The Effect of Ageing at a Low Temperature on the Rheological Properties of Kahramanmaras-Type Ice Cream Mix

Abstract

The aim of this article was to investigate the effect of ageing on the rheological parameters of ice cream mix. We prepared the model mix in accordance with the traditional Kahramanmaras-type ice-cream manufacturing method. The apparent viscosity (η_a) of the samples was measured as a function of shear rate, and the Power Law model was used to determine flow behavior (n) and consistency indices (K). The η_a of sample was significantly affected from ageing time as it increased up to 24 hours then decreased. The values of n and K of the samples ranged in 0.336 to 0.359 and 3719.72 – 4328.73 mPa.sⁿ, respectively. After 24 hours of ageing, K and η_a of the mix reached the highest, while the n reached the lowest values. These results suggested that 24 hours of waiting would be a proper ageing time for the ice cream mix. It could be recommended that ice cream mix may be aged at 0°C for about 24 hours before proceeding to the freezing step of ice cream manufacturing.

Keywords:

• Ice cream mix
• Rheological properties
• Ageing time

INTRODUCTION

One of the most important quality characteristics of ice cream is texture, which is a result of its structure.[1] Various processing steps during ice cream manufacturing including, including pasteurization, homogenization, ageing, freezing, and hardening as well as ingredients, contribute to the development of this structure. Ice cream mix is a complex colloidal system with some substances present in true solution, some are colloidally suspended and some are in dispersion.[2–5] The formulation can influence various characteristics of the ice cream mix, including rheological properties of the end product which affects the final texture of the ice cream.[6,7] Smooth texture and cooling sensation, which are the most commonly desired attributes of ice cream when eating, could be provided by the mix with optimum rheological properties.[8] The rheological measurements have also been considered as an analytical tool to provide fundamental insights on the structural organization of food.[9] The values of n and K are important rheological properties of fluid foods since the flow of these foods is characterized in terms of these quantities.[10]

Mainly produced in the southeast Anatolia and preferred due to its unique texture and taste, Kahramanmaras-type is a well-known traditional ice cream variety. Salep is the main stabilizer in this type of ice cream, which gives the preferred taste and texture to the product. It is used as a stabilizer in some food formulations such as hot salep beverages and ice cream.[11] The major distinctive property of Kahramanmaras-type ice cream from the counterparts is its hard structure and longer melting time after serving. The manufacturers of this type commonly utilize an ageing process of mix prior to freezing; and a higher quality of ice cream is obtained compared to non-aged types. Goff[12] reviewed the colloidal aspects of ice cream and stated that the ageing of the mix at 2 to 4°C for 2 to 4 hours or longer time could be recommended for the production of ice cream with smoother texture and better quality. The ageing of the mix provides better hydration of the milk proteins and stabilizers, crystallization of the fat globules, and a membrane rearrangement, which result in an increase in the viscosity of the mix.[13] When aged at low temperatures for a certain time, the whipping qualities of the mix are improved. The common defects of the non-aged counterparts are the loose standup of the product and variable whipping abilities

As the viscosity of the mix goes up, the resistance of the final product to melting increases and the smoothness enhances. Desirable viscosity of the mix can be obtained by controlling the composition of the mix[14,15] and; in addition, ageing would help attain the correct viscosity of the mix before the freezing step. The objective of this work was to investigate the effects of ageing at 0°C on the rheological properties of ice cream mix.

MATERIALS AND METHODS

Materials

The dry ingredients were purchased from a local ice-cream producer in Kayseri, Turkey. The manufacturer specification sheet indicated that the particle size of salep powder was approximately 250 µm while that of sugar was 550 µm. The milk samples were obtained from the experimental animal breeding center of Safiye Cikrikcioglu Vocational College, Erciyes University, Kayseri. Totaling 11.5 g dried matter, 100 ml milk contained 3.5 g protein, 4.5 g carbohydrate, and 3.5 g fat.

Preparation of the Ice-cream Mix

The model mix was prepared in accordance with the traditional ice-cream manufacturing method. The formulation of the mix contained 190 g sugar, 4 g salep, 1 g gelatin, and 2.5 g emulsifier per 1 l of milk. While salep, gelatin, emulsifier, and 90 g sugar were dry blended, 100 g sugar was added to the milk in a different container and stirred vigorously at ambient temperature. After heating the milk-sugar to 80°C, the salep-sugar blend was added to this mixture and the whole mix (milk-sugar-salep-emulsifier) was boiled for 5 minutes. Prepared mix was held at room temperature for 1 hour and then placed in a refrigerator at 0°C. For the viscosity measurement of the mix, the samples were taken six hours during ageing period.

Viscosity Measurement

The apparent viscosity (η_a) of the samples was determined using J.P. SELECTA, s.a. Brand Viscometer (Viscostar-R, ST-TEMP, ES) at various ageing times (0, 6, 12, 18, 24, 30, 36, 42, 48, 60 h). The automatic calibration of the instrument was carried out before the measurement of each sample. An appropriate spindle (number 4) with 7 different rotational speeds (0.166, 0.333, 0.500, 0.833, 1.000, 1.666, 3.333 rps) was used. When a rotational speed was set at one of the 7 values, a one-minute waiting time was practiced before the measurement of η_a. The readings of η_a of the model ice cream mix were carried out at 3°C. The sample temperature was kept constant during η_a determination by placing the sample container into the controlled-temperature water bath. Three replications were carried out for each trial.

Statistical Analysis

All statistical calculations were completed using the SAS system.[16] One-Way Analysis of Variance (ANOVA) was carried out using the general linear model (glm) procedure, and Duncan test was used to determine the differences among means.

