Impact of protein, lipid and carbohydrate on the headspace delivery of volatile compounds from hydrating powders
The release of volatile compounds, such as aroma, from a food material during hydration is of wide relevance to the food industry. To this end, dry powders of varying chemical composition were hydrated in a controlled system to investigate the impact of varying composition (protein, lipid and carbohydrate) on the delivery rate of volatile compounds to the headspace. Additional lipid and carbohydrate reduced the concentration of volatile compounds in the headspace and accelerated their rate of delivery to the headspace. Protein had no measurable impact. Of the volatile compounds measured, 2,3 butanedione and acetaldehyde were shown to be released slowly into the headspace, and pyrrol, methyl acetate and pyridine were released rapidly; this differential release rate was strongly correlated with hydrophobicity and would indicate that during hydration there is a temporal dimension to the relative abundance of volatile compounds in the headspace.
KeywordsAroma delivery Flavour Powder dispersion Aroma
Human perception of food flavour during the preparation–consumption cycle is driven by a number of interacting sensory modalities; these may include taste, vision, mouthfeel, olfaction, auditory and trigeminal stimuli [1, 2]. It is the combination of these stimuli, prior experiences [3, 4] and the temporal framework within which they innervate the brain that creates overall perception .
Within the olfactory modality, the principle stimulus, aroma, is perceived in one of two ways. The inhalation of volatile compounds through the nose (orthonasal delivery) and the delivery of volatile compounds to the nasal cavity during exhalation (retronasal delivery), both routes deliver volatile compounds to odour-binding proteins  and aroma receptors in the nasal cavity allowing perception to occur through combinatorial coding , but both achieve this in different ways . The resulting impact is a significant difference in perception when comparing the inhaled aroma and the aroma associated with consumption of a food material [8, 9].
The work detailed herein focusses on the headspace delivery of volatile compounds during the hydration of a food powder; this is designed to be representative of aroma inhaled during the preparation of products such as instant coffee, soups, gravies or beverages. The aroma perceived during this stage of the preparation process is a key consumer liking step and is often overlooked when evaluating or designing products .
When a powder is dissolved, volatile compounds are delivered to the headspace via a complex kinetic partitioning process. For a volatile compound to be available in the headspace it has to be available in the continuous phase, and for it to be available in the continuous phase it has to be first dissolved from the dispersed phase into the continuous phase. The rate of delivery of volatile compounds from the dispersed phase to the continuous phase is dependent on a number of factors, which may include interactions with hydrophobic regions such as lipids [11, 12], entropic restrictions to the dispersion of carbohydrates, the breaking of transient bonds with proteins and wetting kinetics of the powder. To simplify a complex process, it is proposed that there are three main components within food systems that play a role in dispersion, these being lipids, carbohydrates and proteins.
The presence of lipid in a dispersing system will play a number of roles. Lipid will change the polarity of the product and act as a sink for hydrophobic volatile compounds [13, 14], and additionally, lipid may be present as natural emulsions [15, 16, 17, 18] or form emulsion droplets [19, 20, 21], oil layers or micelles [22, 23] within the product during dispersion, which will impact the spatial distribution of lipophilic compounds and modify mixing behaviour [24, 25]. Simple carbohydrates drive the partitioning of volatile compounds through salting-in and salting-out phenomena [26, 27, 28]; in an aqueous system, this is associated with the relative availability of water molecules in the continuous phase . Complex carbohydrates can form structures that sterically entrap compounds  and, if present in the glassy state, can form an immobile matrix that restricts the mobility of volatile compounds and limits migration to the gaseous phase [30, 31]. Protein can also reduce the concentration of some volatile compounds in the headspace , although proteins generally have a lesser impact on the partitioning of volatile compounds to the headspace than lipid or carbohydrate. Proteins may modify the availability of volatile compounds through the formation of reversible hydrophobic and hydrogen bonds  or irreversible covalent linkages . Physical long distance networks of lipid, carbohydrate or protein that impart macroscopic structural changes such as complex emulsions , gel networks , carbohydrate structures  and fibrous protein networks (e.g., muscle)  are not discussed within this work, but will clearly impact the delivery of volatile compounds.
