From sauces to soups, thickness is often regarded as key determinant in quality. For stocks, a thin texture is preferred so that it can be used more universally, while sauces must have the necessary nappe in order to be acceptable to serve without being considered too thin. This factor which determines the thickness of a sauce is classified as a product’s resistance to flow, also known as viscosity (Arana, 2012).
A resistance comes from the chemical properties of the liquid being used. Common with almost every food item, water makes up the majority of these liquids. By itself, water has a low viscosity, which can be seen as it moves freely along its path. However, if the water is the solvent for a solution, a solute of some nature makes it harder for water to travel quickly on its path, as now it must move around the new molecules occupying the space, thus moving slower and appearing as a thicker substance (McGee, 2004). Ingredients containing protein such as gelatin have a higher affinity for water than normal solutes (Stevens, 2010). This allows for intermolecular bonds to form easier, as well as effectively trapping water within the molecules. Proteins are then able to swell, creating a more viscous solution which also has a higher melting point than water and allowing it to set above freezing temperature (McGee, 2004).
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Most liquids handled in the kitchen are not affected by external shear forces other than momentum or gravity, giving them the name Newtonian fluids. The viscosity of Newtonian fluids also cannot change due to these forces. If the viscosity changes, those liquids are considered Non-Newtonian (McWilliams, 2012). More specifically, if the force causes a thinning of a product like ketchup, whereby flow is increased, the item is considered to be thixotropic, while those which become thicker, such as pure starch slurries, they are called dilatant fluids.
Previous research into viscosity utilizes a measurement similar to speed, calculating the distance at which a fluid travels over a set period of time, or vice-versa (Meloan, 1980). This can be done using small utensils such as Zhan cups—which allow for liquids to flow from the bottom to measure the rate at which the set amount of liquid moves out of a hole in the bottom—or using larger equipment such as viscometers, or tools which impart a rotational shear force to measure the viscosity of the solution (Arana, 2012). From here, empirical data can be collected to judge the resistance to flow and come to conclusions as to what can cause changes in viscosity.
In two of the three tested experiments in this paper, the measure of concentration and temperature are measured to determine if they have an influence on the viscosity of liquids, conducted through testing different concentrations of sugar solutions as well as the effects of gelatin on stocks. The third experiment speaks of an observational analysis comparing Newtonian and Non-Newtonian fluids and how they behave under shear force.
Materials and Methods
Sugar Solutions: Three differently concentrated sugar solutions were created for the purpose of this experiment, measured out to be 500g each of a 20%, 40%, and 60% sugar in water (w/w) (Escali Primo Scale Model # P115, Burnsville, MN) The accuracy of the measurement was recorded with refractometers. (ReichertRefractometer, Ranges 0 to 30, 0 to 50 and 50 to 90 Brix Model# 137531LO, Buffalo, NY) These solutions were placed into small bain maries. (Vollrath Bain Maries Stainless Steel BAIN MARIE POT 1.25 QT Model# 78710, Sheboygan, WI) When the solutions reached room temperature, 25°C, (Traceable® Dual-Channel Thermometer Type-K SN# 130073099) the amount of time for the solutions to flow through a set volume was measured using a 3mm Zhan ladle (Custom made Zahn ladle using a Vollrath Stainless-Steel 3-oz Ladle Model# 58430 with a 3mm hole, Sheboygan, WI) this process was repeated for the same sugar concentrations at 40°C, which was achieved using an immersion circulator (Sous Vide Professional™ CHEF Series SN# 2D1330856, Niles, IL) controlled water bath.
Veal Stock Samples: Two separate containers of veal stock were used in this experiment, one being a control, while the second contained 4.5% gelatin (w/w) (Escali Primo Scale Model # P115, Burnsville, MN). Both stocks were stored in small bain maries in an immersion circulator controlled water bath set to 80°C until the stocks had achieved that temperature. Using a 3mm Zhan ladle, the amount of time required for the stocks to flow was measured. This was repeated using a 40°C immersion circulator controlled water bath.
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Sensory Analysis: Sensory recordings were also taken from the two samples of veal stock, which were conducted while the liquids were at 40°C. Two plates each were held at room temperature, in refrigeration, (Traulsen 44" Single Door 9.8 Cu. Ft. Undercounter Refrigerator Model# TU044HT) and in a C-Vap machine (Cvap Pod Model# CPOD 200 NN.02, Louisville, KY) set to 80°C. At each temperature, a small amount of the stock was placed in the center of the plate to allow for evaluation based on viscosity, appearance, mouthfeel and the overall flavor of each stock.
