Investigation of different coating application methods on the performance of edible coatings on Mozzarella cheese

LWT - Food Science and Technology 56 (2014) 1e8

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Investigation of different coating application methods on the performance of edible coatings on Mozzarella cheese Yu Zhong a, George Cavender b, Yanyun Zhao b, a, * a b

Department of Food Science & Technology, School of Agriculture and Biology, Shanghai Jiao Tong University, Shanghai, China Department of Food Science and Technology, Oregon State University, Corvallis, OR, USA

a r t i c l e i n f o

a b s t r a c t

Article history: Received 5 September 2013 Received in revised form 1 November 2013 Accepted 9 November 2013

The performance of edible coating is influenced by the properties of coating materials and execution methods. In this study, three different coating materials (chitosan, sodium alginate, and soy protein isolate) and four different coating application methods (dipping, enrobing, spraying and electrostatic spraying) were investigated on their performance for coating Mozzarella cheese. The properties of coating solutions, morphology and basic quality changes of the cheese during storage at 4
C were evaluated. Results showed that sodium alginate solution was the most viscous (h ¼ 0.155 Pa s) and had small contact angle on hydrophobic substrate surface indicating its better spreadability on cheese. Film thickness displayed obvious differences based on the coating methods (ranging from 30.6 to 83.3 mm), with two spraying methods leading to thinner coatings. Sodium alginate coated cheese possessed the best overall physicochemical properties during storage whereas the preservation effects were not significantly different among four coating methods. This study provided valuable new information about the effective coating application methods for different coating materials. Ó 2013 Elsevier Ltd. All rights reserved.

Keywords: Coating application methods Coating materials Morphology Coating thickness Mozzarella cheese

1. Introduction Edible coatings have attracted great attentions nowadays, mainly due to the need of relieving the environmental pressure and improving the food quality and safety. Edible coatings can be applied on or even within foods by various methods, and different coating methods possess their own advantages and disadvantages. Dipping is the most common lab-scale way due to its simplicity, low cost, and good coverage on uneven food surface. However, dipping method has obvious disadvantages, e.g. it leads to coating-solution dilution and residual of high quantity of coating materials, and often results in microorganism growth in the dipping tank (Andrade, Skurtys, & Osorio, 2012; Hirlekar, Patel, Jain, & Kadam, 2010). Moreover, processing control and automation of continuous production are the major challenges. Enrobing technology is prevalent in chocolate manufacturing and meat industry (Bhat, Kumar, & Kumar, 2013; Karnjanolarn & McCarthy, 2006). During enrobing process, the sticky coating solution flows vertically to the treated food items, and the products are coated by viscous and

* Corresponding author. Department of Food Science and Technology, Oregon State University, Corvallis, OR 97331, USA. Tel.: þ1 541 737 9151; fax: þ1 541 737 1877. E-mail address: [email protected] (Y. Zhao). 0023-6438/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.lwt.2013.11.006

gravitational forces. For good product quality and accurate weight control, coating solution viscosity is a key parameter and the surface of food should better be flat. Spraying is another widely used way for applying coatings. This technique offers uniform coating, thickness control, and the possibility of successive applications which does not contaminate the coating solution (Andrade et al., 2012). It has been reported that bovine gelatin is successfully spray-coated onto fresh meat including beef tenderloins, pork loins, salmon fillets, and chicken breasts to improve their storage qualities (Antoniewski, Barringer, Knipe, & Zerby, 2007). Recently, there is a growing effort to adapt electrostatic spraying technology in food industry. Electrostatic spraying, beginning in the paint industry, has a series of superiorities than traditional spraying technique. This method can control the droplet size, increase the droplet coverage and deposition, produce homogenous distribution, and reduce wastage. It has been reported that the application efficiency can be increased up to 80% with 50% less spray dosage by using electrostatic spraying (Maski & Durairaj, 2010). Nam, Seo, Jo, and Ahn (2011) indicated that electrostatic spraying of ascorbic acid at 500 mg/kg could efficiently prevent both lipid oxidation and color change in ground beef (Nam et al., 2011). Ganesh, Hettiarachchy, Griffis, Martin, and Ricke (2012) pointed out that electrostatic spray of food-grade acids and plant extracts are more effective compared with conventional spray in decontaminating Escherichia coli O157:H7 on spinach and iceberg lettuce (Ganesh

