Araştırma : İlkay TOSUN - N. Şule ÜSTÜN
Corresponding author. Tel : +90-362-4576020; Fax : + 90-362-4576034 - E-mail address: suleustun@hotmail.com
Ondokıız Mayıs University, Faculty of Agriculture,
Department of Food Engineering, Kurupelit, Samsun, Turkey
Received 18 December 2001; received in revised form 18 March 2002; accepted 18 March 2002

ELSEVIER

Food Chemistry 80 (2003) 441 - 443 www.elsevier.com/locate/foodchem

Food Chemistry

Nonenzymic Browning During Storage of White Hard
Grape Pekmez (Zile pekmezi)

Abstract

            Effects of storage period on hydroxymethylfurfural, pH and colour were studied in white hard grape pekmez (Zile pekmezi), a Turkish traditional product. Pekmez samples were stored at 20 ⁰C for 8 months. Analysis of variance revealed significant differences in hydroxymethylfurfural, pH, L, a and b values (P<0.01) based on storage time.

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Keywords : Pekmez; Zile pekmezi; White hard grape pekmez;
Hydroxymethylfurfural; Colour.

Çalma Pekmez Yapan Kadınlar

(Yağlıboya Tablo 90X110 cm İlhan TRAK)

            1. Introduction

            Pekmez, which has been produced for a long time in Turkey, is a traditional Turkish food. Because most of its carbohydrate is in the form of glucose and fructose, it easily passes into the blood without digestion. This is nutritionally important, especially for babies, children, sportsmen and in situations demanding urgent energy.

            Pekmez has an important function in the working of the brain, in which glucose is an energy source. Further - more, pekmez confers approximately 293 kcal/100 g of energy and also has important organic acids and mineral materials. There are many publications about production, composition, standardization and health benefits of pekmez (Aksu & Nas, 1996; Batu, 1991a; Batu, 1991b; Batu, 1991e; Batu, 1993; Batu & Aktan, 1993; Batu & Yurdagel, 1993; Bozkurt, Göğüs, & Eren, 1999; Güven, 1982; Karakaya & Artık, 1990; Kayahan, 1982; Nas & Nas, 1987; Tekeli, 1965; Ustun & Tosun, 1997).

            Fresh or dried grapes are particularly used in the production of  pekmez, but apple, prune, watermelon, mulberry, apricot and sugar beet can also be used in pekmez production (Batu, 1991e). The raw materials and processing conditions differ geographically. Concentrated grape juice alone, up to a minimum of 65 % total soluble solids, is liquid pekmez. White hard pekmez is produced by adding some gelling and bleaching agents to liquid pekmez.

Zile Pekmezi'nin Mikzerle Karıştırılması

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            Grape juice is used for the production of hard grape pekmez. Acidity is decreased with the application of CaCO₃ - containing special white soil, or CaCO₃ only, and the grape juice is left to sediment. Acid - decreased and clarified grape juice is concentrated to the desired Brix degree under atmospheric pressure, or by a vacuum method. After the addition of the white of an egg, soapwort extract, former pekmez, or powdered sugar as bleaching agents, and pectin as a gelling agent, the concentrated grape juice is stirred and beaten.

            During this process, the penetration of air, along with the bleaching agents, causes the production of white or cream - coloured pekmez. During storage, the light colour of the product darkens because of the Maillard reaction. As an intermediate product of the Maillard reaction, that originates during production and progresses also in storage of pekmez, hydroxymethylfurfural (HMF) is an important quality index in the Turkish Pekmez Standard (TS, 1989), maximum HMF in ist quality product is declared as 25 mg/kg.

            The purpose of this study was to evaluate HMF formation and its effects on colour during storage and also to determine the changes in pH, that affected HMF formation.

            2. Materials and methods

            2.1 Materials

            White hard grape pekmez samples, produced by a commercial factory in Tokat, Zile, were used. Upon production, the samples arriving to the laboratory were placed in 100 g jars and stored at 20 °C in darkness. Soluble solids, sugar, protein, total acidity," HMF, pH and colour analysis were established in samples before storage. In stored samples, HMF, pH and colour analysis were obtained at 2 - month intervals.

