Received: 23 January 2017
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Revised: 26 March 2017
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Accepted: 27 March 2017
DOI: 10.1111/jfpp.13362
ORIGINAL ARTICLE
Evaluation of PLA nanocomposite films on physicochemical and microbiological properties of refrigerated cottage cheese Wenhui Li1 | Lin Li2 | Heng Zhang1 | Minglong Yuan3 1 Institute of Yunnan Food Safety, Kunming University of Science and Technology, Kunming, 650550, China 2
College of Light Industry and Food Science, South China University of Technology, Guangzhou, 510640, China
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Yuyue Qin1
Abstract A novel antimicrobial packaging system was prepared by incorporating TiO2 or Ag nanoparticles into poly(lactic acid) (PLA) matrix. The effect of PLA, PLA/TiO2, and PLA/TiO2 1 Ag film on the physicochemical and microbiological properties of Yunnan cottage cheese stored at 5 6 1 8C for 25 days was investigated. The low density polyethylene (LDPE) film was used as the control. The
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Engineering Research Center of Biopolymer Functional Materials of Yunnan, Yunnan Minzu University, Kunming, 650500, China Correspondence Yuyue Qin, Institute of Yunnan Food Safety, Kunming University of Science and Technology, Kunming 650550, China. Email:
[email protected] Funding information National Natural Science Foundation of China, Grant/Award Number: 21576126; Scientific Research Foundation of Yunnan Educational Commission, China, Grant/ Award Number: 1405186038
weight loss, pH value, lactic acid bacteria count (LAB), texture, sensory quality, and antimicrobial activity were determined. Cheeses packed by PLA/TiO2 and PLA/TiO21Ag film provided better retention in pH value, LAB, sensory quality, and antimicrobial activity compared with those packed by PLA and LDPE film. Migration of Ti and Ag nanoparticles was lower than the limit of 10 mg/kg as defined by EFSA for food contact materials. The results indicated that the incorporation of TiO2 or Ag nanoparticles into PLA matrix could maintain cheese quality and prolong its shelf life to 25 days.
Practical applications Yunnan cottage cheese is one of the most popular cheeses in China. However, the shelf life of unpacked Yunnan cottage cheese is only 5–7 days under refrigerated condition. PLA film incorporated with antimicrobial TiO2 or Ag nanoparticles has great potential to prolong cheese shelf life. The novel packaging material may be an effective alternative for cheese preservation.
1 | INTRODUCTION
packaging for cheese. Dukalska et al. (2011) compared the effect of traditional packaging material and PLA on a soft cheese. They
In recent years, the research on the production of innovative food
confirmed that the PLA film could be alternative to the conventional
packaging materials has received considerable attention due to the
packaging material due to its good results on cheese shelf life. Conte
growing field of preparation of advanced functional composites and
et al. (2013) reported a PLA coating embedding copper nanoparticles
nanocomposites (Gorrasi et al., 2016). Active packaging for food is
was developed as a new antimicrobial active packaging for fiordilatte
gaining interest due to increased demands on food safety, consumer
cheese. Proliferation of food pathogenic microorganisms was inhibited
convenience, shelf life extension, cost efficiency, and environmental
with a consequent preservation of sensory attributes.
issues (Dalzini et al., 2016). Antimicrobial packaging is an important
In order to further impart new functionalities to PLA-based antimi-
technology to delay or inhibit the growth of pathogenic microorgan-
crobial packaging, some suitable nanoparticles were incorporated into
isms in food stuff. Biopolymer-based films are usually used for the
the PLA matrix (Ahmed, Varshney, Auras, & Hwang, 2010; Conte et al.,
preparation of antimicrobial packaging system combining the advan-
2013). TiO2 nanoparticle has received an increasing interest due to its
tages of biopolymer and the antimicrobial properties of additives (Del
nontoxicity, environmental compatibility, accessibility, and chemical
Nobile et al., 2009).
stability (Mofokeng & Luyt, 2015). It is usually used as a photocatalytic
Poly(lactic acid) (PLA) is a polyester derived from lactic acid mono-
antimicrobial substance for packaging application (Lin et al., 2014).
mer. It is one of the most promising polymers due to its versatility,
It has strong antimicrobial activity against a wide spectrum of microor-
superior mechanical properties, and low cost (Lemmouchi et al., 2009).
