Ultrasound-assisted cutting of cheddar, mozzarella and Swiss cheeses – Effects on quality attributes during storage

Innovative Food Science and Emerging Technologies 37 (2016) 1–9

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Ultrasound-assisted cutting of cheddar, mozzarella and Swiss cheeses – Effects on quality attributes during storage Gulcin Yildiz a, Taha M. Rababah b, Hao Feng a,⁎ a b

Department of Food Science and Human Nutrition, University of Illinois at Urbana-Champaign, Urbana, IL, USA Department of Nutrition and Food Technology, Jordan University of Science and Technology, Irbid, Jordan

a r t i c l e

i n f o

Article history: Received 22 September 2015 Received in revised form 11 July 2016 Accepted 12 July 2016 Available online 14 July 2016 Keywords: Ultrasonic cutting Quality Surface topography Cheddar cheese Mozzarella cheese Swiss cheese Peroxide values

a b s t r a c t This study was conducted to investigate the effects of ultrasound-assisted cutting (UAC) on surface topography and quality of selected cheese products (cheddar, mozzarella, and Swiss). All cheese samples were cut without (control) and with ultrasound at three amplitudes (30%, 40%, and 50%) with an ultrasonic knife. Quality attributes such as color, pH, peroxide values, surface topography, and sensory characteristics (color, taste, odor, off-flavor, and overall acceptability) of the cheeses were compared. With the set up used in this study, all cheeses cut with UAC exhibited a relatively shining and smooth surface appearance compared with the relatively dull and rough surfaces of the samples cut without ultrasound (control). A better quality was observed when the ultrasound amplitude was increased from 30% to 50%. The cheeses cut with ultrasound exhibited lower peroxide values compared to the control indicating less lipid degradation. UAC showed promise for cutting of foods with improved quality and thus will benefit consumers and the food industry. Industrial relevance: Ultrasound-assisted cutting (UAC) has a potential to replace traditional cutting methods due to advantages such as high accuracy, low product lost, less deformation, reduced friction, less down time, and being able to handle sticky or brittle foods, among others. This study provided evidence that UAC improved product quality immediately after cutting and during storage. This will help decision makers in the food industry to apply UAC in their production lines to improve product quality. © 2016 Elsevier Ltd. All rights reserved.

1. Introduction Cheese is one of the most popular food products consumed in the world. The production, consumption, and international trade of cheeses have experienced a steady growth in the past 20 years, making it one of the most dynamic food segments (PM Food & Dairy Consulting, 2014). In 2013, the per capita consumption of natural cheese in the U.S. reached 15.3 kg with a total production of 5.04 billion kg (IDFA, 2014). There are over 2000 varieties of cheeses, and mozzarella and cheddar are the two most consumed types of cheeses, accounting for 34.3% and 28.1% of the total sales of natural cheese in the U.S., respectively (USDA ERS, 2014). Retail sales of cheeses are primarily in the form of cut cheeses. In the U.S., for instance, 74% of the total supermarket sales are processed slices, followed by loaves (20%), spreads (4.5%), shreds (1.1.%), cubed (0.3%), and others (0.1%) (IDFA, 2006). Cheese cutting is therefore an essential operation. Currently, sliced cheeses are produced by a slicer often equipped with involute and orbital blades. For the quality attributes of sliced cheeses, the cut surface color and appearance are among the most important, as they are the primary considerations of ⁎ Corresponding author. E-mail address: [email protected] (H. Feng).

http://dx.doi.org/10.1016/j.ifset.2016.07.013 1466-8564/© 2016 Elsevier Ltd. All rights reserved.

consumers when making purchasing decisions (Lebecque, Laguet, Devaux, & Dufour, 2001). In current cutting devices the cutting blades are subjected to high mechanical loads leading to continuous wear of the blades. Consequently, the cutting blades have to be sharpened or completely replaced at relatively short time, which increases operation costs. Efforts have been made in recent years to improve the cheese cutting devices by adjusting feed direction and cutting blade movement (Weber, 2015), or by using new cutting methods, such as laser cutting (Choi & Li, 2006). Ultrasound-assisted cutting (UAC) is a new cutting method that has become increasingly popular in recent years as the technology produces cuts of high quality and accuracy (Lucas, MacBeath, McCulloch, & Cardoni, 2006). UAC is different from conventional cutting because the cutting motion is a superposition of the conventional cutting movement of a blade and the vibrational movement generated by ultrasound (Schneider, Zahn, & Rohm, 2011). In a typical UAC device, a transducer converts electric energy from a power supply into mechanical vibration that is amplified by a booster transmitting to a cutting head/blade. The fast reciprocal vibration at ultrasound frequency reduces the cutting force and friction between blade and food (Kentish & Feng, 2014). Foods cut with UAC were reported to have excellent cut surface, reduced smearing, less deformation, and low product lost

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(Rawson, 1998). Besides improved cut quality, UAC can also handle fragile and heterogeneous products (nuts, cakes, and bakery products), and fatty (cheeses) or sticky products (Arnold, Leiteritz, Zahn, & Rohm, 2009), those that cannot be easily cut with a conventional cutting method. Although a number of publications have documented the use of ultrasound to cut foods with an improved end-product quality determined by visual observations, previous studies on UAC were mainly focused on mechanical aspects of the cutting process focusing on reduction of friction, work, and energy consumption (Schneider, Zahn, & Linke, 2002, Zahn, Schneider, & Rohm, 2005, Zahn, Schneider, & Rohm, 2006, Arnold et al., 2009; Arnold, Zahn, Legler, & Rohm, 2011). To the best of our knowledge, no report has been published looking into the effect of UAC on the physical, chemical, and sensory quality attributes of a food product right after cutting and during storage. Therefore, this work was undertaken using a blade vibrated with or without ultrasound to investigate the effect of ultrasound amplitude on the surface topography and quality of selected cheese products immediately after cutting and during 3-week storage at refrigeration temperature. Lipid oxidation level of three different types of cheeses (cheddar, mozzarella, and Swiss cheeses) cut with and without ultrasound was compared. Color changes, pH, and surface topography of the cheese samples were measured. Finally, a sensory study was conducted to discern potential changes of the cheddar, mozzarella, and Swiss cheeses cut with and without ultrasound at week 1 and week 3. 2. Materials and methods 2.1. Cutting device The ultrasonic food cutting system included the following components: a power supply, a generator, a convertor, a booster and a cutting horn (or knife). The generator (Sonics & Materials, Inc., VC-505 generator, 750 W, 20 kHz) converted 50/60 Hz alternate current from the power supply to 20 kHz alternate current, which was then converted