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Y. Zhong et al. / LWT - Food Science and Technology 56 (2014) 1e8

et al., 2012). The selection of an appropriate coating method not only impacts the preservation effect of the coatings formed on the food items, but also determines the production cost and process efficiency. Unfortunately, a very few studies have compared the performances of these different coating methods on the same food product. To evaluate the behavior of edible coatings, coating thickness is an important parameter since it can influence the morphology, opacity, mechanical property, and barrier ability of resulted films. Generally, a micrometer is used to measure the thickness of edible films. However, this method can’t directly reflect the thickness of coatings applied on food surface. Hsu, Weng, Liao, and Chen (2005) chose Near-infrared Fourier transform Raman technology to determine the thickness of zein coatings on apples by correlating the Raman intensity ratio of characteristic peaks versus the film thickness (Hsu et al., 2005). Unfortunately, this method needs long analytical process, expensive instrument, and professional spectral analysis skills. In this study, a rapid and easy optical method was developed to observe the coating thickness without employing expensive instruments. The performances of edible coatings not only depend on the coating methods employed, but also the properties of the coating materials (type, amount, density, viscosity, and surface tension). Many natural materials have the potential to make well performing edible coatings, including proteins, polysaccharides, and lipids (AlHassan & Norziah, 2012). Among these nature biopolymers, chitosan, sodium alginate and soy protein isolate are the three most promising coating materials (Janjarasskul & Krochta, 2010). Chitosan obtained by the deacetylation of chitin has good film-forming ability, excellent biocompatibility and wide antibacterial spectrum. Sodium alginate, a natural linear polysaccharide, has good moisture-retention capacity, gel-forming and biocompatibility. Soy protein isolate is digestible and nourishing, and has shown excellent oxygen barrier property (Skurtys et al., 2010). The applications of these three materials as edible coating on various food products have been widely discussed (Elsabee & Abdou, 2013; Pizato, CortezVega, de Souza, Prentice-Hernandez, & Borges, 2013; Ramos, Fernandes, Silva, Pintado, & Malcata, 2012). Hence, it is worthy to study the responses of these different coating materials to the above mentioned coating methods, and their behaviors on a given food item. For the same coating material and coating method, the protection effect of an edible coating will also depend on the composition, surface morphology, size, and shape of the product matrix to be coated. Cheese, a widely consumed food mainly consisting of casein, fat, and water, is an ideal food model to study the diversity of coating process due to its smooth surface, uniform texture, and regular shape. Hence, in the present study, four different coating methods including dipping, enrobing, spraying and electrostatic spraying were studied by employing three kinds of coating solutions (chitosan, sodium alginate, and soy protein isolate) to evaluate the comprehensive coating performances of these methods, especially their impacts on film morphology and their preservation effects, and consequently guide the smart selection of coating materials and successive industrial applications of edible coatings. 2. Materials and methods 2.1. Materials Chitosan with 300 kDa of molecular weight and 88% of degree of deacetylated was purchased from Primex (Siglufjordur, Iceland). Sodium alginate (TICA-alginÒ 400 powder) was obtained from TIC Gums (Belcamp, MD). Soy protein isolate (with 90% protein) was

acquired from Cargill Inc. (Minneapolis, MN). Glycerol from IBI Scientific Inc. (Peosta, IA) and acetic acid from Fisher Scientific Inc. (Fair Lawn, NJ) were used as plasticizer and acid solvent, respectively. Low-moisture Mozzarella cheese (Tillamook County Creamery Assoc., Tillamook, OR) was obtained from a local supermarket in Corvallis, OR, USA, and stored in a refrigerator (4
C) before using. Cheese bricks were carefully cut into square pieces of 40  40  4 mm (approximately 9 g) by a cheese slicer. 2.2. Preparation of coating solutions Three different coating solutions were prepared according to the procedures described in the literature (Kanatt, Rao, Chawla, & Sharma, 2013; Shon, Eo, & Eun, 2010; Sipahi, Castell-Perez, Moreira, Gomes, & Castillo, 2013) and our preliminary experiments: (1) 2 g/100 g chitosan and 0.5 g/100 g glycerol (as plasticizer) (chitosan: glycerol ¼ 1:4) were dissolved in 1 g/100 g acetic acid (CA); (2) 1 g/100 g sodium alginate and 0.25 g/100 g glycerol (sodium alginate: glycerol ¼ 1:4) were dissolved in DI water (SA); and (3) 5 g/100 g soy protein isolate and 1.25 g/100 g glycerol (soy protein isolate: glycerol ¼ 1:4) were dissolved in DI water and heated at 80
C water bath for 45 min (SP). All coating solutions were filtered through nylon mesh to remove insoluble particles. 2.3. Coating methods Four different coating methods were chosen: (1) dipping (D); (2) enrobing (E); (3) spraying (S); and (4) electrostatic spraying (ES). In order to get thin films and control film thickness, the operation time was fixed. Generally, the coating time was set to 30 s for CA and SA, and 20 s for SP except for dipping method. During dipping process, the coating time was 15 s for SP and 20 s for CA and SA. After coating process, all of the samples were drained on stainless steel screens and air-dried in a laminar airflow bench for 1 h, and then were put into plastic “clam-shell” containers with four samples in each box (Industry standard, 9756Z, Pactiv Corp., Mexico) and stored at 4
C for 14 d. Concerning dipping method, each sample was directly dipped into coating solution for the given time mentioned above. For enrobing method, each coating solution was pumped up by a peristaltic pump through a 6 mm inner diameter plastic tube. Dripping speed was controlled at 50 mL/min, and the dripping nozzle ca. 2 cm above each sample was moved around by hand to promote fully coating. Both spraying and electrostatic spraying processes were carried out by an SC-ET Sprayer (Electrostatic Spraying Systems Inc., Watkinsville, GA). The spray gun was perpendicularly fixed ca. 50 cm over a metal tray with 3 samples laying out as triangle on it. During the spraying process, spray rate and feeding pressure were set at 3.8 L/h and 1.8 kg/cm2, respectively. For electrostatic spraying, voltage was set as 7.5 kV and load current of 60 mA. 2.4. Properties of coating solutions (CS) Total soluble solid content (TSS) was measured at room temperature in triplicate by an RA-250 HE digital refractometer (Kyoto Electronics Manufacturing Co. Ltd., Kyoto, Japan). Viscosity was measured using a model DV-III programmable rheometer (Brookfield Engineering Laboratories, Middleboro, MA) with a 91 spindle at steady shear rate (30 rpm), and each solution was measured for six times at room temperature. Density was calculated by the weight of each solution divided by its volume (fixed to 25 mL) and was expressed as g/mL.