ganisms. It has been approved by the US Food and Drug Administra-
PLA has been approved by the Food and Drug Administration (FDA)
tion (FDA) for use in the food industry (Bodaghi et al., 2013). Previous
for use in food-contact materials (Ortiz-Vazquez, Shin, Soto-Valdez, &
study reported that the addition of TiO2 to high density polyethylene
Auras, 2011). Several authors had reported that PLA was used as
could obtain useful photoactive filmable materials for food packaging
J Food Process Preserv. 2018;42:e13362. https://doi.org/10.1111/jfpp.13362
wileyonlinelibrary.com/journal/jfpp
C 2017 Wiley Periodicals, Inc. V
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applications (Gumiero et al., 2013). Nano-Ag is a promising antimicro-
solution was poured onto a polytetrafluoroethylene 200 mm 3
bial material. Silver nanoparticles can attach to the cell membranes and
200 mm dish and dried in a vacuum oven at ambient temperature. The
penetrate into bacteria (Emamifar, Kadivar, Shahedi, & Soleimanian-
PLA film with 3 wt % nano-TiO2 was named as PLA/TiO2 nanocompo-
Zad, 2010; Jiang, Feng, & Wang, 2013). The larger surface area of
site film. The PLA film with 2 wt % nano-TiO2 plus 1 wt % nano-Ag
nanoparticles can effectively inhibit the growth of bacteria. They
was prepared as previous described and named as PLA/TiO21Ag
have been widely used in food packaging, water filtration, textiles,
nanocomposite film.
and health care due to their outstanding antimicrobial properties (Azlin-Hasim et al., 2015). To improve properties of the resulted nanocomposite films, the nanoparticles should be well dispersed into the polymer matrix. The amount of nanofiller should be no more than 3 wt % to prevent agglomeration of the nanoparticles (Bodaghi et al., 2013; Lin et al., 2014). In our previous work, we have prepared PLA/TiO2 composites with different amount of nano-TiO2 (0, 1, 3, and 5 wt %). The best compromise between barrier, mechanical, antimicrobial, and thermal properties could be achieved by the incorporation of 3 wt % nano-filler (Data was not shown in this study). So, 3 wt % nano-filler was selected to use in this study. However, a shortcoming of using nanoparticles in food packaging is that nanoparticles would migrate from packaging materials toward the packaged food. Migration tests must be conducted when a new nanocomposite packaging material is designed (Panea, Ripoll, Gonzalez, Fernandez-Cuello, & Albertí, 2014). The aim of the present study was to evaluate the effect of a combination of TiO2 and Ag nanoparticles in PLA packaging on physicochemical and microbiological properties of cottage cheese, during storage at 5 6 1 8C for 25 days. Migration tests were also performed.
2.3 | Packaging of Yunnan cottage cheese For shelf life test, 20 g of Yunnan cottage cheese (50 mm 3 50 mm 3 5 mm) was packaged in individual pouches (200 mm 3 200 mm) of different packaging materials. For PLA treatment, the cheese sample was packaged by pure PLA film. For PLA/TiO2 treatment, the cheese sample was packaged by PLA/TiO2 nanocomposite film. For PLA/ TiO2 1 Ag treatment, the cheese sample was packaged by PLA/ TiO2 1 Ag nanocomposite film. For low-density polyethylene (LDPE) treatment, the cheese sample was packaged by commercial LDPE film. The cheese samples were stored at 5 6 1 8C for 25 days. At 0, 5, 10, 15, 20, and 25 days of storage, weight loss, pH value, texture, sensory evaluation, and microbiological properties of cheese were analyzed. The thickness of LDPE, PLA, PLA/TiO2, and PLA/TiO21Ag film was 80 6 1 lm, 78 6 3 lm, 92 6 5 lm, and 95 6 5 lm, respectively. The water vapor permeability of LDPE, PLA, PLA/TiO2, and PLA/ TiO2 1 Ag film was 1.44 3 10213, 5.25 3 10213, 7.91 3 10213, and 8.46 3 10213 kg m/m2sPa, respectively.
2.4 | Weight loss and pH value Weight loss of the cheese sample was determined gravimetrically. On
2 | MATERIAL AND METHODS
the first day and each sampling day, the percentage of weight loss was determined. The result is expressed as the percentage weight loss of
2.1 | Materials PLA (Mw 5 280 kDa, Mw/Mn 5 1.98) used in this work was obtained from Natureworks LLC (Nebraska, USA). Nano-TiO2 powder with an average particle size less than 100 nm and purity of 99.5% was obtained from Sigma (St. Louis, MO, USA) and used as received. NanoTiO2 plus nano-Ag powder (a combination of 95% Nano-TiO2 powder plus 5% nano-Ag with average particle diameter of about 10 nm) was
initial weight. The pH value of 20 mL of homogenized cottage cheese samples stored in different packaging was determined by a digital pH-meter (PHS-3C, INESA Scientific Instrument Co., Ltd, Shanghai, China). The reported values are the average of three replicates.
2.5 | Texture analysis
obtained from Shanghai Jianghutio Production Co., Ltd. (Shanghai,
Textural property of the cheese sample was measured using a texture
China). Dichloromethane was obtained from Chengdu Kelong Chemical
analyzer (TA-XT, Stable Microsystems, London, UK) equipped with an
Co., Ltd (Sichuan, China). Yunnan cottage cheese was purchased in a
8-mm diameter cylindrical probe. The cheese samples (50 mm 3
local market (Kunming, China).