Pekmez Topraklı Şıranın Isıtma Sırasında Köpüğünün Alınışı

            2.2. Methods

            General parameters were measured following official methods (AOAC, 1984); pH was measured with a Nel, pH-890 pH meter; titratable acidity was measured with 0.1 N NaOH up to pH 8.1 and expressed as percentage of tartaric acid; soluble solids content was measured with an ATAGO refractometer; protein was analyzed by the Kjeldahl method (Nx6.25).

            Colour was evaluated by measuring Hunter L (brightness, 100 = white, 0 = black), a ( + , red; —, green) and b ( + , yellow; —, blue) parameters by means of a reflectance colorimeter (CR 300, Chromometer, Minolta, Japan). A white tile (No. : 21733001) was used to standardize the instrument.

            Total sugar, invert sugar and sucrose were quantitated by the Lane - Eynon method (Cemeroglu, 1992); The hydroxymethylfurfural was determined quantitatively following the procedure described by the IFFJP (1964), based on the colorimetric reaction between barbituric acid, p-toluidine and HMF, forming a red-coloured complex. The intensity of red colour was measured at 550 nm with a JASCO UV-Visible V-560 spectrophotometer.

Zile Pekmezi Ambalajının Etiketlenmesi

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            Table 1
            Some components of white hard grape pekmez before storage (n = 3)

            2.3. Statistical analysis

            The data were the mean of three independent determinations and were statistically evaluated by analysis of variance (ANOVA) and the means were compared using Duncan's multiple range test at P<0.01, using the MSTAT program to determine the significant differences that could be attributed to the time of storage.

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            3. Results and discussion

            Study of the storage of pekmez was carried out over 8 months. Some compositions of samples before storage are shown in Table 1 and changes reported during storage of samples are shown in Table 2.

            Because of the rich contents of sugar and protein, HMF in pekmez samples showed a significant increase (P<0.01) as storage time increased (Table 2). The colour darkened; consequently L value decreased and a and b values increased parallel with the increase of HMF. These changes were found to be statistically significant (P<0.01). During storage, the pH value increased until the 4th month and decreased from the 6th month; although the pH over the 8-month storage period changed actually only by approximately 0.1 pH units, this difference was found to be statistically significant (P<0.01).

            Higher pH values promote the development of HMF (Eskin, Henderson, & Townsend, 1971). The pH value of pekmez is higher than the original fruit because of the acid decreasing process applied during production. Consequently, the higher pH stimulates the Maillard reaction in this high sugar- and protein - containing medium.

            In high soluble solids - containing pekmez samples, the HMF increased by 41.1 % after 8 months of storage. This is below the maximum HMF value declared in the Turkish Pekmez Standard for the 1st quality product.

    
Şıranın açık kazan yöntemine göre kaynatılarak sıvı pekmez eldesi (A)
Sıvı pekmezin gerekli maya ve yumurta akı katılarak ağartılması işlemi (B)

            Table 2
            Changes in colour, HMF and pH during storage of white hard grape pekmez (n = 3)^a

            ^a For a given storage period, column values with the same letter are not significantly different (P<0.01).

            HMF formation during storage changed colour as a difference from white - light - cream to brown and this was instrumentally established as a decrease in L value and increase in a and b values.

            References

            Aksu, M. I., & Nas, S. (1996). Dut Pekmezi Üretim Tekniği ve Çeşitli Fiziksel ve Kimyasal Özellikleri (Mulberry Pekmez Manufacturing Technique and Physical and Chemical Properties). Gıda, 21 (2), 83 - 88.

            AOAC. (1984), Official Methods of Analysis. Association of Official Analytical Chemists (14 th ed.). Virginia, USA Arlington.

            Batu, A. (1991a). Farklı İki Yönteme Göre Üretilen Kuru Üzüm Pekmezinde Oluşan Kimyasal Değişmeler Üzerine Bir Araştırma (A Research on Chemical Composition of Raisin Pekmez Produced by Using Two Different Methods). Cumhuriyet Üniversitesi, Tokat Ziraat Fakültesi Dergisi, 7 (1), 179 - 189.