Y. Zhong et al. / LWT - Food Science and Technology 56 (2014) 1e8

Surface tension was measured with a FTÅ 10 automated surface tensiometer (First Ten Angstroms, Portsmouth, VA) at room temperature and repeated three times for each solution. Contact angle was measured by a FTÅ 135 Contact Angle and Video Analysis System (First Ten Angstroms, Portsmouth, VA). A 5.0 mL of coating solution was pipetted onto the clean glass surface and polystyrene surface, respectively. Six replications were carried out for each sample. 2.5. Coating thickness A method to clearly observe the coating (film) thickness formed on the surface of Mozzarella cheese was developed in our laboratory. In brief, green fluorescent dry powders were added into coating solution (0.01 g/100 mL solution) and the solution was then homogenizing for 60 s. One day after cheese being coated, cheese samples were observed by a Nikon Eclipse E400 microscope (Nikon Co., Tokyo, Japan) equipped with an extended digital camera (Q imaging, Surrey, BC, Canada) at the lens position of 40 magnitude. A small slice of cheese (ca. 4  4  2 mm) was carefully cut from middle surface of each cheese sample, the cross section was placed parallel and the film thickness was observed with the help of a handmade UV lamp as an extra light source. The vertical length of the fluorescent band determined by camera software was defined as the film thickness, and six random locations were chosen to calculated thickness. 2.6. Microstructure of coated surfaces The microstructure of cheese surfaces was observed by a Leica MZ16 7.1e11.59 stereo-microscope equipped with a Leica DFC 300 video camera (Nikon Co., Tokyo, Japan) at the lens position of 60  magnitude. 2.7. Changes of physicochemical properties of coated cheese during refrigeration storage Cheese samples were placed in a refrigerator (4
C, 85% RH) for 14 d. Four samples from each treatment were taken out at 1, 7, and 14 d for the following test except for weight loss test. Uncoated cheese was chosen as control. For each cheese sample, two circular discs (f 10 mm  4 mm) were gotten out with a puncher for texture test, and the rest sample was used for color measurement. For determining weight loss, three samples were taken out at 1, 7, 10, and 14 d. Weight loss was registered by monitoring the weight change of each sample and calculated as percentage weight lost from the initial sample weight. Texture property of the sample was measured using a TA-XT2i texture analyzer (Stable Micro Systems Ltd., Godalming, UK) and eight random locations (edge and middle on the same cheese) were tested in each sample. The hardness of the cheese was measured by compression mode with an 8 mm diameter cylinder penetrometer probe. The test parameters were: 1.0 mm/s of prespeed and post-speed, 0.4 mm/s of test speed, 50% of distance and rupture test of 1.0%. Color of the cheese surface was monitored by an MS/S-4500L Lab-Scan II colorimeter (HunterLab, Inc., Reston, VA). The colorimeter was pre-standardized by a white (X ¼ 99.86, Y ¼ 99.83, Z ¼ 94.66) and a black ceramic plate. For each treatment, 6 random locations were chosen and the data were reported as CIE L*, a*, and b* values, where L* was 0 for black and 100 for white, a* represented red (þ) to green (), and b* values indicated yellow (þ) to blue ().

3

2.8. Experimental design and statistical analysis A complete randomized experimental design was applied concerning three coating solutions and four coating methods. Considering the property of coating solution, one-way ANOVA was performed to evaluate the significant mean difference (P < 0.05). For comparing the effects of treatment and storage time on the cheese properties, two-way ANOVA was carried out using SAS software (version 13.0, Statistical Package for the Social Sciences Inc., Chicago, IL).

