Sıcaklığa duyarlı poliüretan bazlı ambalaj filmi geliştirilmesi ve taze çilekte uygulanması
Development of thermally sensitive polyurethane based packaging film and application on fresh strawberry
Yazar: İLKNUR İLHAN
Danışman: PROF. DR. GÜRBÜZ GÜNEŞ
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Gıda Mühendisliği Ana Bilim Dalı / Gıda Mühendisliği Bilim Dalı
Derece: Yüksek Lisans, 2019
Konu: Gıda Mühendisliği = Food Engineering

Özet:
Günümüzde gıda endüstrisinde minimum işlem görmüş taze meyve ve sebzeler tüketici tercihlerinde ön plana çıkmıştır. Bu tür ürünlerin raf ömrünü uzatmayı ve tarladan satışa pek çok aşamada gerçekleşen kayıpları azaltmayı hedef alan çeşitli uygulamalar yapılmaktadır. Modifiye atmosferde ambalajlama (MAA) bu uygulamalardan biridir. Ancak MAA'nın taze meyve ve sebze (TMS) ürünlerine uygulanması sırasında bu tür ürünlerin metabolik faaliyetleri sebebi ile çeşitli güçlüklerle karşılaşılmaktadır.
TMS'ler ambalaj içerisinde sıcaklığa bağlı olarak değişen düzeyde solunum ve terleme yapmaya devam etmektedirler. Ambalaj malzemesinin gaz geçirgenliği sıcaklığa bağlı olarak ürünün solunum hızı ile uyumlu bir şekilde değişmediğinden modifiye atmosfer içerisindeki denge gaz kompozisyonu herhangi bir sıcaklık dalgalanmasında kolaylıkla bozulmaktadır. Bu durum ambalaj içerisinde ürüne özgü maksimum CO2 ve minimum O2 limitlerinin aşılmasına neden olmaktadır. Aynı şekilde, ambalajın nem geçirgenliği istenen düzeyde olmadığında ürünün yaptığı terleme sonucu ambalaj içerisinde su damlacıkları birikmekte ve sonuç olarak ürünün bozulma sürec hızlanmaktadır. 
Bu çalışmada, ambalaj içerisinde denge gaz kompozisyonunun sıcaklık dalgalanmasına bağlı olarak değişimini önleyecek, geçirgenliği TMS ürününün solunum hızına paralel olarak değişen şekil hafızalı bir paketleme materyali geliştirilmesi hedeflenmiştir. Geliştirilen malzemenin aynı zamanda nem geçirgenliğinin ambalaj içerisinde su birikimini önlemek amacıyla yüksek olmasına dikkat edilmiştir. Ambalaj materyali olarak gıda ile uyumlu, çeşitli formülasyonlarda poliüretan (PU) filmler sentezlenmiş ve sentezlenen filmlerin yapıları çeşitli karakterisasyon yöntemleri kullanılarak analiz edilmiştir. Gıda ile uyumlu PU ambalaj malzemelerinin sentezinde herhangi bir çözücü veya katalizöt kullanılmamıştır. 
Projede geliştirilen ambalaj filmlerinin endüstriyel anlamda uygulanabilir, ucuz ve dayanıklı olmasına dikkat edilmiştir. Bu amaçla ve bu filmlerin ısıl özellikleri ve mekanik özellikler incelenmiş ve LDPE başta olmak üzere çeşitli ticari ambalaj örnekleri ile karşılaştırılmıştır. Bu tezde PU filmlerin karkterizasyonu için uygulanan FTIR, DSC, Şekil Hafıza, SEM ve Temas Açısı analizlerine yer verilmiştir. 
Yapılan karakterizasyon çalışmalarına ek olarak,  seçilen farklı sıcaklık duyarlılığına sahip filmlerin taze çilek üzerinde uygulaması yapılmış, elde edilen sonuçlar LDPE ile kıyaslanmıştır. Uygulamada solunum hızı tespit edilen ve ambalajanacak miktarı belirlenen çilekler için MAA tasarımı yapılmıştır. Ambaljlanan çilekler farklı depolama koşullarına ve sıcaklık dalgalanmasına maruz bırakılarak belirli aralıklarla tepe gazı ölçümleri alınmış ve böylece geliştirilen PU filmlerin modifiye atmosfer gaz dengesinin korumasındaki etkileri değerlendirilmiştir. 