Non-Newtonian Fluids: Using a Bostwick Viscometer, (CSC Bostwick Consistometer; 30cm, Fairfax, VA) an observational analysis was conducted comparing molasses, which is a viscous Newtonian fluid, to ketchup, which is thixotropic. An oobleck solution was also analyzed, adding notes on the appearance and viscosity achieved as it was moved in a small Cambro. (Cambro 2 Qt. Square Food Storage Container Model# 2SFSCW, Hungtington, CA) Distances achieved by the molasses or the ketchup within a 30 second time frame were used to aid in the analysis.
Statistical Analysis: For testing the sugar samples as well as the veal stocks, ANOVA and Tukey’s HSD post hoc tests were conducted at p=0.05 in order to determine if the data in either experiment displayed a significant difference between the other tested values.
Figure 1: A graph measuring the concentration of sugar against the average flow time for the Zhan ladle tests. Each line designates a different temperature at which the test was conducted. Each mean value is an average of eighteen individual trials of each sugar sample at each temperature.
Sugar Solutions: The results from the ANOVA testing displayed that there is not enough confidence to determine a difference between the 20°Brix and 40°Brix concentrations at either temperature. However, the tests showed that there was a significantly large difference between the values given for the lower two concentrations and the values given for the 60°Brix concentration at either of the tested temperatures.
Veal Stock Samples: The Tukey tests determined there is a largely significant difference between the rate of flow for the gelatin stock compared to the regular stock in the 40°C water bath, as well as between the two stocks in the 80°C bath. However, there is not enough statistical evidence to prove that the two gelatin added stock samples are significantly different in flow times between the two water bath temperatures.
Figure 2: Average flow times for two different stocks held at two separate temperatures. Mean values were taken from eighteen trials at each temperature range per stock type.
Sensory Analysis: Placing either stock on the room temperature plates causes the flow to slow down slightly from being at 40°C. Both stocks also have a similar appearance as well. When placed on the plates that were held in the C-Vap machine set to 80°C, the regular, non-gelatin stock had a very thin consistency and had the appearance of water. The gelatin-added stock had a thicker appearance as it was moved along the plate, had a rich mouthfeel unlike the other tests and a stronger flavor than a basic veal stock. At the refrigerated temperatures both stocks appeared to be slightly slower in movement, but only the stock with gelatin had set up into a gel after a few seconds.
For the Non-Newtonain fluids experiment, the molasses travelled the full 24cm distance of the bostwick viscometer in less than the fully tested 30 second timeframe, while the ketchup barely moved 3cm in the tested time. The molasses also had a somewhat consistent speed as it travelled down the slope, while the ketchup had a bit of acceleration as soon as the test started. The oobleck solution appeared to be liquid from a visual standpoint, but as soon as a shear force was encountered, be it by stirring or placing a hand on the surface, the oobleck had characteristics of a solid, refusing to move and actually gaining enough resistance to allow one to lift the container applying upward force from a spoon. Dancing one’s fingers on the surface of the substance also created a jello-like appearance, which prevented the fingers from becoming wet, but when the pace of the dancing slowed too much the fingers were able to fall in, the solution appearing like a fluid again.
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While it cannot be concluded that the measured concentrations of sugar varied between 20% and 40% concentration, at both tested temperatures the 60% concentration had much higher values associated with the time taken for the Zhan ladle tests (Figure 1). The temperature also played a role in the viscosity of the solutions. With the exception of the 20% concentration, the values for the solutions that were tested at 40°C had achieved significantly faster times than those tested at room temperature (Figure 1). The times achieved by the 40°C tests were more consistent, showing a range significantly lower than the tests conducted at room temperature (Figure 1). This highlights that while heat plays a role on the rate of flow for a liquid, its influence is likely determined by the concentration of that liquid, whereby a higher concentration is effected more, while a lower concentration, which would appear to act more like water to begin with, would not have the same room to weaken in thickening power.
In regards to the veal stock, gelatin was shown to have a significantly greater viscosity than the regular stock. As the temperature was increased in both samples, they both displayed signs of thinning, as well as decreasing the amount of time in each trial (Figure 2). Gelatin displayed the greater difference when the temperature changed, which could be relative to how the higher concentration sugars had a greater overall loss due to temperature increase. This is again likely due to the fact that the stock which did not have the added gelatin was closer to a thin watery state and therefore did not have the same potential to thicken. It brings up an important note of having a sauce be tested for its viscosity at the temperature it will be served, as if the temperature is too low when tested, the sauce will still appear like water when plated, even if the consistency is right at the temperature you tested it. If the sauce is too hot, the product will be reduced more than necessary to achieve a viscosity which will be too thick as it cools.