3. Results and discussion 3.1. Properties of coating solutions The viscosity of coating solution can impact the spreadability and uniformity of liquid coating layer, which in turn affect the thickness and performance of the resulted film. As seen in Table 1, the viscosity of the SA was the highest (ca. 0.155 Pa s) which was about 3 times higher than that of the other two solutions. Low viscosity can provide processing advantage during enrobing or spraying operations, whereas high viscosity provides better application in dipping method (Skurtys et al., 2010). The viscosity values were affected by the nature of polymer, solvent and their interactions. During solution forming process, amine groups of chitosan was protonated in acidic solution, and the repulsion between adjacent ionized amino groups led to chitosan chain expansion, thus chitosan solution behaved as a polycationic electrolyte (Park & Zhao, 2004). Sodium alginate, consisted of D-mannuronic acid and L-guluronic acid, contained substantial hydrophilic groups. With adding water, water molecules immersed into crystal lattices of sodium alginate, distributed between two hydrophilic layers and formed three-dimensional networks (Davidovich-Pinhas & Bianco-Peled, 2010). During SP solution heating, the native conformations of globular protein were disrupted, the inside hydrophobic groups were exposed, some new inter- and intra-molecular interactions were formed, and stereoscopic matrices were subsequently established when the extended peptide chains associated (Pereira, Souza, Cerqueira, Teixeira, & Vicente, 2010). The reason why the viscosity of sodium alginate was the highest even though its concentration was the smallest (1 g/100 g) was hypothesized as more entangled systems were developed and stronger interactions among molecules were formed in sodium alginate solution, which resulted in a tighter and more compact structure. Total soluble solids (TSS) is a general term refers to substances dissolved in water in an aqueous solution. It is routinely used to assess sugar content in the solution. In the present study, TSS of the coating solutions ranged from 1.20% to 4.76%, and with increasing the concentration of solute resulted in an approximately linear increase of TSS value (TSS ¼ 1.1214*Contentþ0.47, R ¼ 0.9572) regardless of the solute types (Table 1).

Table 1 Basic physicochemical properties of film forming solutions.y

y

Coating Solutionz

Viscosity*1000 (Pa s)

TSS (%)

Density (g/L)

pH

CH SA SP

44.18  3.21b ⁕ 155.53  3.87a 46.67  6.65b

3.30  0.00b 1.20  0.10c 4.76  0.15a

1.474  0.014a 1.485  0.002a 1.491  0.003a

4.49  0.04c 6.16  0.01b 7.06  0.04a

Results were the mean value  standard deviation. CH: chitosan; SA: sodium alginate; SP: soy protein isolate. ⁕ Different lowercase letters in the same column indicated significant differences (P < 0.05). z

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Y. Zhong et al. / LWT - Food Science and Technology 56 (2014) 1e8

Density is the intrinsic property of a solution. As shown in Table 1, the density values of three coating solutions were all around 1.5 g/L and displayed no significant difference (P > 0.05) despite of the concentration difference. Sodium alginate and chitosan may possess much more hydrophilic groups and can form more compact structures with water than protein; consequently, the weights of the solutions were similar at the same volume. Since the pH value of a coating solution can affect its stability (such as viscosity change) and functionality (such as antibacterial property), it is necessary to measure the initial pH of the coating solutions. The pH values of sodium alginate and soy protein isolate solutions were approximately neutral, whereas pH of chitosan solution was acidic (pH ¼ 4.5) (Table 1). It was reported that chitosan was insoluble with pH over pKa 6.3 (Chen et al., 2012); while in slightly acidic solution, abundant amino groups in chitosan were protonized which exhibited good biocompatibility and antimicrobial ability. Moreover, Holme, Lindmo, Kristiansen, and Smidsrød (2003) and Holme, Davidsen, Kristiansen, and Smidsrød (2008) indicated that the highest stability of alginate solution is in a pH interval of 5e8 (Holme et al., 2008; Holme et al., 2003). 3.2. Surface wettability of coating solutions Surface tension is a contractive tendency of the surface of a liquid drip that allows it to resist an external force. Comparing three coating solutions, the surface tension of SA was 78.4 N/m, about 11.6 and 26.7 N/m higher than that of CA and SP, which indicated that the cohesion of SA was the highest (Table 2). Choi, Park, Ahn, Lee, and Lee (2002) indicated that the surface tension of 1.5% chitosan and 1% alginate coating solution was 70.9 and 61.5 N/m, respectively (Choi et al., 2002). In order to accomplish favorable preservative goals, coating solution should uniformly spread on the food and thereafter form a film that has adequate adhesion, cohesion, and durability. The most popular measurement of wettability for a liquidesolid interaction is the contact angle q. Generally, lower contact angle means easier spread of liquid on the solid. Both the liquid droplet properties and the interactions between liquid and solid matrix affect the contact angle of the droplet. As contact angle was sensitive to the roughness and levelness of the substrate, smooth and horizontal glass and polystyrene substrates were used to study the wetting behaviors of coating solutions on different polar solids matrix instead of real food surface. The contact angles on glass substrates were all below 55
, indicating the hydrophilic character of the coating solutions. Besides, there were no significant differences (P > 0.05) between the q values of CH and SP on glass. Considering contact angle on polystyrene substrate, all obtained values were lower than 90
which demonstrated that the coating solutions can also spread on hydrophobic substrate. Compared with sodium alginate, chitosan solution was easier to spread on glass matrix (q was 13.4
lower) while the wettability on polystyrene was poorer (q was 9.6
higher). As for SP, the contact angles were lower on both substrates. In aqueous