50 mm 3 5 mm) were kept at ambient temperature for 30–45 min prior to test. The test speed was 1.0 mm/s during penetration. The tex-
2.2 | Film preparation The film production procedure used in this work was similar to Qin et al. (2014). The PLA resins were dried overnight in a vacuum oven at 60 8C prior to film preparation. About 4 g of PLA was added into 80 mL of dichloromethane solution with stirring until PLA resins dissolved completely. Then, 3 wt % nanopartilces based on PLA dry mat-
tural parameters such as hardness (N), springiness (cm), cohesiveness (ratio), and chewiness (J) of the cheese were measured by the method of International Dairy Federation (1991). The texture analysis was carried out three times.
2.6 | Sensory evaluation
ter was added to the polymer solution by vigorous stirring and the
The sensory evaluation was carried out by a panel consisting of 10
resulted solution was sonicated for 2 h to distribute nanoparticles into
experienced assessors from Institute of Yunnan Food Safety, Kunming
the polymer matrix. The homogeneous PLA/nano-TiO2 film forming
University of Science and Technology (Kunming, China). The panel
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evaluated odor, color, texture, and overall acceptability of cottage cheese samples using a 0–10 point scale (0 5 dislike extremely, 10 5 like extremely). A score of 6 was defined as good and limit of marketability. The samples were presented in a random order and served at room temperature. The average data were calculated.
2.7 | Antimicrobial activity For microbiological analysis, Yunnan cottage cheese (10 g) were transferred aseptically into 90 mL of sterile 0.85% (w/v) NaCl solution and homogenized in a stomacher lab blender. Serial decimal dilutions were prepared in sterile peptone water and plated on specific media. The following microbial counts were evaluated: total bacterial count, on plate count agar (Oxoid, London, England) at 30 8C for 48 h; lactic acid bacteria count (LAB), on double layer deMan-Rogosa-Sharp agar (Oxoid, London, England) at 30 8C for 48 h; yeasts and molds, on peptone dextrose agar
Changes in weight loss of Yunnan cottage cheese during storage period
FIGURE 1
(Oxoid, London, England) at 30 8C for 48 h; respectively. All counts were the average of two different samples and expressed as log CFU/mL.
longing the storage period. This indicated that moisture content of cheese decreased with increase of storage period. This was probably
2.8 | Migration test
related to the continuous moisture movement from cheese to surrounding environment.
Migration of TiO2 and Ag nanoparticles from PLA matrix was analyzed using hexane as aqueous food simulant at 40 8C for 25 days (Beigmohammadi et al., 2016). The aforementioned storage time for this test was chosen based on the actual case for use. The PLA/TiO2 and PLA/TiO2 1 Ag nanocomposite film were cut into pieces of 3 cm 3 3 cm and weighed. Then, the samples were immersed in 30 mL of simulant solution. At 0, 5, 10, 15, 20, and 25 days of storage, the amount of TiO2 and Ag nanoparticles in the aqueous food simulant was determined by an inductively coupled plasma atomic emission spectrophotometer (ICP-AES, Optima 8000, Perkin Elmer, USA). The migration test was performed in triplicate and the result was expressed as mg/kg. TiO2 and Ag nanoparticles migration from the nanocomposite films to the cheese surface was determined according to De Oliveira, de Fatima Ferreira Soares, Pereira, and de Freitas Fraga, (2007). The cheese rind was removed from the cheese surface at 25 days of storage. About 5 g of cheese rind sample was mixed with 10 mL of extraction solvent. The homogenized solution was filtered and analyzed by ICP-AES. The migration analysis was carried out three times.
2.9 | Statistical analysis The means of the results were analyzed by the analysis of variance
In general, cheese packed by coatings or films had lower weight loss during storage compared with the unpacked ones (Ehsannia & Sanjabi, 2016). Weight loss of cheese packed by PLA, PLA/TiO2, and PLA/TiO2 1 Ag film was significantly (p < .05) higher than that of LDPE group after 5 days of storage. In this study, cheese samples were packed by packaging films with different water vapor barrier ability. Several factors had been reported to affect water vapor permeability, including film thickness, film composition, and film preparation methods (Martins, Cerqueira, Souza, Carmo Avides, & Vicente, 2010). The commercial LDPE film was manufactured by extrusion method. The water vapor permeability of LDPE film was 1.44 3 10213 kg m/m2sPa. The PLA, PLA/TiO2, and PLA/TiO2 1 Ag film were prepared by solvent casting method in our laboratory. The water vapor permeability of PLA, PLA/TiO2, and PLA/TiO2 1 Ag film was 5.25 3 10213, 7.91 3 10213, and 8.46 3 10213 kg m/m2sPa, respectively. The water vapor permeability of films changed with different fillers. The sequence of water vapor barrier ability of these films was LDPE film > PLA film > PLA/TiO2 film > PLA/TiO2 1 Ag film. The decrease in water barrier properties of PLA/TiO2 and PLA/TiO2 1 Ag film could be due to the existence of nanoparticles in the PLA matrix. The content of filler higher than 2% would lead to some voids in the films and permitted more water vapor transfer (Yang et al., 2013).
