Article: Smooth Sensations -- February 2010
Smooth Sensations – February 2010
R.A. de Wijk, M.E.J. Terpstra, A.M. Janssen and J.F. Prinz, Contributing Editors
Editor’s note: The following article was extensively distilled from a thoroughly referenced and well-diagrammed paper titled, “Perceived Creaminess of Semi-solid Foods,” first published in Elsevier’s Trends in Food Science & Technology. See the end of the article for more information.
Creaminess is an important determinant of the perceived quality of many foods. Research published in 1977 reported that creaminess could be predicted quantitatively from three other sensory attributes: perceived thickness, smoothness and slipperiness. Later studies, primarily with dairy products, showed creaminess predictors could be reduced to just two attributes: thickness and smoothness. However, they have been found to be less accurate predictors of creaminess for other product groups.
A 1993 paper suggested that dairy fat’s contribution to creaminess perception is related to the fat globules’ role in the sensation of smoothness. A high density of evenly sized, small globules in homogenized milks or the high level of butterfat in cream produces a smooth sensation, which, together with the sensation of thickness produced by viscosity, gives a creamy sensation.
In other studies, the amount and type of starch, amount of milk fat and amount of sodium salts in puddings were varied. It was discovered that hedonic scores for creamy texture were higher for puddings that were perceived as thicker, more visually airy, had more mouth coating, and were denser and slower melting. Still other researchers varied fat content, thickener type and flavor concentrations in milk and found thickeners and flavors to have very small effects and fat content to have large effects on creaminess. One study found that an increase in fat from 0.1 to 1.3% enhanced creaminess more than a fat increase from 1.3 to 3.5%, suggesting a non-linear contribution of fat concentration to creaminess.
A 2003 paper investigated the structural and compositional basis of creaminess in food emulsion gels. Oil droplet size, fat content, air bubble size and air content were examined. Of these, oil droplet size and fat content were found to be most important for creaminess. A related paper found higher creaminess for larger fat droplets, but also increased creaminess, when particles smaller than 4–7µm were added. In addition, an inverse relationship between air bubble size and degree of creaminess of aerated foods was reported.
More recently, researchers investigating perceived taste, thickness and creaminess of butter fat-in-water emulsions (stabilized by sodium caseinate) found large effects of viscosity on creaminess, smaller effects of fat content and no effects of fat droplet size (0.5–2mm).
For over six years, the authors of this article have studied creaminess and other texture sensations in semi-solid foods using multidisciplinary research methods, including physico-chemistry, oral physiology, psychophysiology and psychology, with the intention of addressing the following questions:
1. What is creaminess, according to consumers?
2. How does creaminess relate to other food sensations and the food’s functional properties?
3. How is creaminess affected by oral processing?
4. How is creaminess affected by ingredients?
5. Which instrumental measurements relate to creaminess?
6. Can the results with custard be generalized to other semi-solid foods?
Consumers and Creaminess
To better understand creaminess, studies by the authors used model custard systems and, also, commercially available vanilla custard desserts. The model custards had various starch types and varied concentrations of starch and fat. Past studies showed these variables to have the greatest influence on smoothness perception. Modified starches included low and highly cross-linked potato starches, waxy maize-based starch with intermediate cross-linking, and intermediate and highly cross-linked tapioca starches. Carboxymethylcellulose (CMC) also was used as a thickener. All custards contained 6.5% sugar and 0.1% of vanilla flavor. Milk fat contents varied between 0-15%.
The custards were preheated at about 75°C, then subjected to indirect UHT-heat for five seconds at 144°C in a tubular heating system. Rheological measurements performed include flow curve, steady shear rate, dynamic stress sweep and dynamic frequency sweep in shear, as well as squeezing flow measurements. To study the effect of saliva on food rheology, a “Structure Break-down Cell” was developed. Further instrumental measurements included the turbidity of rinse water, infrared reflectance of samples with saliva and friction. Confocal laser scanning microscopy was used to verify microscopic changes in the foods, after saliva was added.
From a consumer’s point of view, creaminess is a multimodal percept that combines texture (e.g., smooth, not rough) and taste (or possibly flavor) sensations. Consumers also relate creaminess to properties related to the bulk (e.g., how compact), as well as to the surface (e.g., “velvety, not oily coating”) of the oral food bolus. Finally, they relate creaminess to a specific rate of degradation of the food in the mouth.
Creaminess seems to be a universal sensation, at least for semi-solid foods, and the sensation of creaminess does not seem to change dramatically with training. This contrasts with the sensation of fattiness, which is judged differently by consumers and trained panelists.
Food Sensations and Functional Properties
The perception of creaminess starts before the food is put in the mouth. The effects of visual texture and color on creaminess were found to be small, but ortho-nasal smell significantly affected creaminess perception. The amount of food also has an impact. Creaminess increased, when the size of a single bite increased from 2 to 11ml, and continued to increase over subsequent bites.
