On the scent: the aroma of raw milk

Pioneering research undertaken by Teagasc, in association with University College Cork, has shown how cows’ diets affect the aroma of raw milk.

Ireland’s predominantly pasture-based feeding system for milk production is relatively unique when compared to most other countries (which use a predominantly concentrate-based system). Many factors influence this decision, but it’s mainly related to the fact that Ireland’s climate makes it more economically feasible to do so.

It has long been believed that diet affects the flavour of raw milk, but this has proven difficult to verify without the identification of the specific compounds responsible. To address this, Teagasc and University College Cork (UCC) used a combination of techniques to successfully complete the first in-depth study demonstrating the impact of diet on the aroma of raw bovine milk.

Pasture vs non-pasture feeding

Previously conducted sensory studies have highlighted subtle differences in milk from pasture and non-pasture diets. Most of these differences relate to visual or textural differences, as these are easier to determine. Differences in colour relate mainly to a higher β-carotene (pigment) content in pasture-derived milk, giving it a more yellow or less white hue. Texture differences can be perceived in the mouth, due to changes in the fatty acid profile of milk.

Whilst flavour is a combination of taste and aroma, aroma has a much greater influence, so to improve their chances of success, the researchers focused on the volatile organic compounds (VOC) found within the raw milks.

Kieran Kilcawley, Teagasc Principal Research Officer and project member, says: “VOC are responsible for odour, so they’re an important area of study.

“We also chose to focus on two main types of raw milk – milk produced from cows outdoors fed perennial ryegrass (GRS) in pasture-based systems, and milk produced from cows indoors fed a total mixed ration (TMR) diet – typically green fodder or silage blended with cereals, protein sources, vitamins and feed additives.”

Gas-chromatography-olfactometry (GC-O) is used to identify odour-active VOC in food products

Identifying active aroma compounds

The VOC profile was extracted from the raw milk using a novel advanced high capacity extraction process. It was then separated and identified using an instrumental technique known as gas chromatography mass spectrometry.

“We identified 99 VOC in these milks – significantly more than any previous international study,” says Kieran. “In addition, we demonstrated that 33 of these VOC were also present in the diet and in the rumen (stomach compartment), highlighting the significance of the direct transfer of VOC from feed to milk.”

The results also highlighted that the VOC content of raw milk from pasture and non-pasture diets were very similar, which was in line with previous studies. Furthermore, the abundance of only 13 VOC varied significantly based on diet alone.

“This finding was very important as our ability to perceive aroma is impacted by both the abundance and the odour threshold (the concentration above which we can perceive it) of each individual VOC,” explains Kieran.

“To determine which VOC influenced the aroma perception of these milks, we used a technique called gas-chromatography-olfactometry (GC-O). GC-O essentially involves trained panellists acting as an additional detection system in combination with the spectrometry detection.

“Using specific processes such as odour intensity (OI) and aroma extraction dilution analysis (AEDA), these panellists can describe individual odours from VOC, but also determine their relative intensity and therefore importance.”

Very few GC-O studies have been undertaken on raw milk, potentially because of its subtle flavour, which makes it difficult to discern aroma characteristics. To overcome this, researchers ensured the VOC were concentrated sufficiently prior to separation by GC, in order for it to be more easily perceived by the olfactometry panellists. This was only achieved by using this novel high capacity sorptive extraction system.

Alternative influencing factors

Approximately 30% of the VOC were deemed odour-active by olfactometry. Even though the raw milks derived from both pasture and non-pasture diets shared many odour-active VOC, enough differences existed in their abundances to result in differences in their respective aromas.

Raw milk derived from a TMR diet was judged to have a higher OI, with many of the odour-active VOC derived from Maillard reactions (a non-enzymatic reaction indicated by heat). This led the researchers to conclude that they were directly transferred from the TMR, but must also have been created as a result of the heat-treatments used to produce the TMR feed itself.

“As the first of its kind, this study is important in our understanding of the relationship between diet and the aroma of raw milk,” says Kieran. “It highlights the significance of both the direct and
non-direct transfer of VOC from diet to milk, that subsequently influence aroma perception and thus flavour.” 

Did you know?

Pasture-based milk production systems are more positively perceived by consumers as they are typically considered more environmentally friendly, organic and better for animal welfare.

Below are some of the aromas most associated with both perennial ryegrass and total mixed ration raw milk:

Roasted

Toasted

Potato

Popcorn

Cheesy

Sweet

Herbal

Fruity

Spicy 

80%
80% of flavour is influenced by aroma molecules.

Acknowledgements

We’d like to thank Deirdre Hennessy (Teagasc) and Joe Kerry and Maurice O’Sullivan (University College Cork) for their contribution to the study this article is based on.

Funding

This study was funded by Teagasc and through the Walsh Scholarship Programme (reference number: 2016071), under project 0044: Profiling Milk from Grass.

Kieran Kilcawley

Principal Research Officer

Food Quality & Sensory Science

Teagasc Food Research Centre, Moorepark, Fermoy, Co. Cork;

School of Food and Nutritional Sciences

University College Cork.

kieran.kilcawley@teagasc.ie

Holly Clarke

Food Quality & Sensory Science

Teagasc Food Research Centre, Moorepark, Fermoy, Co. Cork;

School of Food and Nutritional Sciences

University College Cork.