Mikro/nano bor nitrür ilaveli polilaktik asit (PLA) kompozit filmlerin geliştirilmesi, karakterizasyonu ve gıda validasyonu
Development and characterization of micro/nano boron nitride added polylactic acid (PLA) composite films and food validation
Yazar: İLKNUR ARA
Danışman: PROF. DR. ZEHRA AYHAN
Sakarya Üniversitesi / Fen Bilimleri Enstitüsü / Gıda Mühendisliği Ana Bilim Dalı
Derece: Yüksek Lisans, 2023
Konu: Gıda Mühendisliği = Food Engineering
Dizin: Gıda ambalajı = Food packaging

Özet:
Bu tez kapsamında bir biyopolimer olan Polilaktik asite (PLA) mikro ve nano bor nitrür ilave edilerek kompozit filmler geliştirilmesi ve karakterizasyonu amaçlanmıştır. PLA matrisine % 1, 3, 5, 7, 10 (w/w) oranlarında nano ve mikro bor nitrür ve plastikleştirici (PEG 400) ilave edilerek ve çözelti dökme yöntemiyle kompozit filmler hazırlanmış ve  75°C'de 30 dakika kurutulmuştur. Elde edilen filmler fiziksel (renk ve kalınlık ölçümü), mekanik (gerilme mukavemeti, kopma anındaki uzama oranı ve ısıl yapışma mukavemeti), bariyer (su buharı geçiş hızı, WVTR), antimikrobiyal aktivite ve morfolojik (FE-SEM) özellikler açısından analiz edilmiştir. Özellikle, mekanik, ısıl yapışma ve bariyer özellikler dikkate alınarak seçilen filmler tavuk etinde raf ömrünü belirlemek için kullanılmıştır.   Torba ambalaj haline getirilen mikro/nano PLA ambalajlarda MAP altında (%40 CO2 ve %60 N2) hazırlanan tavuk eti 4±1°C'de 8 gün depolanmış ve renk, pH, nem, toplam mezofilik bakteri ve duyusal değerlendirme analizleri yapılmıştır.
Çözelti dökme yöntemiyle hazırlanan tüm filmler homojen dağılım ve yüzeyden parçalanmadan soyulabilirlik göstermiştir. PLA matrisine eklenen mikro/nano bor nitrür oranı artıkça transparanlık azalmış, beyazlık artmıştır. Mekanik özellikler açısından incelendiğinde kontrol  PLA filmine kıyasla %1 ve %3 mikro bor nitrür ilave edilmesi gerilme mukavemetini %28 ve %11 oranında, %1 ve %3 nano bor nitrür ilavesi gerilme mukavemetini %40 ve %20 oranında arttırmıştır. Ancak %3 üzerinde mikro/nano bor nitrür ilavesi gerilme mukavemetinde azalmaya neden olmuştur. Kontrol filmine kıyasla  %1 mikro/nano bor nitrür içeren filmlerin kopma anındaki uzama oranı artarken %1 üzerinde ilave edilen oranlarda kopma anındaki uzama oranı azalmıştır. Kontrol, %1, %3 nano ve mikro bor nitrür içeren PLA filmler ısıl olarak yapıştırabilmiştir. Diğer filmlerde ise ısıl yapışma zayıf olduğundan sızıntı tespit edilmiştir. Isıl yapışma mukavemeti açısından incelendiğinde kontrol PLA filmi 207,33 N/m, %1 mikro bor nitrür içeren film 285,33 N/m ve %1 nano bor nitrür içeren film ise 305,22 N/m göstermiştir. PLA matrisine %1 nano bor nitrür eklendiğinde filmlerin su buharı geçiş hızı %52 oranında azalmıştır. Mikro ve nano bor nitrür içeren filmlerde disk difüzyon yöntemine göre antimikrobiyal aktivite tespit edilmemiştir. FE-SEM ile incelenen morfolojik yapıda katkısız PLA'nın homojen ve pürüzsüz olduğu tespit edilmiştir. %1 ve %3 oranlarında mikro ve nano bor nitrür içeren PLA filmlerde homojen ve pürüzsüz yapı korunmuştur. %5 ve daha fazla bor nitrür eklenen filmlerde aglomerasyon gözlenmiştir. Nano bor nitrür içeren PLA filmlerin mikro bor nitrür içeren filmlere kıyasla mekanik, bariyer ve morfolojik özellikler bakımından daha iyi sonuç verdiği belirlenmiştir. Raf ömrü çalışmasında kontrol PLA, %1 mikro ve nano bor nitrür içeren PLA filmler kullanılmıştır. Tavuk etlerinin 4. güne kadar karakteristik pembe rengini koruduğu tespit edilmiştir. Toplam mezofilik bakteri sayısı 6. günden sonra bozulma sınırına ulaşmıştır. Duyusal açıdan tüm tavuk etleri 4. güne kadar kabul edilebilir düzeyde bulunmuştur. Sonuç olarak kontrol, %1 mikro ve nano bor nitrür dolgulu filmler 4 gün boyunca tavuğun tazeliğini korumuştur.