Ek olarak depolama koşullarının ve farklı ambalaj filmlerinin çileklerin kalite parametreleri üzerine etkisi incelenmiştir. Bu kapsamda ağırlık değişimi, tekstür, renk, brix, pH ve titre edilebilir asitlik ölçümleri gerçekleştirilmiştir. PU ve LDPE ile hazırlanan ambalajlarda, modifiye atmosferde farklı simülasyon koşullarında depolanan çileklerin, değişen kalite parametreleri kendi aralarında, ticari LDPE ile ve başlangıç koşulları ile karşılaştırılmıştır.
Sonuç olarak sentezlenen PU filmlerin taze çileğin modifiye atmosferde tepe boşluğu gaz dengesinin korunmasında LDPE'ye nazaran daha başarılı olduğu ortaya koyulmuştur. Bununla birlikte geliştirilen PU filmlerin nem geçirgeliğinin çok yüksek olmasının, ağırlık kaybının  artmasına ve pH, titre edilebilir asitlik gibi parametlelerde değişikliklere yol açtığı tespit edilmiştir.

Abstract:
Today, minimum processed fresh fruits and vegetables in the food industry have come to the forefront in consumer preferences. There are various applications aimed that extending shelf life of fresh fruits and vegetables and reducing losses occure in many stages from field to sale. Modified atmosphere packaging (MAP) is one of these applications. However, keeping the metabolic rate of fruits and vegetables under control during transport, storage and sales is an important challenge for the food industry. There are several difficulies faced in the application of MAP to fresh fruits and vegetables due to the metabolic activities of these products. Solving these problems in order to maintain quality parameters and extend shelf life of fresh fruits and vegetables is critical from the food industry point of view to prevent economic losses and food waste.
Fresh fruits and vegetables continue to breathing and transpiration at varying levels depending on the product type and temperature. Since the gas permeability of the packaging material does not change in accordance with the respiration rate of the product depending on the temperature change, the equilibrium gas composition of the modified atmosphere can easily break down at any temperature fluctuation. That situation causes the exceeding the product-specific maximum CO2 and minimum O2 limits inside the package. If the O2 become too low and CO2 become too high in modified atmsphere, various problems occures. For example, when the product-specific critical O2 limit is dropped, the cells switch to anaerobic respiration, accelerate ethylene production, and CO2 accumulate in package at high levels, causing physiological damage to the product. Likewise, when the moisture permeability of the package is not at the desired level, water droplets accumulate inside the package as a result of the increased transpiration rate of the fresh produce and consequently the deterioration process of the product accelerate.
In commercial packaging films the temperature sensitivity of gas permeabilities (Q10) is not sufficient to ensure the stability of modified atmosphere gas composition in case of any break at the cold chain. In many packaging films, especially the temperature sensitivity of CO2 permeability is not compatible with the temprature sensitivity of many fresh produces CO2 production. This temperature sensitivity of product respiration and film permeability are expressed with the Q10 values. It is thought that the shape memory materials, whose barrier properties react rapidly to temperature change, have the potential for packaging of fresh produces. 
In this study, it is aimed to develop a polyurethane (PU) based packaging material which will prevent the change of the equilibrium gas composition occured due to the temperature fluctuations during the storage. It was aimed that the Q10 values of both O2 and CO2 permeability of the developed material were close to the respiratory Q10 values of the selected fresh produce which is strawberry. At the same time, it is aimed to obtain higher moisture permeability packaging films in order to prevent water accumulation inside the package. 
The both the O2 and CO2 permeability of the developed material should change in parallel with the respiration rate of the fresh produces quickly under the temperature fluctuation. According to studies polyurethane shows the shape memory property at room temperature due to it's crystal melting temperature near room temperature. So, the gas permeability of PU films increase at a very high rate with temperature change from refrigerated to near room temperature rapidly and it turns back to the inital state when the temperature decrease. That's why polyurethane films were synthesized in various formulations as packaging material and the permeability properties and heat sensitivities of synthesized films were analyzed in a parallel study to chose the most suitable film to the metabolism of the target fresh produce (strawberry). In the polymer synthesis stage, shape memory polyurethanes were synthesized using poly(ethylene glycol) (PEG), 1,6-hexamethylene diisocyanate (HDI), 1,4-butanediol (BDO) and castor oil (CO) at different ratios. Eight different PU were synthesized by changing the molecular weight of PEG  (400, 1000, 1500), weight ratio of castor oil to PEG (70,50,30) and the hydroxyl ratio of BDO to the total hydroxyls in the reaction (50,40). As a synthessz method 'one step bulk polymerisation' was choosen. Also, no catalyst or solvent was used during synthesis of these PU's to obtain materials suitable to the packaging of food products.
