Viscosity Comparison of Thickened Juices at Two and Ten Minute Intervals

 Lindsey R. Biggs
Lesley C. Cooper
Jane Mertz Garcia*
Edgar Chambers, IV*

Kansas State University

* Faculty Co-Author

 Abstract

The purpose of this study was to compare the physical properties of nectar and honey-thickened juices, which are often clinically recommended in dysphagia diets. The viscosity (thickness) of four brands of powdered thickeners mixed with apple and orange juice was measured at two and 10-minute intervals. Results indicated that: (a) viscosity varied across powdered thickeners, (b) orange juice was typically more viscous than apple juice, and (c) honey thick consistencies became more viscous when allowed to thicken for a longer time period for three of four thickeners. The results are discussed in terms of their clinical implications for people with disordered swallowing (dysphagia). 

Introduction & Background

            Dysphagia, a swallowing disorder, occurs in stroke victims, patients with neuromuscular diseases, head and neck cancer patients, and because of surgical procedures involving the head and neck (Logemann, 1998). It is estimated that more than 15 million Americans have dysphagia due to a variety of conditions that impact individuals across the life span such as traumatic brain injury, cerebral palsy, and Parkinson's disease (Williams & Walker, 1992). Dysphagia caused by neurogenic or structural damage to the upper digestive tract can result in a variety of problems that impact a person�s ability to swallow liquids or foods safely (Logemann, 1983). This is a concern to speech-language pathologists who typically provide diagnostic and therapeutic services to individuals with swallowing disorders (Lubinski, & Frattali, 2001).

Common problems in swallowing liquids for patients with dysphagia include a delayed triggering of the pharyngeal swallow, reduced lingual control, reduced airway protection, and reduced oral awareness (Logemann, 1998). Nutritionists and speech-language pathologists have researched appropriate diets for patients with dysphagia, but finding a diet that will ensure the patient�s safety while receiving a nutritionally balanced diet and adequate hydration is complicated. In fact, aspiration, the entrance of food and liquid into the lungs (Robertson & Pattillo, 1993), presents an additional challenge to clinicians because aspiration can result in a variety of health concerns including pneumonia, chest pain, lung abscess, convulsions due to apnoea, death due to asphyxiation, malnutrition, and dehydration depending on patient's swallowing ability (Penman & Thomson, 1998). 

Thin liquids such as orange juice and apple juice are often reason for concern because they travel quickly through the oral cavity and may not maintain a cohesive bolus, resulting in the possibility of aspiration (O�Gara, 1990). One clinical consideration is to thicken thin liquids with a powdered thickener. Thickened liquids provide greater viscosity, create a more cohesive bolus, and aid in reducing the occurrence of aspiration for patients with dysphagia (Glassburn & Deem, 1998). The more appropriate the level of viscosity, which is the material�s resistance to flow (Miller, 1972), the greater the likelihood of a safe swallow. 

        The National Dysphagia diet describes four levels of liquid consistency including thin, nectar, honey, and spoon-thick. The standards for achieving consistent levels of thickened liquids can be impacted by a variety of factors. For example, there may be differences in the product line from manufacturers (Mills, 1999). In addition, thickening procedures require compliance in preparation procedures. The amount of time elapsed between mixing a liquid and consuming it may vary, especially for patients in a medical facility who are reliant on staff to prepare thickened liquids for them (Pelletier, 1997). Finally, raters have identified differences in thickness and taste for common beverages thickened to nectar, honey, and spoon-thick consistencies (Pelletier, 1997).

The purpose of this study was to assess the physical properties of nectar and honey-thick juices. The viscosity of different brands of powdered thickeners used in modifying liquids in dysphagia diets was compared for orange and apple juice that were allowed to thicken for two or ten minute intervals. The three research questions were as follows: 

1. Are there differences in the viscosity of nectar and honey-thickened juices related to the brand of commercial powdered thickener? 
2. Are there differences in the viscosity of orange juice and apple juice thickened to nectar and honey consistencies?
3. Are there differences in nectar and honey thick juice when allowed to thicken for different time periods (2 minutes versus 10 minutes)?