(ANOVA) using SPSS software package (version 13.0). Duncan’s multiple range test was used to compare differences (p < .05).
3 | RESULTS AND DISCUSSION 3.1 | Weight loss
3.2 | pH value and LAB count The pH was determined on refrigerated cottage cheese stored in different packaging. The pH evolution as function of time was shown in Figure 2a. All the samples showed a sharp decrease during the first 5 days of storage. Then, pH value gradually decreased as the time
Weight loss values for Yunnan cottage cheese during storage period
increased. For LDPE group, the reduction of pH value was significantly
were shown in Figure 1. The moisture content of fresh Yunnan cottage
(p < .05) more rapid than other groups after 15 days of storage. The
cheese was 48.8%. In all of the groups, weight loss increased by pro-
pH value of all samples reached 5.35–5.68 at the end of refrigerated
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3.3 | Texture analysis Textural parameters (hardness, springiness, cohesiveness, and chewiness) of the Yunnan cottage cheese stored at 5 6 1 8C for 25 days were presented in Table 1. The hardness of all groups gradually increased over time. For cheese samples packed by PLA/TiO2 and PLA/TiO2 1 Ag film, the hardness was significantly (p < .05) higher than that of PLA and LDPE groups. There was no significant (p > .05) difference in hardness value between PLA/TiO2 and PLA/TiO2 1 Ag groups during the whole storage period. For PLA/TiO2 and PLA/TiO2 1 Ag groups, hardness reached the highest value of 59.85 and 60.25 N, respectively, after 25 days of storage. The hardness of cheese sample packed by PLA film was significantly (p < .05) higher than that of LDPE group during the whole storage period. The increase in hardness could be attributed to the loss of moisture throughout refrigerated storage (Fontecha, Kalab, Medina, Pelaez, & Ju arez, 1996; Jeon, Lee, Ganesan, & Kwak, 2012; Youssef, El-Sayed, Salama, El-Sayed, & Dufresne, 2015). Zhong, Cavender, and Zhao, (2014) reported that coatings or films generally delay the hardened process of cheese and sodium alginate coatings produced softer cheese texture, which might be due to its favorable water retention ability. In this study, all of the cheese samples were packed by different packaging films. The difference in hardness among the LDPE, PLA, PLA/ TiO2, and PLA/TiO2 1 Ag groups might be due to different water vapor barrier ability of the LDPE, PLA, PLA/TiO2, and PLA/TiO2 1 Ag film. In all of the groups, springiness and cohesiveness did not show any significant (p > .05) difference during the whole storage period. Youssef et al. (2015) also found that values of cohesiveness, springiness, and Changes in pH value (a) and LAB count (b) of Yunnan cottage cheese during storage period
FIGURE 2
storage. There was no significant (p > .05) difference in pH value
gumminess showed few differences in cheese coated by different materials. The chewiness of all groups gradually increased as time proceeding. There were significant (p < .05) differences in chewiness among the four groups after 10 days of storage. The chewiness of
between PLA/TiO2 and PLA/TiO2 1 Ag groups during the whole stor-
Yunnan cottage cheese packed by different packaging materials was
age period. The difference in pH value among different packaging
also affected by storage period due to the loss of moisture. A similar
might be due to the presence of TiO2 or Ag nanoparticles dispersed
trend was also found in hardness (Jeon et al. 2012).
into the nanocomposite antimicrobial film and to the action of active nanoparticles on the microbial community evolution that could change
3.4 | Sensory evaluation
the balance of organic acids (Gorrasi et al., 2016). The results of LAB count were shown in Figure 2b. Counts of LAB
Sensory evaluation such as odor, color, texture, and overall acceptability
gradually increased as the time increased. The LAB counts of all sam-
of cottage cheese were examined on a 10-point hedonic scale. The sen-
ples were found to be 10 –10 CFU/g at the end of refrigerated stor-
sory scores of cottage cheese were listed in Table 2. The values for all
age. The LAB count of PLA and LDPE groups was significantly (p < .05)
considered parameters decreased with increasing storage time for all
6
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higher than that of PLA/TiO2 and PLA/TiO21Ag groups after 20 days
groups. There was no significant (p > .05) difference in sensory scores
of storage. The reduction of pH value was accompanied by an increase
for all packaging treatments on day 5. However, cottage cheese packed
in LAB count during the whole storage time. The acidity levels
by the PLA, PLA/TiO2, and PLA/TiO2 1 Ag film showed significant
increased for all the treatments during storage. The pH and LAB count
(p < .05) higher odor, texture, and overall acceptability scores than that
variations might be due to the increase in lactose conversion to lactic
packed by LDPE film after 10 days of storage. The odor, texture, and
acid and/or other organic acids (Ehsannia & Sanjabi, 2016). This was
overall acceptability scores of PLA/TiO2 and PLA/TiO2 1 Ag groups
also related to the increase in mesophilic microorganism population.