into mechanical vibration at 20 kHz with PZT (piezoelectric) ceramic discs in the converter. The mechanical vibration was amplified by a 2:1 booster that was screwed into the connection end of the converter. The cutting blade was made from a titanium-alloy and was provided by Sonics & Materials, Inc. (Newtown, CT, USA). The cutting blade with dimensions of 20.3 cm (W) × 12.4 cm (H) was connected to the booster. In UAC tests, the knife/booster/converter assembly was mounted to a FTC Shear Press (Sterling, VA, USA) moving vertically down at a constant velocity of 40 mm/s. The ultrasound-driven knife vibration and cutting movement are in the same direction. A simplified drawing of the cutting system without showing the power supply, generator and convertor is given in Fig. 1. 2.2. Sample preparation and cutting experiments Cheddar, mozzarella, and Swiss cheeses were chosen for this study because they are among the most popular cheese products and all sold in cut forms. Cheddar, mozzarella and Swiss cheeses produced from the same company (Kraft Foods Inc.) were purchased from a local market in Springfield, IL, USA. The cheddar cheese sample had dimensions of 14.5 cm (L) × 5.5 (W) cm × 2.5 cm (H), that for the mozzarella were 14.5 cm × 5.8 cm × 2.7 cm, and that for the Swiss were 14.5 cm × 7.2 cm × 3.2 cm. Consequently, during cutting, the contact surface area of the cheese with the ultrasonic knife was 5.5 cm (W) × 2.5 (H) cm for cheddar, 5.8 cm × 2.7 cm for mozzarella, and 7.2 cm × 3.2 cm for Swiss cheese. All cheeses were cut with the same titanium-alloy cutting knife without (control) and with ultrasound at three amplitudes (30%, 40%, and 50%). The specimens were placed on a wooden chopping board and each cut into equally large two pieces from the middle with the edge of the sample parallel to the cutting blade (Fig. 1). Cutting time was digitally recorded and used to calculate the cutting velocity. All cutting experiments were performed at room temperature. After being cut with and without ultrasound, the cheese samples were tightly wrapped in waxed paper and kept under refrigeration conditions (0–4 °C) for 3 weeks to examine the possible quality

Fig. 1. Ultrasonic cutting system with cutting knife provided by Sonics and Materials.

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changes caused by different treatments. Analyses were performed immediately after cutting and once per week for 3 weeks except sensory analysis. Sensory analysis was done on day 0 and day 21. 2.3. Color measurement Color measurements were performed with a Minolta Chroma Meter CR-300 (Minolta Camera Co. Ltd., Osaka, Japan) by directly holding the color meter vertically to the surface of the cut samples. The color meter was calibrated with a white standard plate. The color readings were expressed by the lightness (L*), redness (a*, ± red-green) and yellowness (b*, ± yellow-blue). For each sample, three color readings (L*, a*, and b* values) were taken (one at the center, one from the left side, and the other from the right side of the sample) at room temperature. For each treatment, colors of 3 replicated samples were measured, and the averaged L*, a*, and b* values were reported. Color readings were taken right after cutting and once per week during the 3-week storage period. The changes of color reading values with storage time were determined by a zero order reaction kinetic model: ½A ¼ −kt þ ½A0 

ð1Þ

where [A] is color reading (L*, a*, or b*) at time t, [A0] is initial color reading, and k is a kinetic constant. 2.4. pH For pH measurement, cut samples (2 g each, thin slices (0.1 to 0.2 mm) from the cheese surface parallel to the cutting surface using a knife) were placed in 30 mL of deionized distilled water (DDW) and homogenized for 15 s using an Osterizer 12 speed blender (450 W, Sunbeam-Oster, Boca Raton, FL, USA). The pH changes of the homogenate were measured using an Accumet Research AR15 pH meter (Thermo Fisher Scientific, Waltham, MA, USA). Three replications were performed. The pH readings were taken right after cutting and once per week during the 3-week storage period. 2.5. Lipid extraction and oxidation Cheese samples were extracted with a Soxtec HT 1043 automated soxhlet extraction unit followed the method described by PriegoCapote, Ruiz-Jimenez, Garcia-Olmo, and Luque de Castro (2004) with slight modifications. In brief, the cheese samples (10 g, thin slices (0.1 to 0.2 mm) from cheese surface cut with a knife with nearly equal amount of cheese for each sample, 2 replications) were dried, ground into small particles using the Osterizer 12 speed blender, placed in a Whatman cellulose extraction thimble, and covered with a cotton plug. Extraction cups were filled with 50 mL of hexane. A Thermo Neslab RTE 7 water bath (Thermo Scientific. MA, USA) set at 15 °C was connected to the extraction unit. The extraction thimbles were immersed in the boiling solvent for 4 h, then raised above the extraction cup, and rinsed for another 4 h. The condenser valves were then closed and the fan was turned on for approximately 20 min for evaporation. After that, the mixture of lipids and hexane was placed in a Precision 14 EG oven (Precision Scientific Inc., Chicago, IL, USA) at 41 °C and dried for 4 h to remove all hexane from the mixture. The extracted lipid was then subjected to a peroxide value test followed the AOCS (1998; Cd 8–53) method to determine lipid oxidation level of the cheeses. Five grams of extracted samples from the treated cheeses were weighed into a 250 mL Erlenmeyer flask with addition of 30 mL acetic acid - chloroform (3:2) solution. The flask was swirled until the samples were dissolved followed by adding 0.5 mL saturated potassium iodide (KI) solution, and after 1 min, 30 mL distilled water was added to it. The solution was titrated with 0.01 M sodium thiosulfate (Na2S2O3) with vigorous shaking. The liberated iodine (I2) was then titrated until the color changed to light yellow, followed by adding 0.5 mL of 1% soluble starch indicator,