Table 2 Surface wettability of film forming solutions.y

y

Coating Solutionsz

Surface tension (N)

CH SA SP

66.8  0.9b ⁕ 78.4  0.1a 51.7  1.8c

Contact angle (
) Glass plate

Plastic plate

40.3  3.5b 53.7  1.9a 40.2  3.0b

88.1  2.3a 78.5  2.4b 82.2  3.2b

Results were the mean values  standard deviation. CH: chitosan; SA: sodium alginate; SP: soy protein isolate. ⁕ Different lowercase letters in the same column indicated significant differences (P < 0.05).

z

solution, SP molecules existed in globular forms consisting of a hydrophilic shell and a hydrophobic kernel (Teng, Luo, & Wang, 2012), and behaved as a surfactant which lowered the surface tension of the solution and improved the wettability of the coating. 3.3. Coating thickness Thickness of edible coating must be well controlled to create certain modified atmosphere to suppress respiration rate of horticultural products or delay the quality deterioration of abiosis foods. Both contact method (simple but not actual) and non-contact method might be used to measure the film thickness. Common non-contact methods to directly observe film thickness from food surface include optical or scanning electron microscope, confocal Raman microspectrometry, Fourier transform-Raman spectrometer and surface enhanced Raman scattering method (Skurtys et al., 2010). In this study, a simple visual way to determine the film thickness was developed. Since the three coating materials had similar film-thickness patterns by different coating methods, only the thickness images of CH coating was reported as an example. As illustrated in Fig. 1, the film surface by dipping or enrobing method was compact and smooth without pores or cracks. Taking in account of both spraying methods, film thickness obviously decreased but film surface appeared rough with apparent protuberances distributed randomly. Moreover, film surface by electrostatic spraying was smoother than by spraying method. Six random points were chosen to calculate film thickness. Thickness values by dipping method for CH, SA and SP coatings were ca 66.6, 71.9, and 81.8 mm, respectively, which coincided with the properties of each coating solution. As SA possessed high viscosity and was easier to spread out on hydrophobic surface and SP had low surface tension and was amphipathic, these two coating solutions resulted in higher coating pick-up and thus higher film thickness. Film thickness by enrobing was close to the values by dipping and ranged from 64.6 to 83.3 mm. Spraying led to the smallest thickness with the values being 30.6, 54.2 and 68.5 mm for CH, SA and SP, respectively. Electrostatic spraying enhanced the adhesion of CS on cheese surface and the film thickness increased ca. 10 mm compared with the spraying method. 3.4. Appearance and microstructure of coated cheese surfaces The cheese samples were photographed immediately after being coated to study the coating distribution on cheese (Fig. 2). In order to eliminate the effect of nature light, all the samples were photographed at same locations of the test bed with a fluorescent lamp ca. 1 m right above the samples. Cheese surface was totally covered by coatings using dipping and enrobing method (Fig. 2A). For the two spraying methods, SP completely laid over cheese surface, while there appeared uncoated locations for CH. Spraying can produce fine spray droplets with size distribution up to 20 mm whereas electrospraying is capable to produce uniform particles of less than 100 nm from biopolymer solutions (Skurtys et al., 2010). However, solution wettability can affect coating spreadability. Since CH couldn’t effectively adhere to cheese, cheese was just partial wetting by CH. Besides, the uniformity of SA coating was also poor using spraying method which was mainly due to its high viscosity. After the thin film being formed, all the cheese surfaces looked smooth and homogeneous (Fig. 2B). At 7 d, edible films started wrinkled with greasy appearance (Fig. 2C). At the end of our experiment, the wrinkles of the coatings were more obvious and the cheese color (in the web version) turned to be yellowish-green (Fig. 2D). Stereo-microscope was used to observe the microstructure of coating surfaces. All cheese surfaces showed smooth macroscopic texture at 1 d, but the surface-microstructures differed according