were still higher than 6 score and the cottage cheese samples main-
Some studies also reported a similar trend for changes of pH value and
tained proper characteristics at the end of the storage time. Texture is a
LAB count during cheese storage period (Ricciardi et al., 2015).
very important sensory parameter for cheese, affecting the overall
Because acidity development could lead to flavor and texture changes,
acceptability. The pH value had a major effect on cheese texture (Ehsan-
control of the acidity during storage is very important to maintain the
nia & Sanjabi, 2016). The trend in texture and overall acceptability
cottage cheese quality.
changes was similar to that in pH value. It could be seen from Table 2,
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T AB LE 1
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Effect of different packaging on texture of refrigerated cottage cheese Hardness (N)
Springiness (cm)
Cohesiveness (ratio)
Chewiness (J)
26.70 6 0.57
0.19 6 0.01
0.09 6 0.002
0.41 6 0.01
Day 5 LDPE PLA PLA/TiO2 PLA/TiO2 1 Ag
28.71 6 2.24a 39.50 6 1.23b 51.00 6 2.59c 53.55 6 0.30c
0.15 6 0.02a 0.17 6 0.02a 0.16 6 0.01a 0.18 6 0.01a
0.09 6 0.006a 0.09 6 0.001a 0.08 6 0.001a 0.08 6 0.001a
0.55 6 0.01b 0.45 6 0.00a 0.55 6 0.01b 0.78 6 0.03c
Day 10 LDPE PLA PLA/TiO2 PLA/TiO2 1 Ag
36.70 6 0.54a 41.85 6 2.73b 54.05 6 1.07c 56.40 6 3.72c
0.18 6 0.01a 0.15 6 0.01a 0.17 6 0.00a 0.18 6 0.02a
0.10 6 0.002a 0.12 6 0.005a 0.09 6 0.003a 0.11 6 0.010a
0.73 6 0.01a 3.66 6 0.11b 5.19 6 0.18c 8.51 6 0.08d
Day 15 LDPE PLA PLA/TiO2 PLA/TiO2 1 Ag
37.26 6 1.45a 46.34 6 2.37b 55.87 6 0.88c 57.56 6 1.13c
0.18 6 0.01a 0.16 6 0.01a 0.18 6 0.01a 0.17 6 0.02a
0.11 6 0.001a 0.10 6 0.002a 0.09 6 0.001a 0.13 6 0.005a
1.92 6 0.01a 4.38 6 0.02b 7.35 6 0.05c 9.78 6 0.15d
Day 20 LDPE PLA PLA/TiO2 PLA/TiO2 1 Ag
38.65 6 2.54a 48.70 6 2.48b 56.50 6 0.29c 58.25 6 3.02c
0.20 6 0.01a 0.17 6 0.00a 0.19 6 0.01a 0.16 6 0.02a
0.09 6 0.001a 0.08 6 0.005a 0.09 6 0.008a 0.09 6 0.003a
2.68 6 0.03a 5.72 6 0.14b 7.81 6 0.02c 10.81 6 0.11d
Day 25 LDPE PLA PLA/TiO2 PLA/TiO2 1 Ag
38.30 6 1.03a 48.90 6 3.47b 59.85 6 3.84c 60.25 6 2.86c
0.19 6 0.00a 0.19 6 0.01a 0.21 6 0.01a 0.20 6 0.02a
0.09 6 0.006a 0.09 6 0.001a 0.09 6 0.001a 0.10 6 0.007a
3.75 6 0.08a 6.68 6 0.07b 8.90 6 0.05c 11.95 6 0.70d
Treatments Day 0
a–c
Values followed by different letters in the same column were significantly different (p < .05), where a is the lowest value.