Sensations elicited by custard desserts with varying fat and starch concentration, as well as types of starch, can be summarized by three “sensory dimensions” based on their interrelations. The largest variation was in the “roughness to creaminess” dimension, which relates to the food’s lubricative properties. A second “melting to thickness” dimension relates to the food’s viscosity, and a third “airiness to heterogeneity” dimension relates to the food’s homogeneities.
It was found, for example, that sensations of the rough/creamy dimension varied systematically with friction in low-fat, semi-solid foods. That is, high friction is associated with the roughness and low friction with the creaminess/fattiness. Sensations along this dimension are typically affected by other variables that affect lubrication, such as particles, specific tastants/flavorants and specific thickeners.
Sensations at each of the three dimensions’ extremes either reflect properties related to the surface of the food bolus (e.g., roughness, melting and airiness) or the surface plus bulk (e.g., creaminess, thickness and heterogeneity). Statistical analysis showed that creaminess was predicted well from a combination of sensations reflecting surface (roughness) or surface plus bulk properties (thickness). In addition, flavors such as fatty and dairy flavors and vanilla (in vanilla custard desserts) enhanced creaminess, while off-flavors, such as from starch, suppressed creaminess. These texture and flavor attributes, plus fatty mouthfeel and “afterfeel,” predict well the creamy mouthfeel of a set of nine custards, mayonnaises and white sauces.
Optimal custard creaminess was reached at viscosities similar to those found in commercial custards. In commercial mayonnaises, creaminess was negatively related to thickness (and positively with melting mouthfeel), suggesting that some commercial mayonnaises are too thick and perhaps also too sticky, since thickness and stickiness are associated in semi-solid foods.
Independent variation of flavors, such as vanilla and diacetyl added to custards, also affected creaminess, especially when the flavors resulted in increases in sensations associated with roughness, such as prickliness and off-flavors. Most flavor effects on creaminess disappeared, when nose clips were applied or when tastants were added, suggesting that flavor effects on creaminess were mediated by olfaction and/or nasal irritation and not by gustation.
Food Manipulation in the Mouth and Creaminess Perception
Panelists assessed sensory attributes in a chronological order established during training that was based on the chronology of perception. Flavors were rated first, followed by texture sensations, such as perceived temperature and thickness. Creaminess was rated late in the assessment process. “Late sensations” grew considerably more in perceived intensities than “early” ones, when oral tongue movements became increasingly more complex. For example, an early sensation (such as thickness) required simple up and down movements of the tongue for the sensation to reach maximum intensity. A late sensation (such as creaminess) required up and down movements, in addition to smearing movements of the tongue. Creaminess increased by as much as 75%, with the complexity of oral movements.
Starches, such as those in sauces or custards, are enzymatically degraded by salivary a-amylase. When consumers’ amylase activity was increased by adding amylase or decreased by adding an amylase inhibitor (acarbose) to starch-based custards, significant amylase effects on most sensations, including creaminess, occurred. Studies have shown a reduction of the amylase activity increases creaminess ratings by as much as 100%.
During enzymatic breakdown, parts of the food bolus will be already broken down, resulting in less viscosity, while others will be unaffected. Low-viscosity parts may act as slip planes for the high-viscosity parts. When this slip-plane effect was simulated by adding a layer of fat between the two halves of the bolus, creaminess was suppressed, and heterogeneity was increased. A negative relationship between heterogeneity and creaminess was also found, when unblended cream instead of blended cream was added to custard. Hence, homogeneities in semi-solids, either caused by different viscosities, added fat layers or unblended creams, increase perceived heterogeneity and reduce creaminess.
How is Creaminess Affected by Ingredients?
The three sensory dimensions relating to specific food properties are lubrication (rough/creamy dimension), viscosity (melting/thick dimension) and homogeneities, or lack thereof (airy/heterogeneity dimension). Each of these properties is associated with specific ingredients (e.g., lubrication with fat and viscosity with thickener concentration). However, the ingredient–property relationships are not straightforward. For example, fat primarily affects lubrication, but also viscosity, to a lesser extent. Thickener concentration primarily affects viscosity, but, to a lesser extent, also affects lubrication. Similarly, thickener type affects homogeneities and also lubrication.
Fat’s role on creaminess and fattiness was demonstrated in mayonnaises, custards and white sauces with fat contents of 0-80%. Fat content affects both creamy and fatty mouthfeels, although creamy mouthfeel is typically considered more preferable than fatty mouthfeel. As fat content increases, creamy and fatty mouthfeel both increase. Creamy mouthfeel dominates until fat levels reach about 15%, where creamy and fatty mouthfeel level off, and fatty mouthfeel begins to dominate. In this study, the high-fat foods were always mayonnaises; thus, the effects of fat content could not necessarily be separated from effects of food type.
Different thickener types and concentrations produce various degrees of lubrication, as demonstrated by friction measurements on custards with various starches, CMC and fat levels. In the study, friction was most affected by thickener type and concentration and less by fat concentration (0.5 vs. 3%).