Y. Zhong et al. / LWT - Food Science and Technology 56 (2014) 1e8

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Fig. 1. Thickness of chitosan coatings (films) by different coating methods (A: dipping; B: enrobing; C: spraying; D: electrostatic spraying). The vertical length of the green fluorescent band was defined as the coating thickness, and six random locations were chosen to calculate the thickness. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

to coating methods (Fig. 3). Numerous small particles aggregated on cheese surface by electrostatic spraying and cheese surfaces coated by SP were rougher than the other two coating materials (Fig. 3A). During electrostatic spraying, small droplets speedily jetted on cheese surface layer by layer with short time to spread, which generated aggregation of solution droplets. Jiang, Li, Chai, and Leng (2010) reported that protein molecules form fine strands and twist wormlike chains during film forming process, which could explain rougher surface of protein-coated cheese in

the present study (Jiang et al., 2010). During storage, the roughness of cheese surface enhanced especially in SP coated cheese (Fig. 3B, C). It was reported that cheese was a homogeneous polyblend composed of casein matrix and fine fat globules uniformly dispersed (Guinee, Auty, & Fenelon, 2000). As time went by, fat globules tended to clump and coalesce, which resulted in inhomogeneous texture. Since the compatibilization between casein and SP was better than other two materials, fat migration was more obvious in SP coated cheese.

Fig. 2. Photographs of cheese surface immediately after coating and during storage at 4
C for up to 14 d. CH: chitosan; SA: sodium alginate; SP: soy protein isolate. D: dipping; E: enrobing; S: spraying; ES: electrostatic spraying.

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3.5. Physicochemical properties of cheese during storage 3.5.1. Weight loss In all cases the weight loss increased linearly with storage time of which was possibly due to the continuous moisture movement from cheese to surrounding environment. For example, weight loss rates were 1.54%, 2.12%, and 1.85% per day for uncoated cheese (R2 ¼ 0.99), SA coated cheese by dipping (R2 ¼ 0.99) and SP coated cheese by electrostatic spraying (R2 ¼ 0.99). Generally, coated cheeses had lower weight loss comparing with uncoated cheeses (Bermúdez-Aguirre & Barbosa-Cánovas, 2010; Ramos et al., 2012). However, in the present study, the weight loss of coated cheese was higher than uncoated cheese, especially for two spraying methods. At 7 d, the weight loss of spraying coated cheese was ca. 14%, approximately 25% higher than values of other cheese. We used the cheese weight after 1 day storage for initial weight. At that time, the edible coatings were still wet and moisture of coated cheeses was significantly higher than that of uncoated cheeses, essentially due to the water present in the coating itself. During storage, moisture evaporated from both coating and cheese, and weight loss of coated cheese consequently enhanced. Although spraying methods led to thinner films which should mean less water loss, the films were heterogeneous and some parts of cheese surface were not even covered, weight loss was thus particularly stronger. As for coating materials, SP coated cheese had higher weight loss compared with the other two materials, which can be explained by its less hydrophily. 3.5.2. Hardness Hardness analysis was done based on the compression test in this study. The hardness of uncoated cheese was 75.88 N at the first day (Table 3). After being coated, hardness values of cheese exhibited significant declines (P < 0.05). The hydration of coated cheese might contribute to a decrease in compared with control (Guerra-Martínez, Montejano, & Martín-del-Campo, 2012). The hardness of cheese increased significantly (P < 0.05) over time and the increased degree ranged from 410.2 %to 990.5% within the whole time. Previous study reported that the hardness increase of fresh Panela cheese is correlated with the moisture level during storage (Guerra-Martínez et al., 2012). In the present study, the hardness of Mozzarella cheese increased dramatically with water evaporation, but was more obvious than water loss trend (Table 3). It was also found that edible coatings generally delay the hardened process of cheese and SA coatings produce the softest cheese

Fig. 3. Microstructures of cheese surface during refrigeration storage at 4
C for up to 14 d. CH: chitosan; SA: sodium alginate; SP: soy protein isolate. D: dipping; E: enrobing; S: spraying; ES: electrostatic spraying.

Table 3 Weight loss and hardness changes of coated cheese during storage at 4
C.y Treatmentz

Weight loss (%)

Hardness (N)

7d Control D

E

S

ES

y

CH SA SP CH SA SP CH SA SP CH SA SP

11.31 10.13 11.24 11.22 11.62 10.62 11.66 13.58 13.58 13.82 12.72 14.52 16.42

10 d             

1.13 2.54 3.09 1.98 0.74 2.20 1.99 1.88 4.14 2.54 2.54 1.43 3.10

cBC cC

⁕

cBC cBC cBC cBC cBC cABC bABC cABC bABC cAB cA

15.64 15.57 17.74 16.55 18.01 17.26 18.63 18.16 18.97 20.62 17.10 20.38 22.29

14 d             

0.79 2.90 2.45 1.24 1.19 2.05 2.40 1.36 4.94 2.34 2.10 2.53 2.93

bC bC bBC bBC bBC bBC bABC bABC bABC bAB bBC bAB bA

22.10 22.89 26.12 22.80 24.89 25.69 27.60 23.58 27.38 28.35 24.05 28.18 29.41

1d             

aE

0.91 2.13aE 3.48 aABCDE 0.35 aE 1.64 aBCDE 3.22 aABCDE 3.53 aABCD 1.58 aDE 4.23 aABCD 1.75 aAB 2.31 aCDE 2.93 aABC 1.85 aA