In summary, the impact of ingredient composition on the delivery of volatile compounds from powdered food systems is complex; therefore, to prepare for reformulation and product development challenges, aroma chemists and product developers require an insight into the relative partitioning of volatile compounds between foods and their headspace; this can only be achieved by combining fundamental knowledge, modelling data and applied case studies with the experience of flavourists. The aim of this research program was therefore to develop an understanding of the relative delivery rate of volatile compounds to the headspace from hydrating powders. The specific objective of this study was to identify the impact of ingredient modification (carbohydrates, proteins and lipids) on the intensity (IMAX) and time to reach maximum intensity (TMAX) of volatile compounds delivered to the headspace of a model system containing a standardized dry powder (soluble coffee) dissolving in an aqueous solvent (water).
Materials and methods
Spray-dried coffee powder (n = 3) and sunflower oil were obtained from a local supermarket, Loughborough, UK. Casein and fructose were purchased from Acros Organics (Geel, Belgium).
Model powder matrix
Sample preparation matrix: total sample indicates the sample (g) as added to the dissolution cell, the total sample weight varies such that coffee powder per sample is maintained at 12.525 g in all sample runs (protein = casein, carbohydrate = fructose, lipid = sunflower oil)
Physicochemical properties of the volatile compounds of interest were estimated using KOWWIN v1.67 within EPISuite ver. 3.20 (U.S. Environmental Protection Agency).
Samples were taken on day of opening of commercial products, the product was mixed to homogeneity and triplicate samples (1.5 g) were taken for analysis. Moisture content was measured in predried aluminium trays by evaporation to dryness at 105 °C for 48 h (Convection oven MOV-112F, Sanyo, Japan). Samples were weighed until constant weight. Water activity was determined at 24.6 °C using an AquaLab 3TE water activity meter (Decagon Devices Inc, Pullman, USA).
All samples were presented in randomized order and analysed in triplicate. Data were analysed by XLSTAT 2009 (Addinsoft, USA), using analysis of variance with Tukey’s post hoc test (P < 0.05) to identify significant differences between samples sets .
Results and discussion
The delivery of volatile compounds to the headspace in this closed system (Fig. 1) is driven by two independent factors, firstly the dispersion and dissolution of the powder into the dissolving solvent and secondly the equilibrium partitioning of the volatile compound across the air-product barrier. The rate (TMAX) and intensity (IMAX) of delivery of volatile compounds to the headspace above a dissolving powder was therefore measured to correlate with changes in the dissolution kinetics and the static equilibrium state, respectively (Fig. 2).
After the addition of the sample powder (instant coffee with a moisture content ± SD of 5.58 % ± 0.31 % and water activity ± SD of 0.25 ± 0.00 at 24.6 °C) to the solvent within the dissolution cell (40 °C), the concentration of volatile compounds in the headspace rapidly increased, and then effectively plateaued after 1.0–1.5 min. The curve formed was of a smooth shape, was reproducible and was dependent on the chemical composition of the dissolving powder. A typical headspace hydration curve is illustrated in Fig. 2, shows the development of the total headspace ion count over time and clearly identifies the two extracted parameters, TMAX and IMAX (as previously described).