Thickeners that produce high perceived thickness and limited perceived melting--and that also resist enzymatic breakdown--enhance creaminess, probably more those that are broken down. Indeed, custards thickened by CMC were judged as somewhat more creamy than those thickened by starches. However, CMC also enhances sensations such as fattiness and stickiness. Additionally, enzymatic breakdown of starches may have beneficial effects, such as the release of fat droplets from the starch matrix. These fat droplets migrate to the food bolus surface, where they lubricate and release flavors. These effects are beneficial for low-fat custard creaminess.
Evidence for fat surfacing from a broken-down starch matrix comes from various sources. For example, the lubricative properties of a starch-based custard increase after enzymatic breakdown, and confocal laser scanning microscope images show increased fat at the surface after enzymatic breakdown. Additional research indicates the degree of fat surfacing is not related to the speed of breakdown, and large fat droplets surface more than smaller ones.
Consumer panelists in these studies indicated coatings (i.e., the food that remains in the mouth after swallowing) can have an oily or a “velvety” character. Many of the authors’ studies showed ratings for creamy or velvety and fatty coatings were typically strongly correlated. Differences seemed related to the type of thickener used. For example, in one series of model custards, creamy afterfeel was most intense for custards with a certain potato starch, followed by waxy maize starch and tapioca starch; all three starches elicited similar fatty afterfeel sensations. Creamy afterfeel seems related to speed of the thickener’s enzymatic breakdown, with the starch having the slowest breakdown providing the creamiest afterfeel. Similarly, panelists with relatively low activities of salivary amylase perceived creamy afterfeel as more intense than panelists with relatively high activities, whereas no effect on perceived fatty afterfeel was found. These results suggest that thick coatings, resulting from the use of starches with relatively low enzymatic breakdown, are perceived as creamier than thin ones. In contrast, fatty afterfeel is probably unaffected by the thickness of the coating.
The results of studies by the authors and others on creaminess suggest several ways to boost creaminess in semi-solid, starch-based foods:
* Increase the bulk-viscosity of the food.
* Minimize, but not eliminate, loss of bulk viscosity during oral processing by using starches that show limited mechanical and enzymatic breakdown during oral processing, and/or decrease the activity of salivary amylase via amylase inhibitors, pH and others.
* Use small and stable fat droplets.
* Add flavors associated with creaminess.
* For low-fat foods (0.1-0.5%), use starches specifically to increase the fat concentration at the surface of the food, to enhance lubrication and flavor release.
* For medium-fat foods (up to 10–20%), use specific types of starches that reduce fat surfacing. Fat surfacing may lead to an over-representation of fat at the surface, resulting in coalescence of fat droplets and sensations dominated by fattiness, instead of creaminess.
* For high-fat foods (over 40%), use non-fat ingredients that increase friction, resulting in enhanced creamy afterfeel. pf
The original article, with over 35 references, has been, with permission, condensed and adapted by Claudia D. O’Donnell, chief editor, Prepared Foods. The original article was published in Trends in Food Science & Technology, Vol. 17, R.A de Wijk, M.E.J. Terpstra, A.M. Janssen and J.F. Prinz,“Perceived Creaminess of Semi-solid Foods,” pp. 412–422, copyright Elsevier (2006).
R.A de Wijk is the corresponding author, e-mail: firstname.lastname@example.org. To obtain the original article, go to http://bit.ly/57Mb1n or search “Perceived Creaminess of Semi-solid Foods,” using an Internet search engine.
The relationships between creamy mouthfeel and other sensory attributes, as found in custard desserts, have been verified for commercial yogurts; commercial and model mayonnaises and white sauces; and a wide range of semi-solid foods with different viscosities (from unwhipped cream to mozzarella and mascarpone), different melting properties (from ice cream to Turkish yogurt) and different structures (from smooth yogurt to heterogeneous mozzarella).
Statistical analysis shows similarities and differences in the relationship between creamy mouthfeel and other mouthfeel attributes. In general, creamy mouthfeel associates positively to thickness, fatty mouthfeel, and afterfeel and airiness, and negatively to melting, powdery/grainy/heterogeneity and roughness.
The degree of association varies with food type. For example, the negative association between heterogeneity and creamy mouthfeel was very large for a range of foods, which is probably the result of some very heterogeneous foods (e.g., mozzarella) that were included only in this series. Other foods, such as custards, are typically not heterogeneous, and heterogeneity is, therefore, not associated with creaminess for these foods.
Occasionally, an attribute was positively associated for some foods and negatively for others. For example, rough mouthfeel associates positively to creamy mouthfeel for a wide range of foods (e.g., both low- and high-fat cream cheese and high-fat cream), but negatively for others. Foods with a high association between roughness and creamy mouthfeel also received relatively high ratings for other attributes associated positively to creamy mouthfeel, such as thickness (cream cheese) and fattiness (cream); they had relatively low ratings for attributes associated negatively, such as melting. Possibly, the negative effects of roughness on creamy mouthfeel are off-set by the positive effects of these other attributes.