The sensory evaluations for the stocks also displayed similar connections to some of the known details about gelatin, specifically in its melting point. When set on the refrigerated plate, the gelatin was able to set into a semisolid, even though it was not possible for the regular stock nor for the gelatin at room temperature. This would therefore put the setting point for the gelatin somewhere between 4°C (the average refrigerator temperature) and 25°C (the room temperature), which is higher than the freezing point for water. When placed on the plate that had been sitting in the C-Vap machine, the gelatin-added stock displayed the better coating, looking similar to how a sauce would appear. This carried over into the coating mouthfeel and a stronger flavor release than the regular stock provided.
While most of the data supported the previously known information about viscosity, some of the points remained inconclusive due to being variably close through mean values or standard deviations. Sources of error for the experiment were possible in the concentration of each solution, and potentially in the timing of the Zhan tests. Six teams contributed data to the mean values, but were required to separately create their own solutions. Therefore while we assume that the solutions match the required concentrations for the experiments, the variation could be great enough to create a larger range and not show a significant difference between values for that reason. By having these six teams with six potentially different solutions, the timing of the Zhan tests would not reflect the same values, even if we are to assume that every person who used a stopwatch for the experiment has the same reaction timing and measured using the same point of reference.
With the water being the focus of most liquid-based recipes, concentration is a large factor of many of the foods consumed. Flavoring or enhancing food requires either something to be added through seasoning or water being removed through evaporation, which in both cases increases the concentration of the liquid. Oftentimes this is why an emulsion, regardless of the stability, will appear thicker than a simple vinegar mixture. Emulsions contain polar and non-polar molecules which are forced together through mechanical action to create a thicker product. From what these experiments have shown, this is likely due to having an increase in its concentration, containing more oil than water, which forces the water molecules to move around the oil, which appears as a thicker product at a macroscopic level. If the thickener in question is a protein, such as the tested gelatin, the thickening power is much greater, requiring a concentration of only 4.5% to achieve a product as viscous as with a 40% sugar solution. This shows that gelatin is a highly effective thickener, which also provides great characteristics essential to a sauce, such as mouthfeel and flavor release, which can allow for a great tasting runny sauce to have the desired consistency and texture without imparting too much extra flavor or over reducing the sauce to lose yield, which can save money while maintaining flavor.
In the test of Non-Newtonian fluids, the ketchup displayed a thixotropic behavior, becoming thinner as the shear force of gravity was applied, allowing it to have an acceleration as it traveled. This characteristic is similar to the desired effect of fluid gels thickened with hydrocolloids, whereby they are solid until moved with a spoon, while also giving a shine similar to viscous liquids. For the oobleck, the ability to set into a solid from a liquid state reflects the ability of starches to influence the thickness of the liquid they are combined with, such as in the process of making roux. While Non-Newtonian fluids are not as common in the kitchen as Newtonian fluids, they still serve importance, as it is a property that defines many of the store-bought squeezable ketchup, mustard or mayonnaise brands.
While it cannot be completely concluded that the experiments matched the data from prior known knowledge on viscosity, it was shown through the tests that viscosity of Newtonian fluids can be affected by changing the concentration of the solution or changing the temperature. Temperature was also shown to have a greater effect on the solutions which contained higher concentrations, which plays an important role when determining how much of a thickening agent must be added in order to achieve the desired consistency. This is an important concept in furthering the understanding of how to effectively alter the outcome of food, as the viscosity of a liquid can completely alter the perception as it is consumed and can be altered in more than one way. This allows for a versatility in the kitchen so that a sauce on a plate can be the desired consistency and flavor for overall customer satisfaction.
Arana,I. (2012).Physical properties of foods: Novel measurement techniques and applications. Boca Raton, FL: Taylor & Francis.
McGee,H. (2004).On food and cooking: The science and lore of the kitchen. New York: Scribner.
McWilliams,M. (2012).Foods: Experimental perspectives. Upper Saddle River, NJ: Pearson Prentice Hall.
Meloan,C.E., Pomeranz,Y., & Pomeranz,Y. (1980).Food analysis laboratory experiments. Westport, CT: Avi.
Stevens,P. (2010). Gelatin. In A. In Imeson(Ed.),Food Stabilisers, Thickeners and Gelling Agents. Chichester: Blackwell Publishing.
Effects on Viscosity for Newtonian Fluids
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