75.88 44.28 39.96 75.70 72.34 39.74 33.06 28.45 22.23 40.20 47.88 21.78 37.92

7d             

bA

3.74 6.22 cB 10.93 cBC 11.54 bCD 12.25 bA 15.28 bBC 8.37 bCD 5.10 bDE 4.53 bE 10.83 bBC 9.21 bB 4.54 bE 9.02 bBCD

14 d bC

87.0714.60 174.90  23.87 bA 106.78  30.05 bC 94.89  13.51 bC 45.41  8.33 bD 40.56  16.69 bD 17.38  1.79 bEF 91.13  21.33 bC 17.00  7.17 bEF 34.96  11.66 bDE 149.15  31.51 bB 9.95  2.41 bF 41.14  8.03 bD

420.68 284.79 278.06 349.65 376.52 273.34 200.61 351.86 113.41 325.67 522.13 135.34 340.11

            

109.09 aAB 33.49 aCD 65.73 aCD 63.21 aBC 147.12 aBC 169.73 aCD 42.89 aDE 122.17 aBC 75.80 aE 155.06 aBCD 203.73 aA 49.71 aE 92.43 aBC

Results were the mean value  standard deviation. CH: chitosan; SA: sodium alginate; SP: soy protein isolate. D: dipping; E: enrobing; S: spraying; ES: electrostatic spraying. ⁕ Different lowercase letters in the same row indicated significant differences (P < 0.05) by storage time, and different capital letters in the same column indicated significant differences (P < 0.05) by different coating solutions and methods. z

aBC

bBCD

bCDE

aB

bE

aA

aBC

bE

bE

aCDE

bDE

aBC aB

aDE

aB

abC

bF

bCD

bA

bG

aCD

bG

aEF

bC

bA

cD

bA

bB

bCD

cCD

bB

aBC

aAB

bG

aCD

aA

bF

aBCD

aBCD

aEF

bFG

aDE

aAB

aA

aCD

aA

aA

aBC

bD

aA

aCD

aA

aC

bB

bB

cDEF

bF

bA

cF

bB

bCD

bF

cEF

bBC

aC

aAB

bG

aABC

aA

bEF

aCD

aC

aDE

bEF

aC

ES

S

E

Results were the mean values  standard deviation. CH: chitosan; SA: sodium alginate; SP: soy protein isolate. D: dipping; E: enrobing; S: spraying; ES: electrostatic spraying. ⁕ Different lowercase letters in the same row indicated significant differences (P < 0.05) by storage time, and different capital letters in the same column indicated significant differences (P < 0.05) by different coating solutions and methods. z

y

aB

aDEF

aB

aB

aBC

aB

aA

aEF

aB

aBC

aF

15.17 15.09 16.06 14.84 14.64 17.96 22.85 14.41 19.14 16.44 17.12 17.70 18.46 aCD

17.39 17.47 15.99 18.51 18.93 16.18 22.07 19.23 18.62 19.08 18.90 16.31 19.71 aC

16.96 17.68 15.66 13.24 17.02 13.24 20.84 16.98 14.99 17.75 18.97 16.48 19.23 bC

3.10 3.15 2.61 3.31 3.40 2.72 1.96 3.61 1.90 3.20 3.20cC 2.39bB 2.66bB aFG

1.90 2.01 1.71 2.04 1.85 1.57 1.54 1.98 1.28 1.65 2.20 1.40 1.51 aD

1.70 1.88 1.68 1.36 1.80 1.36 1.89 1.65 1.34 1.41 1.74 1.38 1.50 cDE

57.67 60.16 64.89 57.58 53.93 62.64 67.96 53.65 75.36 55.05 58.09 68.36 61.28 aBC

             72.56 78.08 77.81 74.42 75.03 77.87 76.92 74.36 80.9 78.90 69.2 80.44 77.49 Control D

CH SA SP CH SA SP CH SA SP CH SA SP

76.81 75.33 79.17 80.25 75.78 80.25 78.19 79.62 81.74 80.37 76.02 81.00 77.48

            

aEFG

⁕ 1.68 2.48 bG aBCD 1.53 1.52 aAB 2.29 aFG 1.53 aAB 0.71 aCDE 0.96 aBC 0.95 aA 1.17 aAB 2.10 aFG 1.22 aAB -1.50 aDEF

7d 1d

L* Treatmentz

Table 4 Color changes of coated cheese during storage at 4
C.y

2.68 0.85 1.24 1.76 1.79 0.79 0.87 2.55 1.16 3.11 3.27 2.72 1.68

bF

14 d

            