Moreover, the packaging films developed in the project were considered to be industrially applicable, inexpensive and durable. For this purpose thermal and mechanical properties of these films were examined using various characterization methods and compared with various commercial packaging samples, especially LDPE. In this thesis, FTIR, DSC, Shape Memory, SEM and Contact Angle analyzes were applied for characterization of the developed packaging films. 
The FT-IR analysis was repeated twice, before and after polymerization for the purpose of tracking synthesis and the results were compared. Also, as a result of DSC analysis the suitable PU compositions were elected based on their crystalinities and crytstal melting temperatures. Tg and Tm values were determined from the second heating graph of DSC and from the area below Tm, enthalphy value was determind and crystallinity Xc (%) was calculated. Becide, shape memory properties and thermal sensitivities of the gas permeabilities (Q10) of the films were considered in choosing suitable PU films for product application. Also the contact angle was measured to investigate the hydrofobic-hydrophylic behavior of the materials against water and morphological structure of the materials was observed in SEM images. The porosity and interaction with water were associated with these characterizations.
After the characterization studies, the selected temperature sensitive films (PU 1500-50-40 and PU 1000-30-40) were applied on fresh strawberries and the results were compared with control group which performed with LDPE films. For the product  application, the respiration rate of the strawberry was measured with the prepared jar system. For this purpose, both at 5oC and 20oC, measurement was taken under optimum gas composition and atmosphere composition. In the application, MAP design was made according to determined respiration rates of the strawberries, the amount of strawberry to be packaged and the permeabilities of the packaging films. MAP applied strawberries were exposed to different storage conditions and temperature fluctuations. The fist condition was the 1 week cold storage at 4oC, without any temperatre fluctuation. The second condition that is called as simulation 1 consisted of 3 days cold storage at 4oC plus 1 day high temperature exposure at 20oC and 3 days cold storage. At the thirth condition, the high temperature exposure time was increased to two days. Then the head space gas measurements were taken at determined intervals to evaluate the effects of developed PU films on the maintenance of equivalent modified atmospheric gas compositon.
In addition, the effect of different storage conditions and different packaging films on the quality parameters of strawberries was investigated. In this context, weight change were evaluated by using a sensitive scale. Also the texture, color, brix, pH and titratable acidity measurements were performed before and after the modified atmosphere packaging and 1 week storage. The change in the texture was measured by drilling the strawberries with a 5mm diameter cylindrical probe. In addition, L*, a* and b* values were determied, chroma value, total color change and hue angle were calculated. A microprocessor pH meter was used to determine the pH. Strawberries were cut in half and measured at room temperature from the midpoint of each strawberry. The percentage of total dissolved dry matter was determined using the hand refractometer. Finally, titratable acidity was calculated as citric acid percent based on the NaOH spent in titration and the weight of the strawberry used.
In conculusion, FTIR analysis showed that the peaks of the reagents disappeared and PU peaks were formed. In DSC analysis PU materials having enough crystalinity and the crystal melting temperature which is close to the room tempereture were detected. Additionally, packaging films were successfully obtained by hot press method from the synthesized PU materials and choosen formulations were applied for MAP of fresh strawberries. 
As a result of the product applicaton, it has been shown that the packaging films obtained from the synthesized PU materials are more successful than LDPE in maintaining the head space gas balance in the modified atmosphere. After the temperature fluctuation, the critical increase in CO2 ratio in LDPE packages were observed. On the other hand, there was no statistically significant change in the head space gas composition of PU packages on the basis of day or packaging material. 
However, it has been found that developed PU films have very high moisture permeability that causes the increase in weight loss and changes in some quality parameters such as pH, brix and titratable acidity. Methods for reducing moisture loss can be developed in future studies. For example; it can be investigated whether moisture permeability can be reduced without affecting the temperature sensitivity of gas permeability of the films. Also it can be focused on the more hydrophobic formulations for the product application.