Tüm filmlerin fiziksel, mekanik, yapısal analiz sonuçları ve tavuğun raf ömrüne etkisi dikkate alındığında en uygun formülasyonun % 1 mikro ve nano bor nitrür içeren filmler olduğu belirlenmiştir. Mikrobiyolojik ve duyusal analiz sonuçları birlikte değerlendirildiğinde kontrol PLA, %1 mikro ve nano PLA torbalarda ambalajlanan tavuk etlerinin raf ömrünün +4 C'de %40 CO2 ve %60 N2 MAP koşullarında 4-5 günle sınırlı olduğu belirtilebilir.

Abstract:
The objective of this study was to development and characterization of composite polylactic acid (PLA) films incoporated with micro and nano boron nitride. For this purpose, film solutions were prepared by adding 1, 3, 5, 7, 10 % (w/w) micro and nano boron nitride using plasticizer (PEG 400) to PLA matrix. The films were produced by solvent casting method and dried at 75C for 30 minutes. The obtained films  were characterized for  physical (color and thickness), mechanical  (tensile strength, elongation at break and seal strength), barrier (water vapor transimission rate), antimicrobial activity and morphological structure. The selected films (control PLA, PLA with 1% micro and nano boron nitride) were used to package chicken meat under MAP (40% CO2 and 60% N2) and packaged products were stored at 4±1°C for 8 days for shelf life study.
5% (w/w) PLA solution was prepared using chloroform as the solvent. 10% plasticizer (PEG 400) was added to the solution. Micro/nano boron nitride at 1, 3, 5, 7 and 10% was dissolved with dimethylformamide (DMF). The solutions were mixed with a magnetic stirrer at 60°C for 12 hours at 700 rpm. All films prepared by solvent casting method showed homogeneous and smooth distribution with very good film forming property, as well as easy peelability from the glass petri dishes. The polymer solution prepared in the same way to be used for shelf life analyzes was poured into 20x20 cm glass plates with a thickness of 500 µm. Petri dishes and plates were placed in an oven and dried at 75oC for 30 minutes. After 30 minutes, the films were removed from the oven, peeled from the plates, and placed in an environment of 50±5% relative humidity at 23±2oC.
According to color analysis L*, a*, b* increased as the micro/nano boron nitride concentration increased compared to the control PLA films. Increase in micro and nano boron nitride concentration increased E of the PLA composite films, however, there was no significant difference between E values of PLA films with the same concentration of micro and nano boron nitride except PLA films with 3% micro and nano boron nitride. As micro/nano boron nitride concentration increased in PLA matrix films transparency decreased, the whiteness and thickness increased compared to the control films. 
Mechanical properties as tensile strength and elongation at break were significantly affected by the micro/nano boron nitride concentration. In terms of tensile strength (MPa) in micro boron nitride (BN) films, the highest value was obtained in 1% micro boron nitride added film with 42.12 MPa, and the lowest value was obtained in 19.57 MPa in 10% micro boron nitride added film. Compared to the control PLA films, the tensile strength of 1% and 3% micro BN-PLA films increased by 28%, and 11%, respectively. However, decrease in tensile strength was observed after 5% micro BN concentration. For PLA films incorporated with nano boron nitride, the highest tensile strength (MPa) was 46.06 MPa for 1% nano boron nitride added films and the lowest value was 23.15 MPa for 10% nano boron nitride added films. Compared to the control films, 1% nano BN increased tensile strength by 40%, and 3% nano BN increased by 20%, while a decrease in tensile strength was observed after 5% nano BN concentration.
In terms of the elongation at break (%), the highest values were 2.83% for control PLA film, 2.81% for 1% micro BN and 2.92% for 1% nano BN added films, while the lowest value is 1.59% for 10% micro boron nitride film. However, there was no statistical difference between control PLA films and PLA with 1% micro and nano boron nitride. Elongation or the flexibility of the composite PLA films decreased as the boron nitride concentration was higher than 3%. Adding higher than 3% micro/nano boron nitride did not make a statistical difference in terms of elongation at break comparing to control PLA films.