the overall acceptability of cottage cheese was strongly affected by
3.5 | Antimicrobial activity
odor. The Yunnan cottage cheese is a relatively mild flavored cheese that does not develop much additional flavor during storage. The cottage cheese packed by LDPE and PLA film had a significant (p < .05) higher value in color between 10 and 20 days of storage. This might be because that TiO2 or silver nanoparticles provoked a change of the cheese surface color after a few days of storage. However, odor and to a lesser extent texture are indicators
Total bacterial count, yeasts, and molds of the cheese samples during storage period were shown in Figure 3. The present total bacterial count (4.42 log cfu/g) of fresh Yunnan cottage cheese was not agree € k (2015) with cheese reported by other authors. Akarca, Tomar, and Go reported 5.17 log cfu/g for total aerobic bacteria count in sliced Mozzarella cheese, whereas Papaioannou, Chouliara, Karatapanis, Kontomi-
which most frequently cause unacceptability of the cheese (Ricciardi
nas, and Savvaidis (2007) described 3.3 log cfu/g for total viable count
et al., 2015).
in Greek whey cheese.
Considering a score of 6 as corresponding to good and limit of
It could be seen from Figure 3a, total bacterial count in all groups
marketability, cottage cheese samples packed by the LDPE, PLA, PLA/
gradually increased as the time increased. Total bacterial count in PLA/
TiO2, and PLA/TiO2 1 Ag film achieved a shelf life of 15, 20, 25, and
TiO2 and PLA/TiO2 1 Ag group was significantly (p < .05) lower than
25 days, respectively. The results suggested that PLA/TiO2 and PLA/
that in LDPE and PLA group during the whole storage period. There
TiO2 1 Ag film could improve the quality of cottage cheese products
was no significant (p > .05) difference in total bacterial count between
during refrigerated storage. Similarly, Conte et al. (2013) reported that
LDPE and PLA group during the whole storage period. Antimicrobial
nanocomposite PLA coatings incorporated with copper nanoparticles
nanoparticles were not incorporated into LDPE and PLA film. The pure
could inhibit microbial proliferation and extend the sensory attributes
LDPE and PLA film could not inhibit the growth of microorganisms.
of fiordilatte cheese. Mastromatteo et al. (2015) indicated that a silver-
The incorporation of TiO2 or Ag nanoparticles into polymer matrix
based nanocomposite coating could preserve the microbiological qual-
could effectively inhibit the growth of microorganisms in Yunnan cottage
ity of fiordilatte cheese without compromising the sensory quality. The
cheese. However, there was no significant (p > .05) difference in total
trend in sensory attributes of Yunnan cottage cheese was also accord-
bacterial count between PLA/TiO2 and PLA/TiO2 1 Ag group during the
ance to microbiological trend.
first 15 days of storage. Then, total bacterial count in PLA/TiO2 1 Ag
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Effect of different packaging on the sensory evaluation of refrigerated cottage cheese
Treatments
Odor
Color
Texture
Overall acceptability
10.00
10.00
10.00
10.00
Day 5 LDPE PLA PLA/TiO2 PLA/TiO2 1 Ag
8.89 6 0.29a 9.02 6 0.54a 9.16 6 0.45a 9.05 6 0.32a
9.02 6 0.27a 8.68 6 0.12a 8.64 6 0.25a 8.86 6 0.17a
8.56 6 0.26a 8.45 6 0.12a 8.52 6 0.04a 8.56 6 0.25a
8.35 6 0.19a 8.26 6 0.25a 8.30 6 0.15a 8.35 6 0.17a
Day 10 LDPE PLA PLA/TiO2 PLA/TiO2 1 Ag
7.23 6 0.28a 7.78 6 0.08b 7.87 6 0.10b 7.89 6 0.14b
8.89 6 0.06b 8.72 6 0.03b 8.38 6 0.22a 8.46 6 0.31a
7.68 6 0.23a 8.12 6 0.18b 8.32 6 0.16b 8.24 6 0.09b
7.70 6 0.11a 7.94 6 0.20ab 8.07 6 0.21b 8.06 6 0.10b
Day 15 LDPE PLA PLA/TiO2 PLA/TiO2 1 Ag
6.21 6 0.38a 7.24 6 0.07b 7.72 6 0.18c 7.68 6 0.09c
8.32 6 0.20b 8.09 6 0.04ab 7.90 6 0.16a 7.92 6 0.09a
6.86 6 0.11a 7.88 6 0.17b 7.88 6 0.20b 7.98 6 0.12b
6.85 6 0.08a 7.59 6 0.06b 7.78 6 0.18b 7.75 6 0.10b
Day 20 LDPE PLA PLA/TiO2 PLA/TiO2 1 Ag
6.04 6 0.27a 7.02 6 0.26b 7.12 6 0.27b 7.20 6 0.32b
7.62 6 0.08b 7.54 6 0.26b 7.12 6 0.30a 7.22 6 0.14a
5.32 6 0.06a 6.92 6 0.18b 7.12 6 0.20b 7.02 6 0.17b
6.52 6 0.09a 6.98 6 0.08b 7.09 6 0.24b 7.10 6 0.15b
Day 25 LDPE PLA PLA/TiO2 PLA/TiO2 1 Ag
4.28 6 0.24a 6.46 6 0.03b 6.68 6 0.23b 6.74 6 0.11b
6.78 6 0.15a 6.51 6 0.15a 6.62 6 0.05a 6.60 6 0.22a
4.68 6 0.05a 5.68 6 0.06b 6.98 6 0.18c 6.96 6 0.21c
4.74 6 0.01a 5.45 6 0.12b 6.64 6 0.21b 6.66 6 0.20b
Day 0
a–c
Values followed by different letters in the same column were significantly different (p < .05), where a is the lowest value..
