Moisture absorption and dynamic flavor changes in hydrolysed and freeze-dried pine nut (Pinus koraiensis) by-products during storage
Graphical abstract
Introduction
Pine nuts (Pinus koraiensis) are widely available in Korea and China (Son, Hwang, Kim, Lee, & Kim, 2001). Pine nuts are popular because they are considered nutritious since, in addition to protein, they contain oil, phenolic compounds, vitamins, and sugar (Nergiz & Dönmez, 2004). Because of this, pine nuts are considered to have many beneficial effects. For instance, researchers have shown that pine nut triglycerides and free fatty acids can be used as a safe appetite suppressant (Pasman et al., 2008). Kim, Kim, Han, Park, and Watanabe (2014) reported that the pine nut shell is a potential source of bioenergy. Polyphenols isolated from pine nuts have also been shown to possess anti-proliferative activity (Yi et al., 2015). The majority of the studies has focused on pine nut oils whereas relatively is known about pine nut protein. Pine nut meal protein is the main by-product in pine nut oil industry, but most of them are wasted without further processing. However, pine nut protein hydrolysate powder (PNPHP) is a good antioxidant and immune resource, which enjoys great business prospects. In our previous reports, Yang, Li, Lin, Zhang, and Chen (2017) identified novel peptides from 3 to 10 kDa pine nut meal protein, and explored the relationship between the antioxidant capacity and secondary structure. The DPPH inhibitions of 1–3 kDa and 3–10 kDa pine nut meal protein were all above 35%. This means the pine nut peptide possesses the antioxidant activity. Additionally, Lin, Liu, Liu, and Yu (2017) observed that pine nut meal protein peptides had an ability to enhance the function of immunity, for instance, 100 mg·(kg·d)− 1 PNPHP3–10 kDa enhanced the abdominal macrophage phagocytosis of mice at 30.22%. However, during storage, the peptide powder absorbed water and becomes sticky, nubbly and browning. A similar phenomenon was observed by Sullivan et al. (1974) that browning phenomena occurred frequently during the storage of potatoes. However, the investigation about this phenomenon is rare. Therefore, a study addressing the storage of PNPHP is necessary.
Recent studies have been performed to examine the quality of meat product during storage. Rysman, Hecke, Poucke, Smet, and Royen (2016) studied protein oxidation and proteolysis during storage. The results showed that protein oxidation led to impaired proteolysis. Gas chromatography coupled with a mass spectrometer (GC–MS) is widely used to identify flavor compounds in food. For instance, Khakimov et al. (2016) identified forty-five volatile compounds from fruits which included mango, pineapple, jackfruit, baobab and tamarind fruits. Using GC–MS, Sulejmani and Hayaloglu (2016) assessed the volatile compounds in Kashkaval cheese, under different heating conditions. The fatty acid profile in pine nut has also been studied using GC–MS (Destaillats, Cruz-Hernandez, Giuffrida, & Dionisi, 2010). The aims of this paper were to prepare PNPHP and isolate the PNPHP1–3 kDa and PNPHP3–10 kDa. Following storage of the PNPHP1–3 kDa and PNPHP3–10 kDa for fifteen days at 25 °C and 50% relative humidity (RH), changes in moisture absorption and secondary structure were observed. Following on from this, the flavor compounds present in the PNPHP1–3 kDa and PNPHP3–10 kDa were extracted by solid phase micro extraction (SPME) and a qualitative and quantitative analysis of them was performed using GC–MS. To the best of our knowledge, no reports have been published so far on changes during PNPHP storage. In the northeast of China, the RH is around 50% in summer. Therefore, the present study could be applicable to storage and quality control of PNPHP. Further research is performed about restraining the moisture absorption of PNPHP, such as microencapsulation, vacuum package and hygroscopic agent treatment.
Section snippets
Materials and regents
PNPHP was obtained from the College of Food Science and Engineering, Jilin University (Changchun, China). Ethanol, sodium hydroxide (NaOH), potassium bromide (KBr) and magnesium chloride hydrate (MgCl2 ∙ 6H2O) were purchased from Beijing Chemical Plant (Beijing, China) and were all of analytical grade. Alcalase® Food Grade (FG) 2.4 L, and cyclohexanone were obtained from Sigma Chemicals Co. (St Louis, MO, USA).
Preparation of PNPHP
PNPHP was prepared using the method described by Yang et al. (2017). Briefly, the pine
Moisture absorption changes of PNPHP1–3 kDa and PNPHP3–10 kDa during storage
The weight of PNPHP1–3 kDa and PNPHP3–10 kDa increased during storage at 25 °C and 50% RH (Fig. 1). Compared with PNPHP3–10 kDa, PNPHP1–3 kDa had higher moisture absorption capacity at day 3 and then tended to be stable. Similar research was reported previously. The moisture absorption capacity of < 1 kDa soybean antioxidant peptide powders was the strongest during the storage with 300 min (Lin, Yang, Li, Chen, & Zhang, 2016). However, PNPHP3–10 kDa had higher absorption capacity from day 6 to day 15.
Conclusions
In the present study, PNPHP1–3 kDa and PNPHP3–10 kDa were prepared by enzymolysis and ultrafiltration. The PNPHP1–3 kDa and PNPHP3–10 kDa were stored for fifteen days at 25 °C and 50% RH. At different time intervals over this period, changes in the PNPHP1–3 kDa and PNPHP3–10 kDa were detected, including changes in moisture absorption, secondary structure and the profile of flavor compounds. The results showed that large molecular weight (PNPHP3–10 kDa) had higher moisture absorption capacity from day 6
Notes
The authors declare no competing financial interest.
Acknowledgements
The authors acknowledge the financial support provided by the National 863 Program under Grant (2013AA102206-5).
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