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The benefits of breastfeeding are well established, both for babies and their mothers. Breast milk provides infants with all of the nutrients for the first six months of life, the recommended minimum period for breast feeding in the UK and the USA. Breastfed infants, and those who are fed expressed breast milk, have fewer deaths during the first year and experience fewer illnesses than babies fed formula. It protects them against common childhood infections and diseases such diarrhoea, middle ear infections, chest and urine infections and diabetes. Certain fatty acids present in milk are also building blocks for brain development. From the mother's point of view, breastfeeding is also thought to protect against certain diseases such as ovarian, uterine and breast cancer and the development of weak bones later in life. In addition, some women might be pleased to know that breastfeeding hastens a quicker return to their pre-pregnancy figure. For both parent and infant, breast feeding allows them to grow physically and emotionally closer, strengthening the bond between them. The pressures of modern living can limit the opportunities for mothers to breastfeed their babies directly, but there is always the option of expressing milk for storage and consumption at a later date. There does not appear to be one broadly accepted set of recommended storage conditions, although refrigeration for 2-3 days or freezing at -18°C for up to 3 months summarise the general advice. But what happens to the milk during storage? Several studies have reported changes in the chemical composition and biochemical components, but there have been no significant studies on the aroma profile. The aroma is important as newborns are known to be sensitive to specific smells that are believed to influence food preferences in later life. Now, two German researchers have studied the changes in the aroma profile of stored milk. Andrea Buettner and Johanna Spitzer from the University of Erlangen-Nurnberg and the Fraunhofer Institute for Process Engineering and Packaging (IVV) in Freising used a trained panel of sniffers to characterise the odour of fresh and stored human milk, and identified the associated aroma changes by GC with olfactometric and mass spectrometric detection. Fresh breast milk was stored for 2 months in a freezer at -19°C, the typical temperature of a domestic freezer. The panellists described the fresh milk to be low in overall odour intensity. Some of them detected a weak, hay-like, metallic, sweet or cooked milk odour but others did not. The odour profile of the thawed, stored milk was strikingly different. It had an intense odour that was dominated by metallic and fishy odours. Other, less intense odours included rancid, fatty notes with weak hay-like, buttery and sweaty odours. When mothers tasted their own fresh milk, it was generally described as sweet, fatty and soy-like, with a buttery note but the stored milk was so unpleasant that most mothers spat it out. It was described as unpleasant and nauseating. In comparison, stored cow milk did not produce the metallic or fishy notes. Instead, the odour intensity was only slightly greater than that of fresh cow milk. The hay-like or fishy notes were barely detectable. The human milks were analysed by GC-O and GC/MS, using retention times and mass spectra for confirmation. About 20 odourous compounds were detected in the fresh samples, including saturated and unsaturated aldehydes and ketones, lactones and fatty acids. The stored milk contained extra compounds with strong odours, such as 1-octen-3-one (mushroom), hexanal and (Z)-3-hexenal (green and grassy) and (Z)-1,5-octadien-3-one (metallic, like geranium leaf). The strongest smelling additional compounds with high flavour dilution factors in stored milk, but absent from fresh milk, were 1-octen-3-one, (Z)-1,5-octadien-3-one, trans-4,5-epoxy-(E)-2-decenal and (E,Z,Z)-2,4,7-tridecatrienal (both metallic and blood-like) and (E,Z)-2,4-nonadienal and (E,Z)-2,4-decadienal (both fatty, tallow-like). In addition, some acids with higher flavour dilution factors were found in stored milk such as acetic acid (vinegar), decanoic and dodecanoic acid (musty), phenylacetic acid (honey) and 3-phenylpropanoic acid (beeswax). The formation of some of these odorous compounds in stored milk fits with the metallic note reported by the panellists. However, no fishy compounds were detected, but the researchers noted that many perceived odours are a result of combinations of specific carbonyl compounds. So, the aroma profile of the stored milk was markedly different to that of fresh milk and was distasteful to the respective mothers. On the other hand, the babies did not appear to be concerned, despite their high odour sensitivity. The mothers reported "no significant changes in facial or behavioural response" and all fresh and stored samples were completely consumed. Buettner and Spitzer speculated that some specific aroma compounds might not be detectable by the babies but agreed that a more likely explanation was that "the babies had no prior experience of this flavour and had not yet learned that it might be associated with a possible deterioration of a food that might cause them harm." This is in line with previous studies on hydrolysate milk formula which was "relatively offensive" to adults but acceptable to babies. The researchers concluded that the contributions of the individual aroma volatiles to the overall odour of stored milk can only be confirmed by more extensive quantitative studies on the odour thresholds and by further examination of particular combinations of compounds. Related Links:
The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd. |
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