Moisture absorption and dynamic flavor changes in hydrolysed and freeze-dried pine nut (Pinus koraiensis) by-products during storage

Highlights

• Traced the difference of moisture absorption in PNPHP fractions

• Differences in 15 days' storage were determined by FT-MIR and SPME-GC–MS.

• PNPHP_1–3 kDa mainly generated aldehydes and PNPHP_3–10 kDa generated pyrazines

• PNPHP_3–10 kDa had higher absorption capacity and intensity of flavor compounds.

• The formation of pyrazines was generated because of Maillard reaction.

Abstract

In the present study, two different fractions of pine nut (Pinus koraiensis) protein hydrolysate powder (PNPHP_1–3 kDa and PNPHP_3–10 kDa) were prepared and stored for fifteen days at 25 °C and 50% relative humidity. Changes in moisture absorption, secondary structure and flavor compounds were measured at various time intervals over fifteen days. Results showed that the PNPHP_3–10 kDa had higher moisture absorption capacity from day 6 to day 15 and that the secondary structure of two PNPHP fractions was changed. Major nine and eight flavor compounds were identified from PNPHP_1–3 kDa and PNPHP_3–10 kDa, respectively. PNPHP_1–3 kDa mainly generated aldehydes and PNPHP_3–10 kDa generated pyrazines. Finally, the formation of pyrazines was conjectured. The data suggests that Maillard reaction, microbial fermentation and lipid oxidation occurred during storage. The reactions appeared to be strongest by the sixth day.

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 PNPHP_3–10 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 PNPHP_1–3 kDa and PNPHP_3–10 kDa. Following storage of the PNPHP_1–3 kDa and PNPHP_3–10 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 PNPHP_1–3 kDa and PNPHP_3–10 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.