Impact of airflow rate on time to maximum headspace intensity (TMAX ± SD, min) and concentration on maximum headspace intensity (IMAX ± SD, mV)
Airflow rate (mL/min)
1.03 ± 0.025a
0.939 ± 0.100a
0.678 ± 0.066b
1.26 ± 0.157c
27,600 ± 10,800a
40,000 ± 16,600a
43,500 ± 11,600a
72,500 ± 12,700b
Physicochemical properties of five volatile compounds (CAS number; estimated Log P; boiling point, °C; vapour pressure at 25 °C; solubility, mg/L; K a/w by bond estimation) calculated by KOWWIN v1.67 within EPISuite ver. 3.20
3.1E + 04
9.4E + 04
7.3E + 05
1.0E + 06
2.6E + 05
To investigate the impact of changes of bulk chemical composition (lipid, protein, carbohydrate) on the powder dissolution kinetics and the headspace delivery of volatile compounds, three compounds (sunflower oil, casein and fructose) were added to instant coffee powder at varying concentration levels. The maximum headspace intensity (IMAX) and time to reach maximum headspace intensity (TMAX) were directly compared.
Carbohydrate had a suppressive effect (P < 0.05) on the maximum headspace volatile concentration (Fig. 7) at 15.6 % carbohydrate inclusion, but at higher concentrations no further statistical impact of additional inclusion was demonstrated. The carbohydrate was chosen for its value within a model system, but it does imply that addition of simple sugars to a coffee mix will modify the headspace volatile concentration. A previous study with a reduced concentration range showed no impact of fructose on the partitioning of a range of volatile compounds , but the work used a different simple sugar and did not include the concentration range of interest of this study.
Protein had no statistically significant impact on the headspace volatile concentration in the system under study (Fig. 7). Although there is some indication of suppression of the maximum headspace intensity, which would support previous studies that have shown casein to interact selectively and transiently with some aroma compounds [50, 51].
The addition of carbohydrate reduced (P < 0.05) the time to achieve maximum headspace intensity. This was significant at a concentration of 15.6 % and was surprising as additional fructose would increase both the viscosity of the solution  and the surface tension , thereby reducing mobility and the kinetics associated with mixing phenomena. The results demonstrated contradict this and show that mixing kinetics are enhanced by the addition of fructose and that the delivery of volatile gasses to the headspace occurs more rapidly in a fructose-enriched system. This may be due to enhanced solubility and dissolution kinetics of the powder when enriched in the soluble sugar fructose.
Protein had no impact on the time to maximum headspace intensity (Fig. 8) and there were no underlying trends that could be observed.
In conclusion, the addition of lipid or carbohydrate has a significant impact on the delivery of aroma during hydration. It should be noted that although previous studies have shown similar impacts of lipid on the static partitioning of aroma during the preparation of coffee brews , there are no studies investigating the delivery of aroma during the hydration of instant or soluble coffee.
Kim et al.  showed a suppressive effect of both fat and protein on coffee aroma headspace partitioning, and Bucking and Steinhart  similarly showed that carbohydrates and protein depress the static headspace aroma concentration above coffee brew. Chiralertpong et al.  showed a selective impact of non-dairy creamer on the partitioning of non-polar compounds, although no impact was shown on the polar aroma compounds, and there was no impact of sucrose addition (10 % w/w) on aroma partitioning in a coffee brew preparation.
The impact of lipid and carbohydrate on the availability of aroma during the hydration of dried food powders highlights the importance of a true understanding of aroma chemistry during reformulation and product development exercises. Simple product changes such as fat reduction, nutritional enhancement (e.g., addition of protein) and replacement of carbohydrates for materials of enhanced functionality will not only impact the static partitioning of the aroma compounds into the headspace, but will also have a significant impact on other quality attributes, such as the delivery of aroma during hydration and product preparation.
In summary, both lipid and carbohydrate reduce the headspace volatile concentration significantly and reduce the time that is required to achieve equilibrium. Protein has no impact on the headspace volatile concentration or the time to achieve equilibrium. In addition, the delivery rate of the volatiles to the headspace is dependent on the physicochemical properties (log P) of individual volatiles.
Margarida Carvalho Da Silva and Jenny Drury are acknowledged for their assistance in the moisture content analysis and during proof reading.
Conflict of Interest
The authors declare that they have no conflict of interest.
This article is distributed under the terms of the Creative Commons Attribution License which permits any use, distribution, and reproduction in any medium, provided the original author(s) and the source are credited.
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