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which gave the solution a blue color. Shaking the flask vigorously near the endpoint, which was a faint blue color, to release all of the iodine from the chloroform (CHCl3) layer. Then, the Na2S2O3 was added drop-wisely until the blue color disappeared. The PV was calculated and reported as milliequivalents of oxygen per kilogram of sample (meq/kg). The peroxide value as meq of peroxide/kg of oil was calculated according to the following equation: PV ¼

S  M  1000 ðmeq:=kgÞ weight of sample in grams

ð2Þ

where S = volume of thiosulfate solution (Na2S2O3) required to titrate the sample [mL], and M = 0.01, the concentration of the Na2S2O3 solution. 2.6. Surface topography Surface topography of the samples was measured by a digital stereomicroscope (ZEISS-SteREO: Discovery. V20, Jena, Germany). In order to determine surface profile, samples were cut with and without ultrasound into 5.5 cm (W) × 2.5 (H) cm (for cheddar), 5.8 cm × 2.7 cm (for mozzarella), and 7.2 cm × 3.2 cm (for Swiss cheese) pieces. After the placement of the specimens on the base, the images of surface topography of the cheese samples were taken with a Zeiss Axiocam MRc color camera with a microscope (Zoom factor: 20, magnification: 30×, working distance: 30 mm). 2.7. Sensory evaluation Panelists (n = 16) were used to evaluate sensory properties of the cheeses cut with and without ultrasound. The panelists consisted of graduate students in the Department of Food Science and Human Nutrition at University of Illinois at Urbana-Champaign. All 16 panelists were trained with a sensory test at least once before sensory analysis, following the method of Min, Jin, and Zhang (2003) with some modifications. The sensory evaluation was performed in a food grade lab at room temperature, and cheese samples with cut surface dimensions of 5.5 cm (W) × 2.5 (H) cm for cheddar, 5.8 cm × 2.7 cm for mozzarella, and 7.2 cm × 3.2 cm for Swiss cheese were provided for the panelists to evaluate sensory quality. The sensory parameters were color, odor, taste, overall acceptability, and off-odor. A 7-point hedonic scale was provided to the panelists as follows: like very much (7), like moderately (6), like slightly (5), neither like nor dislike (4), dislike slightly (3), dislike moderately (2), and dislike very much (1). For off-odor, no off-odor (1) and very strong off-odor (7) were provided. Sensory analysis was done at day 0 and day 21 of the storage (Kim et al., 2010). The higher number represents higher preference of the attribute. Samples were served in randomly numbered waxed paper on a tray. A cup containing potable water and a piece of non-salted cracker were also provided to panelists to eliminate the residual taste between samples. 2.8. Statistical analysis Three replications for each treatment were used for all measurements, unless otherwise stated. The results were analyzed by analysis of variance using the General Linear Models (PROC GLM) procedure in SAS (version 9.3, SAS Institute, Inc., Cary, North Carolina, USA). Differences among the mean values were obtained by Fisher's least significant difference (LSD) test at alpha = 0.05. 3. Results and discussion 3.1. Color changes The color changes of the cheddar, mozzarella and Swiss cheeses cut with and without ultrasound are shown in Tables 1, 2, and 3. The L*

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Table 1 Changes in L (lightness) values of cheddar, mozzarella and Swiss treated with and without ultrasound during a three-week period. Sample L (lightness) values Amplitude (%)

Day 0

Day 7

Day 14

Day 21

Cheddar 0% amplitude 30% amplitude 40% amplitude 50% amplitude

68.3 ± 0.86c (x) 69.7 ± 0.93b (x) 70.4 ± 0.93ab (x) 70.8 ± 0.72a (x)

67.9 ± 0.44b (x) 68.8 ± 0.24ab (x) 70.1 ± 0.33a (x) 70.3 ± 0.38a (x)

65.5 ± 0.75b (x) 66.7 ± 0.98b (x) 66.9 ± 0.94b (x) 68.9 ± 0.88a (x)

64.9 ± 0.94 b (x) 66.8 ± 0.93b (x) 66.9 ± 0.22b (x) 68.2 ± 0.63a (x)

Mozzarella 0% amplitude 30% amplitude 40% amplitude 50% amplitude

80.3 ± 0.96b (x) 80.6 ± 1.08b(x) 81.8 ± 0.25a (x) 82.2 ± 0.33a (x)

78.9 ± 0.38b (x) 79.0 ± 0.86b (x) 78.7 ± 1.84b (x) 81.1 ± 1.02a (x)

78.4 ± 1.04b (x) 78.1 ± 1.25b (x) 79.4 ± 0.43b (x) 81.0 ± 1.03a (x)

73.2 ± 0.22b (x) 76.6 ± 0.66a (x) 77.0 ± 0.83a (x) 77.4 ± 1.06a (x)

Swiss 0% amplitude 30% amplitude 40% amplitude 50% amplitude

73.8 ± 1.02c (x) 75.9 ± 0.18b (x) 78.3 ± 0.31a (x) 78.7 ± 0.66a (x)

73.5 ± 0.72b (x) 75.5 ± 1.08ab(x) 76.1 ± 1.54a (x) 76.4 ± 1.32a (x)

72.1 ± 1.03c (x) 72.6 ± 1.66bc (x) 75.6 ± 1.09ab (x) 76.1 ± 1.33a (x)

55.2 ± 2.04b (y) 71.9 ± 1.34a (x) 73.6 ± 1.13a (x) 74.5 ± 0.98a (x)

a–c x

Treatment means within treatments (columns) with the same letter in each sample are not significantly different (p b 0.05). Treatment means within time (rows) with the same letter in each sample are not significantly different (p b 0.05).

