1 Introduction

Nearly 6 million tonnes of tobacco waste are generated annually from tobacco processing due to the rapid development of the tobacco industry [1, 2]. However, in order to achieve the goals of carbon peaking and carbon neutrality, it has become an urgent issue to deal with the harmless treatment of discarded tobacco leaves and reduce environmental pollution and resource waste [3]. Since the mid-twentieth century, reconstituted cut stems have been widely accepted by consumers because of the advantages it offers in terms of production costs, combustion characteristics, tar release, flexible ingredient management, and structural strength [4, 5]. With the rapid development of the tobacco industry and the continuous promotion of the lowering tar and reducing harm of tobacco, reducing the proportion of high-quality leaf is one of the most important research topics in the tobacco industry. However, although it is possible to reduce the negative effects of conventional tobacco on product quality by regulating the composition of reconstituted cut stems, such as impurities, concentration, aroma, and irritancy, due to the substitution of reconstituted cut stem combinations, it cannot completely replace the market share of conventional tobacco stems, and starch is an important indicator for measuring tobacco quality [6,7,8]. Therefore, in order to improve the quality of reconstituted cut stems in tobacco, it is urgent to conduct in-depth research on its starch properties.

Starch, as the main source of carbohydrates in tobacco, has a content of up to 5.5% to 8.5% after baking [9, 10]. However, in the past, the impact on starch in tobacco was mainly concentrated in aspects such as starch content [11, 12]. And little is known about the effects of tobacco processing on starch structure. It is well known that tobacco is subjected to high temperatures and high humidity during the blending process, which can lead to changes in the physical and chemical properties of the starch. In particular, the changes during the heat treatment of starch were particularly significant and most pronounced [13,14,15]. When tobacco is heated during the blending and roasting process, the energy provided breaks the hydrogen bonds in the crystalline regions of the starch granules. It leads to the swelling of starch granules by water absorption, the destruction of starch granules, and the consuming zone of birefringence, leading to a decrease in the degree of crystallinity and affecting the quality of tobacco products [16, 17]. Therefore, the study of the physicochemical properties and chemical structure of starch in reconstituted stalk filaments can further optimize the roasting process. It can rationalize the conversion and consumption of starch, thus improving the smoking quality of tobacco products. Structural analysis of the key macromolecule (starch) of reconstituted filaments can help to improve the quality of reconstituted cut stems. This enhances its share in tobacco products and facilitates the broadening of the use of reconstituted cut stems.

This study focuses on coke reduction and harm reduction, aiming to further improve the advantages of reconstituted cut stems in terms of appearance and morphology, restructuring plasticity, and other aspects of tobacco products. Efforts are made to improve the comprehensive utilization of tobacco raw materials in the processing process, effectively reduce the loss of tobacco leaves, and reduce the waste of resources. This study aims to separate the key starch macromolecules in the reconstructed cut stems by combining sample processing methods such as acid hydrolysis, salt precipitation, and organic solvent precipitation. Modern analytical techniques such as Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy (Raman), X-ray diffraction (XRD), small angle X-ray scattering (SAXS), and scanning electron microscopy (SEM) will be used to qualitatively and quantitatively analyze the content and molecular structure of the key starch macromolecules in the reconstructed cut stems. To provide technical support for improving the quality of reconstituted cut stalks by clarifying the qualitative and quantitative methods suitable for characterizing the major macromolecular starch components of reconstituted cut stems.

2 Materials and methods

2.1 Materials

The reconstituted cut stems, Fujian Sanming superior tobacco leaves, and Yunnan long stems (blank cut stems) were provided by Jiangsu Tobacco Industry Co., Ltd. After crushing the tobacco samples, they were sieved through a 100-mesh sieve for later use. Sodium hydroxide and hydrochloric acid were purchased from ** structure of the reconstituted stems starch by making a logarithmic plot of the SAXS scattering intensity I(q) ~ q (Fig. 5b). The results showed that the α-values of the starch structures of the three different sources of tobacco components were in the range of 1 < α < 3, indicating that they all present mass fractal structures (Table 3). At this time, the fractal dimension Dm = ɑ and the Dm value of the reconstructed cut stems is the largest, followed by the cut stems and tobacco leaves, and the orderliness of their scatterers is gradually weakened. It shows that the starch aggregate structure becomes denser, with stronger molecular chain interactions and higher crystalline order, making it less susceptible to damage, as a result of the processing of reconstituted cut stems [36,37,38].

Fig. 5
figure 5

SAXS plots (a) and SAXS I*q2 ~ q plots (b) of different tobacco sample sources

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Table 3 SAXS plots ratio parameters of starch structure from different tobacco sample sources
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3.6 Raman spectroscopic analysis of starch structure

We used a Raman spectrometer to test the helical molecular structure of related starches derived from different tobacco samples. The results show (Fig. 6) that reconstituted cut stems and high-quality tobacco leaves have vibration absorption peaks of sugar compounds at 1500–1200 cm−1. This is caused by the characteristic stretching vibration of the C-O bond in the glycosidic bond and the bending vibration of the C-H chemical bond of the methine and methylene groups. This is related to the single helix structure of amylose, but the blank cut stems did not have such a significant absorption peak here [39, 40]. The starch in the untreated blank cut stems as compared to the reconstituted cut stems and good quality tobacco may be due to the high amount of cellulose, hemicellulose, lignin, and other components in the composition interfering with the starch, resulting in the absence of significant absorption peaks.

Fig. 6
figure 6

Roman plots of different tobacco sample sources

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4 Conclusion

This study explores the content, physical and chemical properties, and characteristic structures of starch from different tobacco sources in the tobacco industry. It was found that the starch content in the reconstituted cut stems was between that of tobacco leaves and cut stems, indicating that reducing the starch content during the processing of the reconstituted cut stems was more conducive to high-value processing of the reconstituted cut stems. Simultaneously, the evaluation of the multiscale structural characteristics of the starch granules shows that the starch of the reconstituted cut stems, compared to the blank cuttings, has a rocky granular structure, with more short-range order on the surface and more order in the structural domain. In addition, the processed reconstituted stems improve the crystallinity of starch and also have the typical B-type crystal structure of starch, with stronger molecular chain interactions and high crystalline orderly arrangement. The successful conduct of this study provides a research basis for the development of reconstituted cut stems in tobacco products. At the same time, it also helps to improve the product quality of reconstituted cut stems, improve the bioavailability of tobacco materials, reduce the cost of raw materials, and reduce the waste of resources and environmental pollution. In addition, the subsequent research on other macromolecules can provide a theoretical basis for the development of high-quality reconstituted cut stems, improve the effects of reconstituted cut stems on cigarette physical indexes and smoke, and realize their integrated application in tobacco products.