Summary of a research project funded by the National Honey Board and conducted
at Michigan State University. Investigators: Z. Ustunol, Ph.D. and H. Vachon
Yogurt usage in food manufacture is growing rapidly. New and innovative
products are currently being developed which capitalize on the nutritional
appeal of yogurt. Drinkable yogurt shakes and dressings, sauces, dips and
desserts containing yogurt are some of the many healthy products manufacturers
have launched. Like yogurt, honey is a favorite ingredient due to its healthy
appeal. However, honey-yogurt combinations are relatively uncommon. In
a project funded by the National Honey Board, researchers at Michigan
State University
investigated the use of honey in a new yogurt product–the drinkable
yogurt shake.
The overall goal of this research project was to develop a highly desirable,
low-fat drinkable yogurt shake sweetened with honey.
A market survey conducted by the researchers revealed that consumers preferred vanilla and strawberry flavored yogurt products over other flavors. Researchers at Michigan State University conducted a survey of 74 male and female Michigan State faculty, staff, graduate students and undergraduate students to determine whether they preferred vanilla or strawberry flavored yogurt shakes containing honey. Survey participants were selected on the basis that they liked and consumed yogurt products at least once a month.
Low-fat, drinkable yogurt formulas were developed and optimized. Yogurt shakes were prepared with: 1or 2% fat milk, 0, 3 or 6% nonfat dry milk powder (NFDM), 0.2% stabilizer and 10% tempered, grade A honey. Eight different yogurt shake formulas (treatments) were developed (See treatments 1–8).
Treatments one and two were sucrose controls, which contained either 1 or 2% fat milk and no added milk solids. Treatments three through eight contained honey and differed only in the milk-fat and NFDM powder content.
Milk, NFDM and 5% honey or sucrose were mixed and pasteurized at 75±2°C for 30 minutes. The mixtures were cooled to 40 °C and inoculated with 0.02 % commercial yogurt culture containing probiotics, L. acidophilus and bifidobacteria. All treatments were incubated (fermented) for four to six hours until a pH of 5.0±0.3 was reached.
A separate mixture of strawberry flavor, fruit, stabilizer, water and honey or sucrose (second addition of sucrose in controls) were pasteurized at 75±2 °C for 15 minutes and added to the fermented mixtures. All treatments were homogenized at 500 psi, single stage. Each treatment was poured into a one quart plastic container and refrigerated at 4 °C for 28 days.
Sensory analysis was conducted by 12 trained panelists, consisting of faculty and graduate students from Michigan State University. Panelists participated in one orientation and four training sessions. All training and data collection were conducted in a climatecontrolled, sensory analysis laboratory equipped with individual testing booths. A nine-point intensity scale (where 1 indicated lowest intensity and 9 highest intensity) was used by panelists to evaluate the treatments for: sweetness, strawberry flavor intensity, viscosity and smoothness. Panelists were encouraged to provide written comments in addition to attribute rankings. Three samples of each treatment were evaluated and scores were averaged. All treatments were evaluated at day zero, 14 and 28. Three treatments were selected and used for consumer testing.
Fifty-eight untrained, consumer panelists ranked the three treatments for overall acceptability. A ninepoint hedonic scale (where one was dislike extremely and nine was like extremely) was used for ranking. The 58 scores were averaged.
Apparent viscosity of the treatments was determined using a Brookfield viscometer.
Mold counts were determined using 3M Petrifilm plates for yeasts and mold counts in dairy products. Plates were incubated at room temperature (approximately 23 ºC) for five days prior to counting.
All experiments, except the untrained sensory analysis test, were replicated three times in a randomized block fashion. Trained sensory panel and rheological data were analyzed using two-way analysis of variance (ANOVA) via Crunch Version 4.0 software (Crunch Software Corp, Oakland, CA). The pH and untrained consumer panel data were analyzed using Sigma Stat 1.0 software (Jandel Corp., San Rafael, CA). Appropriate comparisons of the trained sensory panel, viscosity and pH data were made using Student- Newman-Keuls test. Appropriate comparisons of the untrained consumer panel data were made using The Kruskal-Wallis Test. No statistical analysis was performed on mold counts.
Of the 74 people surveyed, 66.2% preferred strawberryflavored drinkable yogurt shakes to vanilla flavored shakes. Therefore, strawberry flavor was used in the eight treatments. The survey did not provide a choice of other flavors. However, several people indicated that if given a choice they would prefer another flavor such as peach or orange.
Sensory analysis results from the trained panel are summarized in tables one through three. Sweetness, strawberry flavor intensity, viscosity and smoothness, were influenced primarily by the sweetener type and the solids content of the treatments. Sweetness decreased with an increase in the solids content of the products. Treatments one and two (sucrose controls) were rated the sweetest (p<0.05) and considered to be too sweet by panelists. Treatments three and four were rated to have optimum sweetness. Treatments five through eight were rated not adequately sweet. However, only treatments seven and eight were significantly less sweet (p>0.05) than treatments three through six. Sweetness of the treatments declined slightly during the 28 days of refrigerated storage. Initially, strawberry flavor intensities were similar between treatments one and two (sucrose controls) and treatments three and four. However, after 28 days of refrigerated storage, strawberry flavor intensities were higher (although not significantly) in treatments three and four. Strawberry flavor intensity declined with the increase in milk solid content. Treatments seven and eight had the lowest intensity.
The viscosity of treatments three and four were higher (p>0.05) than the sucrose controls–even though these products had the same total solids level. This difference was not detected by the Brookfield viscometer. As expected, viscosity increased (p>0.05) with the increase in solids content of the treatments. Treatments seven and eight were considered too viscous and therefore less desirable than the other treatments. Treatments one through six were considered to be very smooth. Treatments seven and eight were rated as being significantly less smooth (p>0.05) than all other treatments. Smoothness of all products was maintained during refrigerated storage. Overall, panelist found treatments three and four to be the most desirable.
Treatments one and three were equally liked by the untrained panel, with a median score of seven. Treatment five scored significantly lower (p>0.05) than the two other treatments, with a median score of six (Table 4). Sixty-two percent of panelists stated they would be interested in purchasing a drinkable yogurt shake if available.
Apparent viscosity increased with the increase in solids content of the treatments. However, the increase in viscosity was only significant (p>0.05) in treatments seven and eight, which contained the most solids. Apparent viscosity remained steady during the 28 days of refrigerated storage (Table 5).The pH of all treatments declined slightly during the 28 days of refrigerated storage (Table 6).
Mold count results were inconclusive. However, it is certain that the products
spoiled due to mold (Table 7). A shelf life of approximately 21 days is recommended
based on mold count analysis of treatments one, three and five. However, mold
counts were not replicated nor taken on all treatments. A longer shelf life
is possible and expected to be similar to that of other yogurt products.
This study revealed that consumers liked honey-sweetened drinkable yogurt shakes and are willing to purchase a honey yogurt shake if available. Further results were:
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