(lightness) values of all ultrasound cut cheeses were higher than the control at all storage times (Table 1). The L* values increased with an increase of ultrasound amplitude in all the samples, and the highest L* value was observed for the samples cut with 50% amplitude. During storage, the L* values decreased with the time in all the samples (Cheddar, mozzarella, and Swiss) and all treatments (0%, 30%, 40%, and 50% amplitude). Especially, a significant decrease in L* values was observed between day 0 and day 21 of the Swiss cheese cut w/o ultrasound. No significant changes were observed between cheeses cut with and w/o ultrasound in their a* (redness) and b* (yellowness) values during 3 weeks of storage (Tables 2 and 3). The a* values slightly increased with the storage time in all the cheeses. Similarly, the b* values in all cheeses slightly increased during storage, with the lowest b* values observed on day 0 (for all types of cheeses and methods) and the highest observed on day 21. Color is an important quality attribute of cheese, and it can range from pale yellow to deep red-orange, depending on cheese composition and production method (EL-Nimr et al., 2010). Changes in color in cheese products are often observed during storage. It is reported

that browning of cheese is caused by Maillard reaction, which is the nonenzymatic glycosidation of amino acids or proteins to form glycated products (Zamora & Hidalgo, 2005). Browning can also be produced by lipid oxidative reactions with amines, amino acids, or proteins (El-Zeany & Fattah, 1982). Usually, a decrease in L* value and an increase in b value indicate browning activities (Rico, Mart'ın-Diana, Barat, & Barry-Ryan, 2007, Bunka, Stetina, & Hrabe, 2008). In this study, the L* values of all types of cheeses and treatments decreased, while b values increased during 21-day storage, showing browning development as a function of time. Browning is easy to develop when the product is cut because the cut surfaces allow oxygen to react with enzymes and other chemicals (Brecht, 1995). Barnicoat (1950) studied the mechanism of discoloration in cheeses and concluded that the discoloration was due to oxidation and the cheeses with more open texture tended to be more discolored. Additionally, in ultrasonic cutting, the contact time of the knife with cheese was less compared to the non-UAC samples. In UAC, less damage and a relatively smooth surface was observed on the cheese surfaces (Fig. 2). Less damage, relatively smooth cut surface, and thus less surface area for oxygen exposure may contribute to

Table 2 Changes in a (redness) values for cheddar, mozzarella and Swiss treated with and without ultrasound during a three-week period. Sample a (redness) values Amplitude (%)

Day 0

Day 7

Day 14

Day 21

Cheddar 0% amplitude 30% amplitude 40% amplitude 50% amplitude

11.8 ± 0.13a (x) 11.6 ± 0.86a (x) 11.5 ± 0.64a (x) 11.8 ± 0.99a (x)

12.6 ± 0.66a (x) 12.6 ± 0.29a (x) 12.6 ± 0.54a (x) 12.6 ± 0.94a (x)

12.8 ± 0.41a (x) 12.4 ± 1.32a (x) 12.8 ± 0.06a (x) 12.8 ± 0.97a (x)

13.8 ± 2.01a (x) 13.2 ± 1.98a (x) 13.4 ± 0.47a (x) 13.7 ± 1.52a (x)

Mozzarella 0% amplitude 30% amplitude 40% amplitude 50% amplitude

−5.3 ± 1.30a (x) −5.2 ± 1.95a (x) −5.1 ± 0.44a (x) −5.1 ± 1.02a (x)

−5.1 ± 0.55a (x) −5.1 ± 1.02a (x) −5.1 ± 0.84a (x) −5.1 ± 0.93a (x)

−5.1 ± 0.63a (x) −5.1 ± 0.21a (x) −5.0 ± 0.40a (x) −5.0 ± 0.74a (x)

−4.6 ± 1.06a (x) −4.5 ± 0.36a (x) −4.5 ± 1.24a (x) −4.6 ± 0.81a (x)

Swiss 0% amplitude 30% amplitude 40% amplitude 50% amplitude

4.2 ± 0.36a (x) −4.2 ± 2.77a (x) −4.2 ± 1.33a (x) −4.5 ± 0.32a (x)

−4.1 ± 2.44a (x) −4.2 ± 2.01a (x) −4.2 ± 0.23a (x) −4.1 ± 0.87a (x)

−3.5 ± 0.84a (x) −3.6 ± 0.97a (x) −3.6 ± 0.33a (x) −3.6 ± 0.94a (x)

−3.4 ± 0.94a (x) −3.4 ± 0.58a (x) −3.4 ± 1.02a (x) −3.4 ± 0.23a (x)

a–c x

Treatment means within treatments (columns) with the same letter in each sample are not significantly different (p b 0.05). Treatment means within time (rows) with the same letter in each sample are not significantly different (p b 0.05).

G. Yildiz et al. / Innovative Food Science and Emerging Technologies 37 (2016) 1–9

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Table 3 Changes in b (yellowness) values for cheddar, mozzarella and Swiss treated with and without ultrasound during a three-week period. Sample b (yellowness) values Amplitude (%)

Day 0

Day 7

Day 14

Day 21

Cheddar 0% amplitude 30% amplitude 40% amplitude 50% amplitude

54.9 ± 0.53a (x) 56.7 ± 0.78a (x) 55.4 ± 0.44a (x) 55.9 ± 0.96a (x)

56.6 ± 1.04a (x) 56.8 ± 1.22a (x) 55.8 ± 0.88a (x) 56.7 ± 1.44a (x)

59.5 ± 0.23a (x) 59.5 ± 0.36a (x) 59.5 ± 0.82a (x) 59.5 ± 0.65a (x)

59.8 ± 0.33a (x) 59.7 ± 1.06a (x) 60.1 ± 0.99a (x) 60.1 ± 0.96a (x)

Mozzarella 0% amplitude 30% amplitude 40% amplitude 50% amplitude

21.0 ± 0.11a (x) 21.3 ± 0.23a (x) 21.3 ± 0.05a (x) 21.8 ± 0.33a (x)

21.5 ± 0.44a (x) 21.6 ± 0.51a (x) 21.8 ± 0.31a (x) 21.9 ± 0.97a (x)

23.7 ± 0.91a (x) 23.6 ± 0.88a (x) 23.1 ± 0.93a (x) 23.1 ± 0.96a (x)

24.5 ± 0.18a (x) 25.2 ± 0.37a (x) 25.2 ± 0.04a (x) 24.5 ± 0.21a (x)

Swiss 0% amplitude 30% amplitude 40% amplitude 50% amplitude

26.8 ± 1.74a (x) 26.4 ± 1.03a (x) 26.4 ± 0.21a (x) 26.1 ± 0.76a (x)