            Batu, A. (1991b). Pekmeze İşlenecek Kuru Üzüm Şıralarına Uygulanan Ön İşlemler Üzerine Bir Araştırma (A Research on Preliminary Processes Applied to Raisin Juice That Will be Produced Into Pekmez). Cumhuriyet Üniversitesi, Tokat Ziraat Fakiiltesi Dergisi, 7 (1), 191 - 202.

            Batu, A. (1991c). Zile Pekmezi Üretim Teknolojisinin Geliştirilmesi ve Kimyasal Bileşiminin Belirlenmesi Üzerine Bir Araştırma (A Research on Developing Hard Grape Pekmez Production Technology and Defining Chemical Composition). Cumhuriyet Üniversitesi, Tokat Ziraat Fakültesi Dergisi, 7 (1), 171 - 178.

            Batu, A. (1993). Kuru Üzüm ve Pekmezin İnsan Sağlığı ve Beslenmesi Açısından Önemi (The Importance of Raisin and Pekmez on Human Health and Nutrition). Gıda, 18 (5), 303 - 307.

            Batu, A., & Aktan, N. (1993). Üzüm Pekmezlerinde Asit ve pH Değerleri Üzerinde Bir Araştırma (A Study on Acid and pH Values of Concentrated Grape Juice). Gıda ve Yem, 1 (4), 38 - 43.

            Batu, A., &. Yurdagel, Ü. (1993). Değişik Katkıların Kullanımı ile Beyaz Katı Kuru Üzüm Pekmezi Eldesi Üzerine Bir Araştırma (A Study on Production of a White Hard Raisin Pekmez by Using Different Gelling end Bleaching Agents). Gıda, 18 (3), 157 - 163.

            Bozkurt, H., Göğüş., F., & Eren, S. (1999). Nonenzymic Browning Reactions in Boiled Grape Juice and Its Models During Storage. Food Chemistry, 64, 89 - 93.

Zile Asma Üzümleri

TGRT KEŞİF Programı - Sunucu Yeliz PULAT - 11.09.2001

            Cemeroğlu, B. (1992). Meyve ve Sebze İşleme Endüstrisinde Temel Analiz Metotları. Ankara: Biltav Yayınları.

            Eskin, N. A. M., Henderson, H. M., & Townsend, R. J. (1971). Bio­Chemistry of Foods. New York: Academic Press Inc.

            Güven, S. (1982). Bazı Geleneksel Gıdalarımızın İşlenmesi ve Teknoloji Geliştirmenin Önemi (Production of Some Traditional Foods and The Importance of Developing Technology). Türkiye 3. Gıda Kongresi Gıda Teknolojisi Derneği Yayın No. : 4, Ankara.

            IFJJP. (1964). International Federation of Fruit Juice Producers Methods. Zug, Switzerland: Analyses; Fruit-Union Snisse Assoc.

            Karakaya, M., & Artik, N. (1990). Zile Pekmezi Üretim Tekniği ve Bileşim Unsurlarının Belirlenmesi (Hard Grape Pekmez Production Technology and Determination of Its Components). Gıda, 15 (3), 151 - 154.

            Kayahan, M. (1982). Üzüm Şırasının Pekmeze İşlenmesinde Meydana Gelen Terkip Değişmeleri Üzerine Araştırmalar (A Study on Compositional Changes During the Production of Pekmez from Grape Juice). Ankara Üniversitesi Ziraat Fakültesi Yayınları 797, Ankara.

            Nas, S., & Nas, M. (1987). Pekmez ve Pestilin Yapılışı, Bileşimi ve Önemi (Production, Composition and Importance of Pekmez and Pestil). Gıda, 12 (6), 347 - 352.

            Tekeli, S. T. (1965). Ziraat Sanatları. Ankara Üniversitesi, Ziraat Fakültesi. Yayın No. 237, Ankara.

            TS (1989). Üzüm Pekmezi Standards TS 3792. Türk Standardları Enstitüsü, Ankara.

            Üstün, N. Ş., & Tosun, İ. (1997). Pekmezlerin Bileşimi (Composition of Pekmez). Gıda, 22 (6), 417 - 423.

Ağartılmış ve Katılaştırılmış Pekmezin Kutulanması.