group was significantly (p < .05) lower than that in PLA/TiO2 group after
It could be seen from Figure 3a that PLA/TiO2 1 Ag film exhibited
20 days of storage. The PLA/TiO2 nanocomposite film was prepared by
higher antimicrobial activity than the other three packaging films. Mix-
the addition of 3 wt % nano-TiO2 into PLA matrix and the PLA/
ing nano-TiO2 with a metallic form of nano-Ag improved the bacteri-
TiO2 1 Ag film contained 2 wt % nano-TiO2 plus 1 wt % nano-Ag. The
cidal activity of polymer packaging film due to the unique structural
nanofiller content and state of dispersion are very important factors
characteristic of nano-Ag dispersed on TiO2 surface. TiO2 could serve
determining the antimicrobial ability of nanocomposite packaging films.
as a solid anti-aggregation support to maintain the dispersion of nano-
Díez-Pascual and Díez-Vicente (2014) reported that the minimum TiO2
Ag and enhance its antimicrobial performance (Emamifar et al. 2010).
content required for effective microbial growth inhibition was 3.0 wt %.
Yeasts and molds are important microbial contaminants in dairy
On the other hand, the homogeneous nanoparticle distribution in the
products. They could cause sensorial change by hydrolysis of fats
nanocomposites was related to the strong microbial inactivation.
when they reach 7–8 log cfu/g in cheese (Temiz, 2010). The count of
The polyphenylsulfone/TiO2 nanocomposite film could effectively inhibit
yeasts and molds in fresh Yunnan cottage cheese is 2.86 log cfu/g. It
the growth of two food pathogenic bacteria (E. coli and S. aureus).
could be seen from Figure 3b, yeasts and molds in all groups increased
The antimicrobial mechanism of nano-Ag is related to membrane
over time. The use of PLA/TiO2 and PLA/TiO2 1 Ag film was more sig-
damage due to free radicals derived from the nano-Ag surface (Kim
nificantly (p < .05) efficient than LDPE and PLA film in inhibiting yeasts
et al., 2007). Furthermore, nano-Ag might accumulate in the bacterial
and molds growth. The yeast–mold count of the cheese samples
cytoplasmic membrane, causing a significant increase in membrane per-
packed by LDPE and PLA film sharply increased and was higher than 7
meability and leading to cell death (Rhim, Wang, & Hong, 2013). Jiang,
log cfu/g after 20 days of storage. However, cheese samples packed
et al. (2013) found that a small percentage (2.15 wt %) of metal nano-
by PLA/TiO2 and PLA/TiO2 1 Ag film only reached population above
particles in the composite packaging materials was enough to signifi-
5.90 and 5.60 log cfu/g at the end of storage period, respectively.
cantly enhance inactivation of E. coli. Alginate/nano-Ag coating had
There was no antimicrobial activity in the pure LDPE and PLA film.
contributed to a certain decrease in the microbial counts of shiitake
PLA/TiO2 and PLA/TiO2 1 Ag film could effectively suppress the
mushrooms. Rhim et al. (2013) reported that agar/nano-Ag composites
growth of yeasts and molds. Similar results on the inhibition of yeasts
with 2 wt % of Ag content showed strong antimicrobial activities
and molds growth by antimicrobial packaging films had been reported
against both Gram-negative and Gram-positive bacteria.
for cottage cheese (Gonçalves, Junqueira, Dos Santos Pires, Soares, &
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The amount of nanoparticles migration into hexane simulant solution. (a) Ti migration from PLA/TiO2 and PLA/ TiO2 1 Ag film; (b) Ag migration from PLA/TiO2 1 Ag film
FIGURE 4
Changes in total bacterial count (a), yeasts and molds (b) of Yunnan cottage cheese during storage period
FIGURE 3
jo, 2009), gorgonzola cheese (De Oliveira, et al. 2007), and fiordiArau
that the initial release of Ti might be from nanoparticles within the film
latte cheese (Conte et al., 2013). The shelf life of unpacked Yunnan
surface layer. The subsequent release of Ti from the interior part of the
cottage cheese is only 5–7 days under refrigerated condition and could
sample had to migrate out through the polymer matrix (Huang et al.,
be prolonged by active antimicrobial nanocomposite films.