Methods

Materials and Equipment

The viscometer used was model RV-8-0, Viscometers, UK, Ltd. For nectar consistency, spindle size three was selected with the speed of the viscometer at 50 rpm. For honey consistency, the spindle size was six at 10 rpm. Four common commercial thickeners were selected. The brands selected for the study were Thick and Easy, Thick It, Thick Up, and Thicken Right. Thin liquids included Mussleman�s apple juice and Tropicana Pure Premium Original Low-pulp orange juice. Juices were refrigerated and only removed prior to their use in the study. Measured temperatures ranged between 5.7 and 8.4 degrees Celsius at the time each juice was removed from refrigeration.  

Sample Preparation & Measurement Procedures

Similar procedures in preparation were used for each juice and powdered thickener to achieve nectar and honey consistencies. Manufacturer�s guidelines were used to determine the appropriate amount of powdered thickener for four fluid ounces. That amount was tripled for a 12-ounce beverage in order to produce a blended consistency with an adequate volume for its measure of viscosity. That amount of powdered thickener was measured three times for each brand to obtain its average weight in grams. The average in grams was then used for all subsequent samples. Next, 12 ounces of each juice was poured into a 600-ml beaker and measured in grams. This process was repeated three times to obtain an average weight for each juice that was then used for each subsequent sample. The thickening agent was slowly poured into the juice and continuously stirred with a spoon for 30 seconds at 145 beats per minute. A metronome was used to help maintain a constant mixing rate. Approximately 325 ml of the blended juice was then poured into a second 600 ml beaker. Its temperature was measured and the sample was allowed to thicken for two minutes (time 1) or 10 minutes (time 2). Temperatures ranged from 7.8 to 10.3 Celsius for thickened samples. Viscosity (resistance to flow) was then measured by recording the stable value at approximately five seconds after the initiation of spindle rotation. Three separate samples were prepared and measured for each brand of thickener, consistency, and juice type at two and ten-minute intervals. 

Results

        The mean scores and standard deviations in centipoise for nectar and honey-like juices at two and ten minutes are presented in Table 1. Higher numbers reflected more viscous juices; lower numbers reflected less viscous juices. Preliminary review of the data showed that centipoise measurements varied across juices, brands, and consistencies. It was apparent that honey was more viscous than nectar-thickened juices at any time interval. Viscosity measures for nectar were generally similar at two and ten minutes, unlike results for honey.

Table 1. 
Viscosity Measurements in Centipoise and Standard Deviations for Nectar and Honey-Thickened Orange Juice and Apple Juice Measured at 2 and 10-Minute Intervals

To determine statistical significance of these differences, the data were submitted to a three-way analysis of variance test (Brand x Juice x Time) with centipoise as the dependent variable. Nectar and honey thickened juices were examined separately. Nectar results showed a significant main effect for brand (F=76.72; p<.001) and a significant main effect for juice (F=32.47; p<.001). There was no main effect for time (F=0.19; p=.666), and there was one significant two-way interaction for Brand x Juice (F=32.47; p<.001). Results showed that nectar juices did not significantly change in their viscosity when allowed to thicken for ten-minute intervals compared to two minutes. The 3-way analysis of variance test for honey showed significant main effects for brand (F=42.62; p<.001), juice (F=89.72; p<.001), and time (F=29.27; p<.001). There was also a significant interaction of Brand x Juice x Time (F=5.87; p=.0026). 

Further analysis was done to understand the nature of each interaction. Post-hoc analysis of the Juice x Brand interaction showed that nectar orange juice was more viscous than apple juice for all powdered thickeners except Thicken Up, in which there was no statistical differences in viscosity across juices. With regard to orange juice, Thick & Easy and Thicken Right were more viscous than Thick It and Thicken Up. Thick & Easy and Thicken Right did not differ from one another and Thick It and Thicken Up were statistically similar. Apple juice results showed that all brands but Thicken Up and Thick & Easy were statistically different in centipoise from one another. The significant contrasts are presented in Table 2 and illustrated in Figure 1.