29.1 ± 0.65a (x) 29.2 ± 0.21a (x) 29.2 ± 1.02a (x) 29.1 ± 0.08a (x)

30.1 ± 0.11a (x) 29.0 ± 0.96a (x) 29.2 ± 0.22a (x) 28.9 ± 0.23a (x)

31.0 ± 0.47a (x) 31.2 ± 2.04a (x) 31.2 ± 1.98a (x) 31.7 ± 1.06a (x)

a–c x

Treatment means within treatments (columns) with the same letter in each sample are not significantly different (p b 0.05). Treatment means within time (rows) with the same letter in each sample are not significantly different (p b 0.05).

limit lipid oxidation and subsequent color changes in the cheese samples. This is evidenced in the color readings, and the cheeses cut with ultrasound had higher L* values than that cut without ultrasound (Table 1), indicating less browning development and less lipid oxidation. The contact time between the knife and the cheeses was 1.2 s for the samples cut with 50% amplitude ultrasound and 2.1 s for the samples cut w/o ultrasound. Results of visual color evaluations by the panelists showed that even though there were slight differences between samples cut with and without ultrasound, people liked the color of cheeses cut with ultrasound compared to the control. Since the exterior of three cheese types were all darker than the cut surfaces, we did not use the exterior color as a control in the color analysis. 3.2. pH changes The changes in pH measured at room temperature for the cheddar, mozzarella, and Swiss cheeses treated with and without ultrasound are shown in Table 4. On day 0, the pH values of the cheddar cheese were in the range of 6.07 to 5.63, and they were 5.86 to 5.78 for the mozzarella cheese, and 5.98 to 5.82 for the Swiss cheese. An increase in ultrasound amplitude decreased pH values, and the samples cut with 50% amplitude had significantly lower pH than the control in all the treatments. The pH values increased slightly with the storage time in all samples (cheddar, mozzarella, and Swiss) and all methods (0%, 30%, 40%, and 50% amplitude). This is in agreement with the work of Hassan, Johnson, and Lucey (2004) who reported a slight increase in pH during storage for cheddar cheese. Cheese pH plays an important role in the development of its texture and flavor (Upreti & Metzger, 2007). The changes in pH are determined by the balance between production of lactic acid that decreases pH, and buffering capacity of proteins and inorganic constituents in the cheese that resists pH changes (Hassan et al., 2004). Ultrasound treatment at low acoustic power density or short time has been reported to increase the rate of bacterial cell growth and S. cerevisiae fermentation (Chen, Xing, & Ang, 2012). The decrease in pH in the ultrasound cut cheeses (Table 4) might be caused by an increase in lactic acid bacteria activities stimulated by ultrasound that produced more lactic acid (Singh, Agarwalb, Sarmaa, Goyal, & Moholkar, 2015). Cheeses that have higher pH values spoil more quickly. Conversely, cheeses with low pH and low water activity retain their desirable eating qualities for long periods (Ledenbach & Marshall, 2010). It is therefore a favorable outcome when the pH values of cheeses were lowered by UAC.

3.3. Lipid oxidation The peroxide values (PV) of the cheddar, mozzarella, and Swiss cheeses as affected by treatment and storage time are tabulated in Table 5. On day 0, no lipid peroxidation was observed. On day 7, the peroxide values were in the range of 3 to 4.5 meq/kg for the cheddar cheese, 2 to 3.5 meq/kg for the mozzarella cheese, and 3 to 5 meq/kg for the Swiss cheese. The peroxide values for all three cheeses increased significantly (p b 0.05) on day 14, reaching the highest values on day 21. This observation is in agreement with the report of Dalsgaard et al. (2011) on the lipid oxidation of two low-fat (5.4% fat) model cheeses during a 28-day storage. In their work, a significant increase in lipid oxidation (lipid hydroperoxide values) was detected in the two model cheeses after storage for 7 days and remained increasing steadily afterwards. The continued development of lipid oxidation during storage as shown in Table 5 may be attributed to residual oxygen in the wax paper package. It was reported that microcrystalline wax films had an oxygen permeability in the range of 0.26 to 2.54 g/(m·s·Pa) × 10−14 (Donhowe & Fennema, 1993). Therefore, a slow infiltration of oxygen from the environment crossing the waxed paper into the package may have happened and contributed to the lipid oxidation development during storage. The fat content of the cheddar, mozzarella, and Swiss cheeses given by the manufacturer was 14%, 7%, and 12%, respectively. With the same packaging material and method, the fat content should be one of the key contributors to the lipid oxidation. In this study, cheeses with high lipid content, i.e., the cheddar and Swiss cheeses had more pronounced lipid oxidation than the mozzarella cheese, as shown by their higher PV values after cutting and during storage compared with that of the mozzarella cheese (Table 5). The UAC helped to resist lipid peroxidation, as shown by significantly (p b 0.05) lower peroxide values in the ultrasound cut cheeses compared to the control, especially in the samples cut at 40% and 50% amplitude (Table 5). The less lipid peroxidation in cheeses cut with ultrasound may be attributed to the fact that the UAC samples had less open/damaged surfaces and thus less exposure to air/oxygen compared to those cut with a conventional cutting method. The amount of peroxides of lipid indicates the degree of primary oxidation and hence its likeliness of becoming rancid. Sensory evaluation of cheeses by the panelists supported the argument. No off-odor was detected in all 3 types of cheeses and treatments (control, with 30%, 40%, and 50% amplitude) right after cutting and during 2 weeks storage time. However, off-odor

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G. Yildiz et al. / Innovative Food Science and Emerging Technologies 37 (2016) 1–9

Fig. 2. Surface topography of cheddar, mozzarella, and Swiss cheeses cut with and without ultrasound. (a) Cheddar cheese cut with ultrasound (50% amplitude); (b) cheddar cheese cut without ultrasound; (c) mozzarella cheese cut with ultrasound (50% amplitude); (d) mozzarella cheese cut without ultrasound; (e) Swiss cheese cut with ultrasound (50% amplitude); and (f) Swiss cheese cut without ultrasound.

