Analysis of crystallized lactose in milk powder by Fourier-transform infrared spectroscopy combined with two-dimensional correlation infrared spectroscopy Yu Lei a , Qun Zhou a , Yan-ling Zhang a , Jian-bo Chen a , Su-qin Sun a, * , Isao Noda b a Key Laboratory of Bioorganic Phosphorus Chemistry & Chemical Biology (Ministry of Education), Department of Chemistry, Analysis Center, Tsinghua University, Beijing 100084, PR China b The Procter & Gamble Company, West Chester, OH 45069-7053, USA article info Article history: Received 30 October 2009 Received in revised form 11 December 2009 Accepted 11 December 2009 Available online 22 December 2009 Keywords: Milk powder Infrared spectroscopy (IR) Two-dimensional infrared spectroscopy (2D IR) Crystallized lactose abstract Infrared (IR) spectroscopy is used in combination with two-dimensional (2D) correlation IR spectroscopy to conduct rapid non-destructive quantitative research in milk powder without additional separation steps. The experiments conducted in both FT-IR and 2D FT-IR spectra suggest that characteristic spectro- scopic features of milk powder containing different carbohydrate can be detected, and then determine the type of carbohydrate. To predict the approximate content of lactose while the carbohydrate is lactose, different amount of crystallized lactose has been added to the reference milk powder. The correlation coefficient could be used to determine the content of crystallized lactose in milk powder. The method provides a rapid and convenient means for assessing the quality of milk powder. Ó 2010 Elsevier B.V. All rights reserved. 1. Introduction Dairy is an inseparable part of our daily lives. Milk powder has a far longer shelf life than liquid milk and is widely used because of the reduced transport and storage costs. It is frequently used in the manufacture of infant formulas and various dairy foods. However, some complicated and time consuming techniques are still used in the quality control of milk powder. For example, China still typi- cally uses the semi-micro Kjeldahl nitrogen determination method that has been in use since 1985 to analyze the protein from milk powder. Likewise, the Rose–Gettlieb method (1985) is used to test the liquid milk. As for the sugar analysis, JH Lave and Egnon method (1985) is still applied [1]. Fortunately, because of the intro- duction of some new techniques, such as HPLC, JH Lave and Egnon type methods are nowadays employed as supplementary tech- niques. Obviously, these out-of-dated methods fail to meet some of the current detection challenges encountered in the food quality assurance. For example, milk adulteration poses a major threat to China’s nutritional needs. There is usually a certain amount of amorphous lactose in the milk powder, but the lactose becomes crystallized when stored for a long period of time. Some manufacturers are di- rectly adding some crystallized lactose to the milk powder to save the cost. In the traditional method, we must convert the lactose first and then determine the nature of the adulterant. The process unfortunately is cumbersome and complex. Based on the impor- tance of the quality control of milk powder and there is only a lim- ited amount of studies on this subject at home and abroad [2–6], we want to create a rapid and effective detection method to better control the quality of milk powder products. Fourier-transform infrared spectroscopy (FT-IR) is one of the most widely used methods to identify chemical compounds and elucidate chemical structures. It is adopted by the food industry to test the quality of food products based on the fingerprinting fea- tures of IR profiles. Currently, this technology is used in many fields, such as food science, traditional Chinese medicine, chemical industry applications, and so on [7–12]. By interpreting the charac- teristic peaks of chemical functional groups, the representative chemical constituents (e.g., fat, protein, and carbohydrates) of the milk powder could be revealed. Two-dimensional (2D) correlation analysis [13] provides a convenient way to monitor the relation- ship among different functional groups in molecules. The so-called generalized 2D correlation spectroscopy has become a surprisingly powerful tool for the detailed analysis of spectral data of various complex systems [14]. In this paper, IR and 2D-IR spectroscopy are used to study the type of carbohydrate and the crystallized lactose content of milk powder without additional separation steps. To predict the approx- imate lactose content in commercial milk powder, different amounts of crystallized lactose have been added to a milk powder without crystallized lactose to obtain the reference spectra. If any 0022-2860/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2009.12.030 * Corresponding author. Tel.: +86 10 62781692; fax: +86 10 62782485. E-mail address: sunsq@mail.tsinghua.edu.cn (S.-q. Sun). Journal of Molecular Structure 974 (2010) 88–93 Contents lists available at ScienceDirect Journal of Molecular Structure journal homepage: www.elsevier.com/locate/molstruc