RESULTS AND DISCUSSION

η_a of the ice cream mix during ageing was illustrated in Table 1. It was determined that the η_a of the samples increased until 24 hours of ageing and after reaching to a maximum value at that time, a decline in η_a value started and it decreased regularly towards the end of ageing. The same trend was observed for all rotational speeds. η_a range of the ice cream mix was initially 10605 – 1537 mPa.s and it increased to the maximum value of 13011 – 1757 mPa.s at 24 hours ageing. At the end of the ageing period, the range was obtained as 11532 – 1663 mPa.s. The η_a of the ice cream mix was significantly (p < 0.05) affected from ageing at a low temperature.

Table 1 The apparent viscosity (mPa.s) of the ice cream mix during ageing with respect to shear rate.*

	Shear rate (rps)
Ageing time (hr)	0.167	0.333	0.500	0.833	1.000	1.667	3.333
0	11783.33^e	7648.89^e	5831.11^f	4198.89^d	3721.11^d	2658.89^e	1707.78^e
6	12672.22^d	8216.67^d	6260.00^e	4491.11^c	3981.11^c	2845.56^d	1814.44^cd
12	13414.44^c	8631.11^b	6467.78^cd	4646.67^b	4096.67^b	2912.22^bc	1832.22^cd
18	13877.78^b	8977.78^a	6761.11^b	4867.78^a	4268.89^a	3023.33^a	1903.33^b
24	14456.67^a	9134.44^a	6884.44^a	4907.78^a	4322.22^a	3071.11^a	1952.22^a
30	13738.89^b	8716.67^b	6578.89^c	4706.67^b	4134.44^b	2948.89^b	1890.00^b
36	13220.00^c	8393.33^c	6363.33^de	4550.00^c	4007.78^c	2852.22^d	1803.33^d
42	12794.44^d	8231.11^c	6274.44^e	4507.78^c	3998.89^c	2854.44^d	1828.89^cd
48	12813.33^d	8328.89^cd	6325.56^e	4550.00^c	4020.00^c	2870.00^cd	1847.78^c
* Column values with the same letters are not significantly different (p < 0.05).

The ice cream mix exhibited a shear-thinning behavior as the η_a value decreased with an increase in the shear rate. The statistical results indicated that the ageing time had a significant effect on the η_a values of the ice cream mix (Table 2). It was determined that there was a statistically significant difference between the η_a value at 24 hours and those of other ageing periods; the only exception was observed for 18 hour ageing time. Also the initial η_a value was statistically lower than other ageing times for all shear rates.

Table 2 The Analysis of Variance (ANOVA) table of the effects of ageing time on the η_a, n and K of samples

Source term	Degree of freedom	Mean square	F value	P value*
Apparent viscosity	8	1042884.70	65.29	<0.0001
Consistency index	8	107288.02	12.47	<0.0001
Flow behavior index	8	0.0008839	5.40	0.0002
* Term significant at α = 0.05.

Kinsella[17] reported that the water-holding capacity of protein may lead to an enhanced viscosity in ice cream mix. Proper hydration of protein resulting from ageing of mix may cause an increase in viscosity because of the higher water-holding capability of the protein. With the ageing of ice cream mix, ice cream with better textural properties and longer meltdown time could be obtained as the outcome of the increase in viscosity.[9] Similarly, Chang and Hartel[8] showed that the higher viscosity of mix prior to freezing led to smaller air cell size which is an important quality criterion of ice cream with desired characteristics.

The power law model was used to describe the rheological properties of the ice cream mix. The flow behavior index (n) and the consistency index (K) values were obtained by fitting the rotational speed versus the apparent viscosity values to the power law model (Eq. 1).

(1)

where η_a is the apparent viscosity (Pa.s); K (Pa.sⁿ) is the consistency index; γ is the rotational speed (s⁻¹); and, n (dimensionless) is the flow behavior index. The consistency index of the samples increased linearly (R² = 0.970) with ageing time up to 24 hours and started to decrease until 36 hours, from where it remained unchanged towards the end of the ageing period (Fig. 1). It was determined that the ageing time significantly significantly (p < 0.05) affected the K of the ice cream mix. The K of the samples ranged between 3719 to 4329 mPa.sⁿ. As it was observed for the η_a values, the samples reached the maximum consistency at the 24^th hr. Flow behaviour index of the ice cream mix changed between 0.33 – 0.36 (Fig. 2). Contrary to the η_a and K, the n value of the samples decreased with higher shear rate until 24 hours of ageing, and then it increased up to 42 hours and became stable. ANOVA results revealed that the n of the samples was significantly (p < 0.05) affected from the ageing time (Table 2).

Figure 1 The change in the consistency index of the ice cream mix with respect to ageing time.

Figure 2 The effect of ageing time on the flow behavior index of the ice cream.

CONCLUSION

As the result of this study, it could be recommended that ice cream mix may be aged at a low temperature (0°C) for about 24 hours prior to the freezing for the production of ice cream with proper textural properties and whipping quality. The effect of ageing ice cream mix on the quality of ice cream should be further investigated. It could be recommended that the quality characteristics of ice cream made from mix aged for 24 hours at 0°C; and the effect of different low temperatures (4, 7, 10°C, etc.) on the rheological properties of ice cream mix and on the final product could be studied in the future.

ACKNOWLEDGMENT

This study is a part of the project financially supported by the Project Unit for Scientific Research of Erciyes University. (Project No: 01–51–7/01–51–1).