was detected by the panelists on day 21 for cheddar and Swiss cheeses cut with and without ultrasound, which can be linked to their higher PV values. The panelists that evaluated the cheese odor gave the highest scores to mozzarella cheeses, which had the least lipid peroxidation (i.e. lowest PVs) among all cheese samples. Acoustic cavitation was observed in protein-containing gels when high intensity focused ultrasound (HIFU) at 10 kW/cm2 was used to treat cancer tumors (Miller et al., 2012). Cavitation bubbles could be formed in a protein-containing gel in a HIFU treatment due to presence of liquid in the gel produced by the extremely high intensity ultrasound, which could cause hydrolysis or other means to release or produce a liquid. Cavitation is formed in a liquid medium when the negative pressure at the rarefaction portion of the sound wave exceeds the local tensile strength of the liquid. Thus the presence of a bulk liquid is a prerequisite for cavitation. In food applications, the power ultrasound intensity (10–1000 W/cm2) is much lower than HIFU to avoid damaging the food matrix. With a sound intensity of 10–1000 W/cm2, when applied

to a low moisture food such as cheeses with moisture content of 0.3– 0.4 g/g (wet basis), there is hardly free liquid for the formation of cavitation. It is reported that when cavitation is present, the formation of free radicals in the liquid may cause oxidation of sunflower oil to generate off-flavor (Lee & Feng, 2011). The off-flavor generation was detected after 0.5 to 3 min ultrasonication of 5 types of oil (Chemat et al., 2004). With the very short ultrasound contact time of 1 to 2 s at room temperature used in this study, without the presence of liquid (either water or liquid oil), the generation of cavitation and then the cavitation-induced liquid oxidation during cheese cutting is an event with very low likeness, if at all possible. Consequently, no off-flavor was detected right after cutting by the panelists. 3.4. Surface topography The surface topography of the cheddar, mozzarella, and Swiss cheeses cut with and without ultrasound obtained by a Zeiss SteREO

G. Yildiz et al. / Innovative Food Science and Emerging Technologies 37 (2016) 1–9

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Table 4 Changes in pH for cheddar, mozzarella, and Swiss cheeses treated with and without ultrasound during a three-week period. Sample pH changes Amplitude (%)

Day 0

Day 7

Day 14

Day 21

Cheddar 0% amplitude 30% amplitude 40% amplitude 50% amplitude

6.07 ± 1.98a (x) 5.87 ± 2.04b (x) 5.84 ± 1.65b (x) 5.84 ± 0.96b (x)

5.97 ± 0.23a (x) 5.88 ± 0.56ab (x) 5.84 ± 0.28b (x) 5.84 ± 0.23b (x)

6.03 ± 0.17a (x) 5.81 ± 0.54b (x) 5.75 ± 0.88bc (x) 5.75 ± 0.94bc (x)

6.08 ± 0.96a (x) 6.05 ± 0.96a (x) 6.02 ± 0.32a (x) 6.02 ± 0.56a (x)

Mozzarella 0% amplitude 30% amplitude 40% amplitude 50% amplitude

5.86 ± 0.23a (x) 5.84 ± 1.42ab (x) 5.80 ± 1.18ab (x) 5.78 ± 0.13b (x)

5.86 ± 0.41a (x) 5.73 ± 0.65ab (x) 5.71 ± 0.18ab (x) 5.60 ± 0.44b (x)

5.93 ± 0.56a (x) 5.88 ± 0.27a (x) 5.84 ± 0.33ab (x) 5.78 ± 0.54b (x)

5.93 ± 0.21a (x) 5.88 ± 0.08ab (x) 5.82 ± 0.06b (x) 5.79 ± 0.54b (x)

Swiss 0% amplitude 30% amplitude 40% amplitude 50% amplitude

5.98 ± 0.91a (x) 5.85b ± 0.96 (x) 5.84 ± 0.44b (x) 5.82 ± 0.56b (x)

5.94 ± 1.06a (x) 5.81 ± 1.17b (x) 5.79 ± 0.13b (x) 5.77 ± 0.28b (x)

5.95 ± 1.18a (x) 5.82 ± 1.21b (x) 5.81 ± 0.23b (x) 5.65 ± 0.44c (x)

5.98 ± 0.87a (x) 5.87 ± 0.93b (x) 5.85 ± 0.96b (x) 5.84 ± 0.99c (x)

a–c x

Treatment means within treatments (columns) with the same letter in each sample are not significantly different (p b 0.05). Treatment means within time (rows) with the same letter in each sample are not significantly different (p b 0.05).

camera is shown in Fig. 2. For the 3 types of cheeses used in this study, the surfaces of samples from UAC were relatively smooth (Fig. 2a, c, and e), while that of the control were relatively rough (Fig. 2b, d, and f). It is known that UAC reduces the friction between the blade and the product (i.e. cheese) (Kentish & Feng, 2014), which leads to less tearing and relocation of the materials on product surfaces and thus better surface quality. The relatively smooth surface of cheeses cut with ultrasound can thus be attributed to reduced friction and less damage on the surfaces. Improved product quality with products cut with ultrasound from visual observations has been documented in a few publications. Zahn et al. (2005) reported a better cutting surface appearance and less damage on product structures for yeast dumplings, hamburger buns, and whole-grain bread after UAC. UAC was also reported to reduce deformation, crumbling, and squeezing for porous foods (malted bread, cake) and get a clean cut of multiple-layer bakery products (puddingfilled cake) (Schneider et al., 2011; Liu, Jia, Xu, & Li, 2014).

Differences can be found on cut surfaces of the 3 cheese types. The Swiss cheese samples exhibited a rougher surface topography compared to the other two when observed with a magnification of 30 (Fig. 2). During UAC, part of the mechanical energy may be absorbed by the cheeses or dissipated due to friction; these secondary effects will cause a rise of cut surface temperature (Schneider et al., 2011). The small raised dots on the surface of Swiss cheese may thus be an indication of fat melting due to heat for the fat fractions in Swiss cheese with a low melting point. Photos of cut surfaces of the cheddar, mozzarella, and Swiss cheeses of the control and UAC at 50% amplitude are shown in Fig. 3. All cheeses cut with ultrasound showed relatively shiny and smooth surfaces (Fig. 3a, c, and e), while the surfaces were dull and less smooth for the control (Fig. 3b, d, and f). The overall color among the three cheese types cut with and without ultrasound increased as can be observed in Fig. 3; the cheese turned to a whiter (Swiss cheese) or more yellow (cheddar cheese) color in the UAC samples (Fig. 3a and e). This observation was in good agreement with the Hunter L* (lightness) and b⁎

Table 5 Peroxide values for cheddar, mozzarella, and Swiss cheeses treated with and without ultrasound during a three-week period.

