Storage stability of freeze-dried, spray-dried and drum-dried skim milk powders evaluated by available lysine
Introduction
During recent years the application of skim milk powder (SMP) in the food industry has increased due to its long shelf-life and various functional properties. Based on a report by the Food and Agriculture Organization of the United Nations, the annual production of skim milk powder reached 1.9 million tons in 2013, which was a 3.3% increase compared to the previous year (Griffin, 2013). Considering its composition, which is mainly proteins and carbohydrates and bearing in mind that it is usually stored for a significant period of time before being used, the possibility of skim milk powder undergoing Maillard reactions during storage cannot be ruled out. These controversial and complex chemical reactions may lead to the formation of pro-inflammatory molecules, specifically advanced glycation end products (AGEs), under certain circumstances (Bengmark, 2007, Goldberg et al., 2004, Henle, 2005, Nguyen et al., 2013, Siciliano et al., 2015). Therefore, understanding and monitoring this reaction is of vital importance. This can be achieved by means of optimizing the parameters of processing as well as controlling the conditions during transport and storage.
Available lysine is one of the markers that can be employed to understand the early stages of the Maillard reaction and its content is decreased with respect to the increased reaction rate. This reduction in the amount of available lysine is considered to be the first step of the Maillard reaction and is due to binding with lactose, the main carbohydrate in milk (Contreras-Calderón et al., 2009, El and Kavas, 1997, Hurrell et al., 1983, Malec et al., 2002, Mehta and Deeth, 2016, Ramirez-Jimenez et al., 2004, Pereyra Gonzales et al., 2010, Rutherfurd and Moughan, 2008, Schmitz et al., 2011;). In order to quantify the available lysine, different approaches can be adopted depending on the type of sample material. Regarding skim milk powder, a convenient and reliable method is the dye-binding method using Acid-orange 12 which was validated in our previous work for casein (the main protein in milk), bovine serum albumin and a wide range of skim milk powders (Aalaei, Rayner, Tareke, & Sjöholm, 2016).
There have been only a few studies done on the storage of skim milk powder at mild temperatures (37 °C, 50 °C, 60 °C), (Pereyra Gonzales et al., 2010) and in those studies the sample was always commercial. In this study, the skim milk was obtained from a known source (local dairy) and extra effort had been put into manufacturing the different powders in the pilot plant where all the processing parameters were controlled. In order to obtain a clear picture of the impact of processing on the progress of the Maillard reaction, three types of skim milk powders produced by a pilot-scale freeze-dryer, spray-dryer and drum-dryer were investigated, and their available lysine contents were quantified. Subsequently, the storage effect was studied by storing the above-mentioned skim milk powders at various temperatures and relative humidities in highly controlled atmospheres for 200 days. The storage variables included two temperatures (20 °C, 30 °C) and two relative humidities (33%, 52%) and available lysine was measured at intervals during 200 days.
Therefore, the aim of this project was to understand the effects of different drying technologies and various storage conditions on the level of remaining available lysine in order to determine the extent of early Maillard reactions. The hypothesis was that the impacts of the processing and further handling, including the storage and transportation, are more profound than previously thought and mostly underestimated. In other words, this study aims to increase the awareness that the Maillard reaction does not necessarily need extreme conditions to occur and it can take place at temperatures and humidities that are common in many places. The storage conditions investigated in this study can easily occur when consumers store an opened package at room temperature and consume the product intermittently over a long time. This was the criterion behind the selection of storage variables in the way that they closely resembled typical climate conditions during transport and storage, particularly at home.
Section snippets
Chemicals and instruments
In order to analyze available lysine, the following chemicals were used: sodium acetate anhydrous reagent grade (CAS: 127-09-3) was purchased from Scharlau (Sentmenat, Spain); Acid Orange 12 (Crocein Orange G) (CAS: 1934-20-9 and MW = 350.32) was supplied by Tokyo Chemical Industry (Tokyo, Japan); potassium dihydrogen phosphate for analysis (CAS: 7778-77-0) and magnesium nitrate hexahydrate for analysis (CAS: 13446-18-9) were obtained from Merck (Darmstadt, Germany); propionic anhydride 99%
Loss of available lysine due to differences in drying methods
A non-heat treated skim milk, which had 3.54% ± 0.09 available lysine based on dry matter, was used as a reference sample in this study. Application of three different dryers on the skim milk (0.1% fat) resulted in skim milk powders with different levels of available lysine. Freeze-dried SMP had the highest available lysine content followed by spray-dried SMP and drum-dried SMP. The available lysine contents in freeze-dried, spray-dried and drum-dried skim milk powders were 3.49% ± 0.07,
Discussion
Available lysine in this study was only used as one marker to monitor the Maillard reaction in the skim milk powders produced and stored differently. With respect to the effect of the drying method, freeze-dried SMP had the highest available lysine content followed by the spray-dried and drum-dried SMP. Considering the mechanism of the freeze-drying, which involves the drying of the frozen material under vacuum and room temperature, the results of this study correspond to what was expected. The
Concluding remarks
The findings of the current study emphasize the importance of the drying conditions and the storage quality with respect to skim milk powders. Freeze-drying had the lowest impact on the loss of available lysine among the drying technologies studied in this work, followed by spray-drying and drum-drying. The decrease in available lysine content during 200 days of storage follows almost the same pattern regardless of the type of skim milk powder. In order to obtain a better understanding of the
Acknowledgements
The authors greatly appreciate the Swedish Research Council for funding this study.
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2022, NFS JournalCitation Excerpt :Extensive glycation of the amino acids in milk powder and the formation of irreversible protein aggregates by intermolecular covalent crosslinks are consequences of spray-drying and storage [46]. Aalaei and coworkers [47] analysed lysine availability (used as a marker for glycoxidative damage) in spray-dried stored skim milk powder. They reported a loss of 7.45% of available lysine after spray drying and registered a decrease up to 42.6% during storage, depending on the temperature and humidity.