The produced control and composite PLA films were heat sealed at  100°C for 5, 10, 15 seconds and tested for heat seal integrity by dye penetration test using rhodamine B solution. Leakage was observed in all films sealed for 5 and 10 seconds at 100°C. Leakage was observed in the films containing 5%, 7%, 10% micro/nano boron nitride sealed at 100°C for 15 seconds. No leakage was observed in the control PLA, 1% and 3% micro/nano boron nitride, sealed at 100°C for 15 seconds and these films were used for shelf life study.
Control PLA, 1% BN, 3% BN, 1% NBN, 3% NBN films-heat-sealed at 100 °C for 15 seconds did not show any dye leakage in the sealed area. The highest heat seal strength was obtained as 305.22 N/m for 1% nano BN film. The lowest value was  obtained with 153.78 N/m for 3% micro BN film. Compared to the control films, heat seal strength of the 1% nano BN film increased by 47.3% and the 1% micro BN film by 37.7%.
In terms of water vapor transmission rate (WVTR), the lowest value was obtained in the film with 1% nano BN, as 8.68 g/(m2/day), and the highest value was obtained in the control film with 13.23 g/(m2/day). The addition of 1% nano BN to the PLA matrix resulted in a significant reduction (52%) in the water vapor transmission rate.There is no significant difference between 1% and 3% micro BN and 3% nano BN added films. All results show that when micro/nano boron nitride is added to the PLA matrix, the water vapor transmission rate of the films can be reduced by up to 50% increasing the barrier property to water vapor which is important for food packaging.
Antimicrobial activity against S. aureus and E. coli was not detected by disk diffusion method in any of the films containing micro and nano boron nitride.
When the FE-SEM images of the films were examined, the surface roughness increased and homogeneity decreased in both dimensions with the addition of more than 3% boron nitride.
Considering all the analysis results, films containing control PLA films, PLA films with 1% micro/nano boron nitride were selected to determine the shelf life of chicken meat. The PLA films were converted to bags by heat sealing and chicken meat was packaged in the prepared bags under modified atmosphere packaging and stored at 4C for 8 days. The packaged products were monitored for color, pH, moisture, total mesophilic bacteria and sensory analysis at cold storage at every two days.  
L* values of chicken meats in control PLA films decreased significantly during storage (p≤0.05). L* values of chicken meats in BN films did not change significantly during storage days (p>0.05). The L* values of the chicken meats in the NBN films decreased significantly after the 4th day of storage (p≤0.05). The a* values of the chickens in the control, micro BN and nano BN groups did not change significantly after the 4th day (p>0.05). It explains that after the 4th day of storage, the characteristic light pink color of chicken meat turns red/brown. The b*(yellowness) values of the chicken meats in the control, micro BN and nano BN groups did not change significantly during entire storage (p>0.05).
The pH value of chicken meat packaged in control PLA, PLA/BN and PLA/NBN slightly increased during storage. 
The moisture content of chicken meat remained constant throughout the entire storage period. Although the chicken looked dried on the surface, it did not affect the overall moisture content of the product.
Total mesophilic bacterial count (TMB) of chicken breast increased during storage for all packaging groups. TMAB reached critical limit of 6 log cfu/g in the 4th day for control PLA package and PLA package with micro BN, and 5th day for PLA packages with nano BN packages. All products were spoilt at storage of 6 days.
It was observed that chicken meats packaged with all three materials received acceptable scores until the end of the 4th day in terms of sensory qualities (appearance-color, smell, texture, taste) and overall acceptability. Although texture was acceptable on 6th day of storage for all packages, the other sensory attributes and overall acceptability were not acceptable for most of the applications.
Considering microbiological and sensory analysis, the suggested shelf life of chicken meat packaged with kontrol PLA, %1 micro ve nano boron nitride incorporated PLA bags under MAP (40% CO2 ve 60% N2)  at +4 C was 4-5 days. Although the packaging films used were thinner than the petroleum based materials used in the commercial poultry packaging, they are still comparable in terms of shelf life.
In conclusion, PLA based biocomposite materials containing boron nitride could be suggested as an alternative to petroleum-derived plastic for food packaging. In particular, nano boron nitride improved the mechanical and water vapor barrier properties of PLA. It has been determined that boron nitride added films can be used in MAP applications, which is very widely used food packaging technology. The use of biodegradable materials such as PLA, especially in products with a short shelf life such as chicken meat, will contribute to reducing waste management problems, which are a serious problem in terms of environmental pollution. PLA-boron nitride composite films could be further improved by incorporating active agents especially antimicrobial components to extend the shelf life of food products with a short shelf life and high enonomic value.