-butanedione), 2-methylbutanal, (Z)-4-heptenal, 3- butenylisothiocyanate and (E)-2-nonenal. Higher temperature heat treatments, which extend the shelf-life of the milk, tend to result in a cooked flavor from sulfur compounds, such as hydrogen sulfide, produced by protein breakdown. Ketones and lactones are also produced. Sterilisation of the milk at even higher temperatures leads to marked chemical changes, producing browning and Maillard reaction products (e.g. the furanones, including maltol), as well as the ultra heat treated (UHT)-like products.
A large number of other flavor defects can occur in milk, including those from:
- Milk production in the cow itself (e.g., due to metabolic disturbances, infections, or taints from feed)
- Improper handling and storage of milk after the cow is milked
- Contamination of the product during processing
- Taints from packaging or other odorous products stored nearby.
Table 1 lists a number of milk defects and the associated volatile compounds causing them.7.
|Flavour Defect||Contributing compounds|
|Feed flavor||Dimethyl sulfide, acetone, butanone, ethanol, 1-propanol, 2-propanol|
|Weed taints||Methyl mercaptan, dimethyl sulfide, dimethyl disulfide, indole, skatole|
|Cowy flavor||Dimethyl sulfide, acetone|
|Putrid flavor||Methyl mercaptan, dimethyl sulfide, dimethyl disulfide|
|Unclean flavor||Dimethyl sulfide|
|Cooked flavor||Hydrogen sulfide, methyl mercaptan, dimethyl sulfide, dimethyl disulfide|
|Heating flavour (sterilisation / caramelisation)||Maltol, furaneol, 2,5-dimethylpyrazine, 2-aminoacetophenone, 2-pentanone, 2-heptanone, 2-nonanone, 2-undecanone|
|Heating flavor (UHT ketone-like)||2-Pentanone, 2-heptanone, 2-nonanone, 2-undecanone|
|Light-induced flavor (activated)||Methyl mercaptan, dimethyl sulfide, dimethyl disulfide, 3-methylthiopropanal|
|Oxidation flavor (general)||Acetone, butanone, 2-pentanone, 2-heptanone, 2-nonanone, 2-undecanone, hexanal, heptanal, octanal, nonanal, decanal|
|Oxidation flavor (metallic)||Methyl vinyl ketone, ethyl vinyl ketone, amyl vinyl ketone|
|Oxidation flavor (tallowy)||Hexanal, heptanal, octanal, nonanal, decanal, 2-heptenal, 2-octenal, 2-nonenal, 2-decenal|
|Oxidation flavor (green, cucumber)||3-Hexenal, 2,6-nonadienal|
|Oxidation flavour (fatty, fried)||2-heptenal, 2-octenal, 2-nonenal, 2-decenal, 2,4-heptadienal, 2,4-decadienal|
|Oxidation flavor (fishy)||2,4,7-Decatrienal|
|Malty flavor||2-Methyl propanal, 2-methyl butanal, 3-methyl butanal|
|Acid flavor||Acetic acid, lactic acid|
|Lypolytic rancidity||Butanoic acid, pentanoic acid, hexanoic acid|
|Fruity flavor||Butanoic acid, hexanoic acid, ethyl butanoate, ethyl 3-methylbutanoate, ethyl hexanoate|
The microbial processes occurring during the production of cheeses yield a diverse range of volatile aroma compounds which are generally at higher concentrations than in milk. As of 1991, 213 compounds had been identified in the headspace of cheddar cheese. Fermentation produces the volatile fatty acids (C2-C6), lactic and succinic acids. Esters, ketones, small branched aldehydes, and sulfides are often observed at lower concentrations, although the presence and levels of these vary depending on the type of cheese due to the particular microbe(s) used to prepare it.
Yogurt and other sour milk products aroma
The lactic acid bacteria used to create yogurt produce a variety of compounds that contribute to its aroma. These compounds include acetaldehyde, diacetyl, dimethyl sulfide, acetic acid, lactic acid, 1- octen-3-one and 1-nonen-3-one. Acetaldehyde and diacetyl dominate the aromas of other sour products because they are also produced by the action of lactic acid bacteria.
The aroma of butter is dominated by three VOCs: diacetyl, butanoic acid and δ-decalactone. A rather delicate balance between these volatiles must exist for the butter to maintain a favourable aroma.