Infrared-Spectral Characteristics of Camellia oleifera Shell / Meal during Composting

The compost products of Camellia oleifera shell/meal mixed at different mass ratios were characterized by Fourier-transform infrared spectroscopy (FTIR) at different composting stages to monitor the structural changes of their components. The results showed that the amount of Camellia oleifera meal significantly affected the composting rate of the shell, but did not change the degradation order and decomposition of the related compounds. During the composting process, microorganisms used the highly decomposable carbon source materials, such as proteins and sugars, first to grow and multiply, and then decomposed hemicellulose, cellulose and lignin by oxidative cleavage after these nutrients were consumed to a certain extent. The decomposition products were then condensed into more stable humic acids. The degradation rates of the compounds were directly proportional to the amount of Camellia oleifera meal. The compounds in Camellia oleifera shell were composted faster with higher amounts of Camellia oleifera meals, resulting in less lignocellulose in the final products.


Introduction
Camellia oleifera is the largest woody oil crop in China [1], and the history of cooking oil extraction from Camellia oleifera seed can be tracked back to 2300 years ago [2].The outer shell of fresh Camellia oleifera fruit accounts for more than 60% of its total weight [3].Camellia oleifera meal is the residue left after the oil extraction from the seeds, and its weight is three times of that of the oil [4].
The production of both by-products, Camellia oleifera shell and meal, of tea oil extraction have been significantly increasing with the rapid development of the tea oil industry.These by-products are rich in organic compounds and nutrients, which makes them excellent renewable resources.However, Camellia oleifera shell is mainly composed of cellulose, hemicellulose and lignin that are hardly degraded under natural conditions [5].The Camellia oleifera meal contains large amounts of biotoxic substances, such as tannins and saponin.Therefore, the high production of Camellia oleifera shell and meal and their low utilization efficiency have imposed great threats to the environment.
Composting has been demonstrated an efficient approach to utilizing Camellia oleifera shell and meal.During composting, the unstable Camellia oleifera shell and meal are converted into stable humus substances by aerobic fer- mentation under artificially controlled conditions including water, C/N ratio and air, resulting in good soil improvers and organic fertilizers [6].The rational development and utilization of Camellia oleifera shell and meal by composting can not only reduce waste pollution, but also partially alleviate the shortage of agricultural resources, fostering the environmental and economic developments.
Fourier-transform infrared spectroscopy (FTIR) provides the infrared spectrum of absorption or emission of a compound based on the constant vibrations and rotations of its atomic groups, which gives the composition information of atomic groups, and thus facilitates the understanding of structure of a compound.FTIR analysis has exhibited great advantages including low sample loss, easy operation, fast detection and good stability [7] [8], and thus has been widely used in medicine, agriculture and chemical production.Monitoring composting processes with an infrared spectroscopy can provide more information of the structural changes of the compounds in composting stack [9] [10] [11].For example, Huang et al. successfully applied FTIR to the structural analysis of compounds in livestock manure composting [7].They found that the contents of cellulose, hemicellulose and lignin were correlated with the composting temperature and germination index (GI) of the seeds, which could be used as an indicator of compost maturity [7].Duan et al. studied the infrared-spectral characteristics of organic waste during composting and found that FTIR could provide detailed information on the degradation behavior of composting chemicals, and thus could be used for composting control and real-time monitoring [10].However, different materials are composted differently, and there is not a universal compost maturity indicator established yet [12].So far, to the best of our knowledge, the application of FTIR in the compost of Camellia oleifera shell/meal mixture has not been reported.
In the present work, the composted Camellia oleifera shell/meal was characterized by FTIR to investigate the effects of the ratio of shell to meal of on the degradations of various compounds during composting, aiming to establish the scientific foundation for the utilization of Camellia oleifera shell and meal and the corresponding compost maturity indicators.

Composting
The composting was conducted in an insulated and highly ventilated ecological composter (73 cm × 115 cm × 80 cm, 220 L, BIOLAN).Four experimental groups with different shell/meal ratios were designed, and ech experiment was conducted in 3 replicates (Table 2).The initial C/N ratio of each group was adjusted to 30 with urea, and the contents of both EM and saponin-degrading bacteria were 1.5%.The initial moisture content was adjusted to 55% with water.
The raw materials were stir thoroughly and put in the composter to initiate the aerobic compost fermentation.The temperature of compost materials and room temperature were measured every day at 3 pm.The material in the composter was turned over every 7 days in the first two weeks, and every 14 days thereafter.
For each sampling, 200 g samples were taken from the upper, middle and lower positions of the composter, respectively, mixed well, sealed and stored in a 4˚C refrigerator before use.