2.08 2.23 1.13 3.54 3.10 6.12 2.77 4.81 2.81 3.83 4.34 5.72 7.32

cEF

1d

a*

            

0.18 0.10 0.10 0.17 0.08 0.17 0.08 0.13 0.16 0.08 0.11 0.12 0.23

aC

7d

            

0.26 0.12 0.19 0.14 0.14 0.16 0.08 0.17 0.09 0.22 0.22 0.13 0.18

aEF

14 d

            

0.15 0.29 0.14 0.32 0.21 0.44 0.25 0.46 0.17 0.41 0.37 0.40 0.44

bC

b*

1d

            

0.90 0.31 0.95 1.04 1.38 1.04 0.85 0.54 0.94 0.78 0.79 1.56 0.69

aCD

7d

            

0.80 0.22 0.70 0.48 1.41 2.25 0.65 0.90 1.00 1.86 0.33 0.93 0.93

aCDE

14 d

            

8.29 1.12 0.39 1.94 1.94 2.68 0.94 2.09 1.05 2.42 1.36 2.14 1.96

bDE

Y. Zhong et al. / LWT - Food Science and Technology 56 (2014) 1e8

7

texture especially applied by two spraying methods, which may attribute to its favorable water retention ability. 3.5.3. Color Coating solution had obvious impact on color changes while the effect of coating method was inconspicuous (Table 4). Cheese became more luminous after being coated, especially for SA coated cheese with luminosity increasing more than 3%. The luminosity of cheese significantly decreased (P < 0.05) during storage. At the end of the storage, the luminosity of uncoated cheese decreased 24.9%, and the luminosity-change values of SA coated cheese were less than 21.9%. Bermúdez-Aguirre and Barbosa-Cánovas (2010) explained the reason for luminosity decline as microbial growth on the cheese surface (Bermúdez-Aguirre & Barbosa-Cánovas, 2010). The greenness of cheese generally showed less significant difference within 7 d and then enhanced at 14 d with CH coated cheese being greener. It was indicated that greenness values of cheese ranged from green region (a) to red region (þa) and had increased or decreased trends during 4
C storage by different cheese varieties (Bermúdez-Aguirre & Barbosa-Cánovas, 2010). The b* values in Table 4 showed major contribution to the yellow region. The yellowness of cheese enhanced (P < 0.05) or unchanged (P > 0.05) during 7 d storage, whereas the yellowish values generally decreased at last except the value change trend of SA coated cheese. It was also reported that low fat cheeses have a slightly greenish tint associated with fewer light-scattering centers when being cooled (Fife, McMahon, & Oberg, 1996). 4. Conclusions Spraying and electrostatic spraying coating methods led to thinner film thickness whereas film structures were more homogeneous using dipping and enrobing coating methods. Considering the properties of coated cheese, SA solution resulted in better overall qualities which can be explained by its better spreadability on hydrophobic cheese surface, but coating method did not have significant influence on the property changes of cheese during refrigeration storage. As two spraying methods produced thinner film with almost equal preservation abilities compared with dipping or enrobing methods and had advantages for saving raw materials and better process control, their applications in food industry would lower the production cost and help the implementation of full automatic production. However, proceeding parameters of spraying methods especially for electrostatic spraying are needed to optimize in the future to control deposition uniformity of the coating and obtain the best preservation effect. References Al-Hassan, A. A., & Norziah, M. H. (2012). Starch-gelatin edible films: water vapor permeability and mechanical properties as affected by plasticizers. Food Hydrocolloids, 26(1), 108e117. Andrade, R. D., Skurtys, O., & Osorio, F. A. (2012). Atomizing spray systems for application of edible coatings. Comprehensive Reviews in Food Science and Food Safety, 11(3), 323e337. Antoniewski, M. N., Barringer, S., Knipe, C., & Zerby, H. (2007). Effect of a gelatin coating on the shelf life of fresh meat. Journal of Food Science, 72(6), E382eE387. Bermúdez-Aguirre, D., & Barbosa-Cánovas, G. V. (2010). Processing of soft Hispanic cheese (“Queso Fresco”) using thermo-sonicated milk: a study of physicochemical characteristics and storage life. Journal of Food Science, 75(9), S548e S558. Bhat, Z. F., Kumar, P., & Kumar, S. (2013). Effect of skin, enrobing and refrigerated storage on the quality characteristics of chicken meat balls. Journal of Food Science and Technology-mysore, 50(5), 890e899. Chen, C., Liu, M., Gao, C., Lü, S., Chen, J., Yu, X., et al. (2012). A convenient way to synthesize comb-shaped chitosan-graft-poly (N-isopropylacrylamide) copolymer. Carbohydrate Polymers, 92(1), 621e628. Choi, W., Park, H., Ahn, D., Lee, J., & Lee, C. (2002). Wettability of chitosan coating solution on ‘Fuji’apple skin. Journal of Food Science, 67(7), 2668e2672.

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