2011). The ability of the PLA nanocomposite film to extend Ti release period could improve the antimicrobial ability in the food packaging
3.6 | Migration test
(Girdthep et al., 2014). The maximum amount of Ti migration from PLA/TiO2 film and PLA/TiO2 1 Ag film was 112.92 and 127.38 lg/kg,
The nanoparticles migration from the films to hexane simulant solution
respectively. It could be seen that the amount of Ti migration was a
was considered as a simulation of nanoparticles release to fatty-food in
small percentage when compared with that incorporated. This sug-
this study. The migration test was performed according to European
gested that the antimicrobial activity of nanocomposite packaging film
Food Safety Agency (EFSA), which is procedure for the evaluation of
could be exerted by direct contact rather than continuous release of
the additives migration from packaging materials coming into contact
the active component to food stuff (Lantano et al., 2014).
with food stuff.
The amount of Ag migration increased with storage time (Figure 4b).
Release behavior of nanoparticles is an important factor to evalu-
As could be seen from Figure 4b, the release of Ag ions was almost a
ate the applicability of the PLA nanocomposite film (Girdthep et al.,
linear function of storage time. The maximum Ag migration amount for
2014). The amount of Ti and Ag migration from PLA/TiO2 and PLA/
the PLA/TiO2 film was 135.58 lg/kg. The particle size of nano-TiO2
TiO2 1 Ag film into hexane simulant solution was shown in Figure 4a,b.
powder is 100 nm and that of nano-Ag is about 10 nm. The lower the
Ti migration from PLA/TiO2 and PLA/TiO2 1 Ag film gradually
nanoparticle dimension, the higher the release rate. This was because of
increased with storage time proceeding. The extent of Ti migration
the higher surface-to-volume ratio (Conte et al., 2013). Similar behavior
from PLA/TiO2 film was lower than that from PLA/TiO2 1 Ag film after
was also reported by other researchers with polymer-based nanocompo-
15 days of storage. Ti migrated from both film samples with a rapid ini-
site films regarding the migration of metallic nanoparticles like Ag
tial release rate within the first 15 days of storage. The result showed
(Girdthep et al., 2014), Cu (Conte et al., 2013), and Ti (Lin et al., 2014).
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The concentration of Ti migration from PLA/TiO2 and PLA/ TiO2 1 Ag film to cheese samples was 14.59 6 1.28 and 12.27 6 2.30 lg/kg, respectively. The quantity of Ag ions migration from PLA/ TiO2 1 Ag film to cheese was only 20.04 6 1.38 lg/kg at the end of storage period. Although the amount of Ti migration was higher than that of Ag ions, it was still far below the migration limit of 10 mg/kg as defined by EFSA for food contact materials. For PLA/TiO2 film, the amount of Ti migration to cheese samples (14.59 lg/kg) was much lower than that in hexane simulant solution (112.92 lg/kg). This was because that the liquid environment in this study would lead to overestimate the result when compared with the real amount that might migrate from the film sample to a solid food product as cheese (Lantano et al., 2014).
4 | CONCLUSION In conclusion, the addition of Ti and Ag nanoparticles to PLA packaging had antimicrobial effects on Yunnan cottage cheese. For PLA/TiO2 and PLA/TiO2 1 Ag groups, weight loss and hardness of cheese were higher than that of PLA and LDPE group. Adding antimicrobial nanoparticles to PLA packaging materials delayed the decrease in pH value. Application of the PLA/TiO2 and PLA/TiO2 1 Ag nanocomposite film could prolong the shelf life of Yunnan cottage cheese to 25 days without negative effects on sensory attributes. Total bacterial count, yeasts, and molds of the cheese samples were effectively inhibited by the PLA/TiO2 and PLA/TiO2 1 Ag nanocomposite film. The migration of Ti and Ag nanoparticles was lower than the limit of 10 mg/kg as defined by EFSA for food contact materials. So, the PLA/TiO2 and PLA/ TiO2 1 Ag film with nanoparticle fillers could be used as antimicrobial nanocomposite food packaging films without food safety risk.
AC KNOW LE DGME NT The study was financially supported by the National Natural Science Foundation of China (No. 21576126) and Scientific Research Foundation of Yunnan Educational Commission, China (No. 1405186038).
OR CID Minglong Yuan Yuyue Qin
http://orcid.org/0000-0003-3126-2540
http://orcid.org/0000-0002-6738-4550
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How to cite this article: Li W, Li L, Zhang H, Yuan M, Qin Y. Evaluation of PLA nanocomposite films on physicochemical and microbiological properties of refrigerated cottage cheese. J Food Process Preserv. 2018;42:e13362. https://doi.org/10.1111/ jfpp.13362