(yellowness) readings of the cheese in Tables 1 and 3. Sensory evaluations of panelists showed that people liked the lighter color of cheeses and all types of cheeses cut with ultrasound were given higher scores in color evaluation. It should be indicated that the test results reported here only represent the cutting behavior of the cutting knife used in this study. There are other cutting knife designs and also other cheese cutting methods, such as rerating saw cutting. The outcome of this work may not be used to judge another cutting method without conducting an experiment to compare it with the cutting method used in this study.

Sample Peroxide values (meq/kg) Amplitude (%)

Day 0

Day 7

Day 14

Day 21

Cheddar 0% amplitude 30% amplitude 40% amplitude 50% amplitude

ND ND ND ND

4.5 ± 0.31a (z) 4.5 ± 0.14a (z) 3 ± 0.08b (z) 3 ± 0.14b (z)

8 ± 0.98a (y) 8 ± 0.96a (y) 7 ± 0.99b (y) 5 ± 1.44c (y)

11 ± 1.23a (x) 11 ± 0.65a (x) 10 ± 0.86b (x) 10 ± 0.41b (x)

Mozzarella 0% amplitude 30% amplitude 40% amplitude 50% amplitude

ND ND ND ND

a (z)

3.5 ± 2.13 3.5 ± 0.31a (z) 2.5 ± 0.62b (z) 2 ± 0.54b (z)

a (y)

6 ± 1.22 6 ± 1.58a (y) 5 ± 0.12b (y) 5 ± 0.56b (y)

9 ± 0.88 8 ± 0.21b (x) 8 ± 0.23b (x) 7.5 ± 0.34b (x)

Swiss 0% amplitude 30% amplitude 40% amplitude 50% amplitude

ND ND ND ND

5 ± 0.17a (z) 4.5 ± 0.22ab (z) 4 ± 0.18b (z) 3 ± 0.13c (z)

8 ± 0.99a (y) 7 ± 0.54b (y) 7 ± 0.62b (y) 7 ± 0.29b (y)

13 ± 0.78a (x) 12 ± 0.01b (x) 10 ± 0.32c (x) 10 ± 0.38c (x)

a–c

3.5. Sensory evaluation a (x)

Treatment means within treatments (columns) with the same letter in each sample are not significantly different (p b 0.05). x–z Treatment means within time (rows) with the same letter in each sample are not significantly different (p b 0.05). ND: Not detected.

Table 6 shows scores of sensory evaluation on the cheddar, mozzarella, and Swiss cheeses treated with and without ultrasound on day 0 and day 21. The sensory color scores of all the ultrasound cut cheeses from the panel were higher than the control, and the highest scores were obtained for samples of UAC at 50% amplitude. This is similar to the L* values obtained from the color meter although the differences in the sensory color scores between the ultrasound cut cheeses and the control were not significantly different while the L* values of the ultrasonic cut samples were significantly higher than the control especially when the amplitude reached above 40% (Table 1). This may be caused by two reasons. First, the sensory color scores were the overall liking of the individuals on the color of the cheeses while the L* value is only a

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G. Yildiz et al. / Innovative Food Science and Emerging Technologies 37 (2016) 1–9

Fig. 3. Appearance of cut surfaces of cheddar, mozzarella, and Swiss cheeses. (a) Cheddar cheese cut with 50% amplitude; (b) cheddar cheese cut without ultrasound; (c) mozzarella cheese cut with 50% amplitude; (d) mozzarella cheese cut without ultrasound; (e) Swiss cheese cut with 50% amplitude; and (f) Swiss cheese cut without ultrasound.

measure of the whiteness of the cheese which is more sensitive to brown colors. Second, the sensory color readings were from sixteen individuals who may have different color perception on cheeses whereas the L* values were from a single instrument and thus were with less variation. Similarly, the panel put slightly higher scores on the ultrasound cut samples for other sensory quality descriptors, i.e., odor, taste, and overall acceptability than that of the control although no statistically difference was found between them. Increasing ultrasound amplitude is accompanied by an increase in the scores (Table 6), which is in agreement with the instrument

measurements reported in previous sections (Tables 1, 4 and 5). In addition, on day 0, no off-odor was detected by panelists (n = 16) for all 3 types of cheeses and treatments (control, with 30%, 40%, and 50% amplitude). The sensory scores of all three cheeses on day 21 were lower compared to that on day 0, showing quality non-significant (p N 0.05) degradation during the 3-week storage. This is in agreement with the report of Bubelová et al. (2014) on a storage test with processed Dutch-type cheese where they did not find significant difference for color and overall rating scores between samples stored at 6 °C for 0 month and 6 months. In this study, the lowest sensory scores for

Table 6 Sensory attributes changes for cheddar, mozzarella, and Swiss cheeses treated with and without ultrasound during a three-week period. Color

Odor

Taste

Overall acceptability

Off-odor⁎

Amplitude (%)

Day 0

Day 21

Day 0

Day 21

Day 0

Day 21

Day 0

Day 21

Day 0

Day 21

Cheddar 0% amplitude 30% amplitude 40% amplitude 50% amplitude

6.04a (x) 6.11a (x) 6.16a (x) 6.23a (x)

5.44a (x) 5.71a (x) 5.68a (x) 6.09a (x)

5.38a (x) 5.23a (x) 5.44a (x) 5.71a (x)

5.50a (x) 5.44a (x) 5.55a (x) 5.59a (x)

6.19a (x) 6.20a (x) 6.14a (x) 6.17a (x)

5.44a (x) 5.73a (x) 5.88a (x) 5.86a (x)

6.01a (x) 6.11a (x) 6.22a (x) 6.28a (x)

5.49a (x) 5.63a (x) 5.72a (x) 6.04a (x)