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Figure 1. 
Viscosity Measures in Centipoise of Nectar-Thickened Orange Juice and Apple Juice for each Powdered Thickener 

Significant contrast for the Juice x Brand x Time interaction for honey-like consistencies are displayed in Table 3. Results indicated that orange juice was more viscous than apple juice, regardless of the time interval between measurements. The exception was Thicken Up, as the orange juice and apple juice centipoise measures were similar to one another. Time did not interact with juice for three of the powdered thickeners. The samples measured after 10 minutes for Thick It, Thicken Up, and Thicken Right were always more viscous than those measured after 2 minutes. A different pattern occurred with Thick & Easy in that the two-minute honey sample was more viscous than all other Thick & Easy honey samples. In comparing the different thickeners for orange juice after two minutes, Thick & Easy and Thicken Right were significantly thicker, while Thick It was more viscous than Thicken Up. A different pattern resulted at 10 minutes in that Thicken Right exceeded all other powdered thickeners in centipoise and Thick It was greater in centipoise than Thicken Up. Apple juice reflected a similar trend for two and ten minute measurements across the four brands. For both time intervals, Thicken Right was more viscous than Thick & Easy. Other comparisons of centipoise did not reach a level of statistical significance.

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Discussion

This study examined the physical properties of nectar and honey-thickened orange juice and apple juice measured at two and ten minutes for four brands of powdered thickeners. The specific brand of powdered thickener, type of juice, and time span between preparation and viscosity measurement were all components of primary interest.

The results showed that viscosity levels varied across all brands of commercial thickeners, for different juices and levels of consistency. Furthermore, three of the products thickened two types of juices in different ways. Differences were noted even though all powdered thickeners are made from a similar product (i.e., food starch). The results from this research are consistent with Mills (1999), who also commented on the variability of thickened liquids. The current study and Mills found variability in viscosity, which might help explain why raters perceive differences in taste and texture (Pelletier, 1997). 

Although there were statistical differences between powdered thickeners and juices, there is not clear evidence that these differences are meaningful to patients� swallowing safety. Currently, there are no established guidelines that identify acceptable viscosity ranges for nectar or honey-like liquids (Pat Felt-Gunderson, personal communication, April 19, 2002). 

            The viscosity measurements for orange juice were consistently higher than apple juice across all brands of commercial thickeners, with the exception of Thicken Up. Although apple juice and orange juice are both considered thin liquids, these juices responded differently due to their individual composition (e.g., acidity of orange juice).

            Centipoise measurements at two and ten-minute intervals were not found to be a significant factor when measuring the viscosity of nectar-thickened juices; however, time proved to be a significant factor in the viscosity measurements of honey-like juices. This is clinically significant to people with disordered swallowing. Speech-language pathologists make recommendations about levels of diet modifications depending on the patient�s specific swallowing problems. Therefore, variable consistencies could impact swallowing in ways that are not anticipated. Thus, it is important not to under or over thicken liquids because this could result in severe harm to the patient.

            There are possible factors that could limit this study�s findings. For instance, this study compared viscosity at two time periods (2 & 10 minutes). Other time variables could be a factor. For example, thickened juices may continue to thicken after the ten-minute interval that was measured in this study. Also, it is not known if samples thickened at a consistent rate or if significant changes occurred at distinct time intervals (e.g., after 5 minutes). This study examined two common fruit juices. Viscosity results might be different for other liquids (e.g., milk or soda) due to variations in their properties that might cause them to thicken differently. In addition, heated liquids, such as coffee, might react differently to powdered thickeners than those that are refrigerated. Lastly, viscosity measures of pre-thickened liquids should be considered in future research. In summary, the differences between juices and powdered thickeners in this study substantiate the importance of continued research to better understand the physical properties of thickened liquids for the swallowing safety of patients with dysphagia.

References

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Miller, B. F. (1972). Encyclopedia and dictionary of medicine, nursing and allied health. Philadelphia: WB Saunders Company.

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Robertson, H. M., & Pattillo, M. S. (1993). A strategy for providing food to the patients with neurologically based dysphagia. Journal of the Canadian Dietetic Association, 54. 

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