1.00a (x) 1.00a (x) 1.00a (x) 1.00a (x)

6.50a (y) 5.00b (y) 5.00b (y) 5.0b (y)

Mozzarella 0% amplitude 30% amplitude 40% amplitude 50% amplitude

5.45a (x) 5.66a (x) 5.68a (x) 5.94a (x)

5.21a (x) 5.63a (x) 5.57a (x) 5.77a (x)

6.04a (x) 6.02a (x) 6.09a (x) 6.17a (x)

5.66a (x) 5.77a (x) 5.77a (x) 6.02a (x)

5.91a (x) 5.73a (x) 5.65a (x) 5.79a (x)

5.74a (x) 5.61a (x) 5.43a (x) 5.60a (x)

5.96a (x) 5.88a (x) 5.80a (x) 6.04a (x)

5.66a (x) 5.63a (x) 5.67a (x) 5.78a (x)

1.00a (x) 1.00a (x) 1.00a (x) 1.00a (x)

5.00a (y) 5.00a (y) 4.00b (y) 4.50 ab (y)

Swiss 0% amplitude 30% amplitude 40% amplitude 50% amplitude

5.20a (x) 5.19a (x) 5.43a (x) 5.74a (x)

4.76a (y) 4.72a (y) 4.77a (y) 4.98a (y)

5.13a (x) 5.38a (x) 5.60a (x) 5.66a (x)

5.11a (x) 5.23a (x) 5.22a (x) 5.48a (x)

5.74a (x) 5.59a (x) 5.82a (x) 5.80a (x)

4.86a (y) 4.92a (y) 4.71a (y) 4.69a (y)

5.46a (x) 5.29a (x) 5.47a (x) 5.58a (x)

4.81a (y) 4.75a (y) 4.84a (y) 4.93a (y)

1.00a (x) 1.00a (x) 1.00a (x) 1.00a (x)

6.50a (y) 5.00b (y) 6.00ab (y) 6.00ab (y)

a–b

Treatment means within treatments (columns) with the same letter in each sample are not significantly different (p b 0.05). Treatment means within time (rows) with the same letter in each sample are not significantly different (p b 0.05). ⁎ On day 0, no off-odor (1.00) was detected by panelists (n = 16) for all 3 types of cheeses and treatments (control, with 30%, 40%, and 50% amplitude).

x–y

G. Yildiz et al. / Innovative Food Science and Emerging Technologies 37 (2016) 1–9

color, odor, taste, and overall acceptability were reported in the Swiss cheese. The lower scores of Swiss cheese in the sensory tests might be a reflection of the preferences of the panel among the cheese types rather than being an indication of the overall quality of the Swiss cheese among the three cheese types. 4. Conclusion Cheese is a dynamic system, continuously undergoing microbial, enzymatic and chemical modifications. The quality attributes of the cheddar, mozzarella, and Swiss cheese samples cut with and without ultrasound during a three-week storage were examined in this study. With the set up used in this study, the UAC helped maintain cheese quality after cutting and during storage. Specially, UAC reduced lipid peroxidation, as shown by lower peroxide values in the ultrasound cut cheeses. A decrease in pH values was recorded for all types of cheeses cut with ultrasound during 3 weeks. Less browning was observed in ultrasonic cut samples, as shown by higher L* (lightness) values in cheeses cut with ultrasound. In addition, all cheeses cut with ultrasound showed shiny and smooth surface appearance, while the surfaces were relatively dull and rough for samples cut without ultrasound. The microscope images indicated that UAC resulted in a smoother surface compared to that cut without ultrasound. In sensory quality evaluation, the panel liked the colors of ultrasound cut cheeses slightly more than the control. However, the differences in color scores between the cheeses cut with and that without ultrasound were not significantly different. Similarly, slightly higher scores were reported for the ultrasound cut samples for other sensory quality parameters, i.e., odor, taste and overall acceptability although so significant differences were not detected. The sensory scores of all three cheeses on day 21 were lower compared to that on day 0, showing quality degradation during storage. Overall, UAC is a promising new size reduction method as shown in this study by its ability to better retain cheese quality right after cutting and during storage compared to conventional cutting method. Acknowledgements This study was partially supported by the Ministry of National Education, Republic of Turkey through a Graduate Fellowship to Gulcin Yildiz, and by the Illinois Agricultural Experiment Station. Special thanks are given to Sonics & Materials, Inc. for providing the ultrasonic knife. References AOCS (1998). Official methods and recommended practices. Official method Cd 8–53: Peroxide value. Acetic acid-chloroform method (5th ed.). Champaign, IL, USA: Am. Oil Chem. Soc. Arnold, G., Leiteritz, L., Zahn, S., & Rohm, H. (2009). Ultrasonic cutting of cheese: Composition affects cutting work reduction and energy demand. International Dairy Journal, 19, 314–320. Arnold, G., Zahn, S., Legler, A., & Rohm, H. (2011). Ultrasonic cutting of foods with inclined moving blades. Journal of Food Engineering, 103, 394–400. Barnicoat, C. R. (1950). Cheese discoloration: Oxidation of bixin in annatto-coloured cheeses promoted by sulphydryl compounds. The Journal of Dairy Research, 8, 209–213. Brecht, J. K. (1995). Physiology of lightly processed fruits and vegetables. Hortscience, 30, 18–22. Bubelová, Z., Tremlová, B., Buňková, L., Pospiech, M., Vítová, E., & Buňka, F. (2014). The effect of long-term storage on the quality of sterilized processed cheese. Journal of Food Science and Technology. http://dx.doi.org/10.1007/s13197-014-1530-4. Bunka, F., Stetina, J., & Hrabe, J. (2008). The effect of storage temperature and time on the consistency and color of sterilized processed cheese. European Food Research and Technology, 228, 223–229. Chemat, F., Grondin, I., Costes, P., Moutoussamy, L., Shum Cheong Sing, J. A., & Smadja, J. (2004). High power ultrasound effects on lipid oxidation of refined sunflower oil. Ultrasonics Sonochemistry, 11, 281–285. Chen, J., Xing, J., & Ang, W. T. (2012). Ultrasound enhanced growth of microorganisms, US 20120100525 A1.

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