FTIR Analysis
The samples collected from composters were analyzed with a Fourier-transform infrared spectrometer (Nicolet iS50, Thermo Fisher Scientific, USA) using KBr pellets.Briefly, 0.001 g sample was dried at 105˚C for 1 h, mixed with 0.1 g KBr powder, and pressed into a thin and transparent disk by the pressed-disk technique for FTIR measurement.For each measurement, a 32-scan absorption interferogram was collected with the resolution of 4 cm −1 in the range of 400 -4000 cm −1 at ambient temperature.Each measurement was repeated three times, and the peak positions and heights were measured in the software Origin 8.0.

Data Analysis
The spectra were plotted in EXCEL and Origin 2017 to determine peak position and height.

Temperature Change of Camellia oleifera Shell/Meal during Composting
As shown in Figure 1, the temperatures in the composters containing the Camellia oleifera shell/meal mixtures of different mass ratios increased dramatical- ly at first, remained at high temperatures with minor fluctuations for a while, decreased to room temperature and remained at the room temperature eventually.This process can be divided into three stages: the heating stage in the first The compost stack temperature change was caused by the microbial activities that were also affected by the temperature.Therefore, the compost stack temperature can be used to evaluate the composting progress as an indicator of compost maturity.However, the temperature cannot directly reflect the changes in the composition of the stack [13].Therefore, the compost samples were further analyzed by FTIR to determine the composition change of the stack, aiming to explore the possibility to monitor the composting progress of the Camellia oleifera shell/meal mixture with FTIR.

IR Spectra of Raw Materials
The Camellia oleifera shell sample exhibited three strong absorption peaks at   These characteristic IR peaks suggest that the Camellia oleifera shell and meal mainly contain carbohydrates, such as cellulose, hemicellulose, lignin, polysaccharides, and so on, proteins, amides, silicates etc.The peaks of Camellia oleracea meal at 2926 cm −1 , 1737 cm −1 , 1513 cm −1 , 1445 cm −1 and 1049 cm −1 are stronger than those in the shell sample, suggesting that the meal sample contains more polysaccharides, fatty acids and amides than the shell sample.No multiple complex bands of -NH 4+ at 2400 -2200 cm −1 were found in the shell sample, indicating that it contained high amounts of lignocellulose and low amounts of proteins [13] [14].

FTIR Analysis of Camellia oleifera Shell and Meal Compost
The IR peak positions of the compost give the structural information of its compounds, and the peak intensities can be used to evaluate the degradation progress of these compounds [15].As shown in Figures 3-8, the compost samples of different ratios of shell to meal exhibited similar IR spectra, suggesting that they contained similar functions groups, consistent with the IR spectra of garden compost waste reported by Xu et al. [16].However, the peak intensities were varied significantly, yet regularly, during the composting.The peak intensity at 3400 cm −1 was weakened significantly at the heating and high temperature stages from day 0 to 30, indicating that the easily decomposable components  including proteins and polysaccharides were degraded, consistent with the high temperatures during this period.The microorganisms used these proteins and polysaccharides to multiply and grow [16], which produced heats to raise the temperature of the stack.The peak intensity was slightly increased thereafter, and remained constant from day 40 to 60, indicating that the proteins and polysaccharides were completed degraded at the heating and high temperature stages.The increased absorptions at 1420 cm −1 and 1640 cm −1 indicated that some of the degradation products were converted into humus in the composter [16].These results suggest that, during the composting, the microorganisms used the easily decomposed carbon sources including proteins and polysaccharides first for growth and multiplication, and after these nutrients consumed, degraded the hemicellulose, cellulose, lignin etc. via oxidative cleavage.The degradation products were then further condensed into relatively stable humic acids [16], a major component of composting product.The composting processes of the mixtures containing different amounts of Camellia oleifera meal are similar.The variations in their IR peak intensities indicate that the amount of Camellia oleifera meal does not affect the degradation order of their compounds, and does affect the degradation degrees of these compounds.
The relative intensity changes of peaks at 1640 cm −1 for aromatic carbon, 1030 cm −1 for polysaccharide carbon, 1420 cm −1 for carboxyl carbon and 3400 cm −1 for aliphatic carbon can be used to evaluate the degrees of decomposition of different compounds, as well as their degrees of aromatization, during the composting [16].As shown in Figures 3-8 and Table 4, the peak height ratios of peak 3400 cm −1 to peak 1640 cm −1 (3400/1640) of the 1/3, 1/4, 1/5 and 1/10 groups decreased 49.8%, 31.9%,25.9% and 23.6%, respectively after the composting, indicating that the amount of Camellia oleifera meal significantly affected the relative contents of aromatic and aliphatic carbons.The most significant declines of 1/3 and 1/4 group were found on day 40, and those of 1/5 and 1/10 groups appeared on day 60.The maximum decline in the peak height ratio of peak 1030 cm −1 to peak 1640 cm −1 (1030/1640) was found to be 47.1%, 49.4%, 62.7% and 52.7% for the 1/3, 1/4, 1/5 and 1/10 groups, respectively, indicating that the contents of aromatic carbons increased and those of polysaccharides decreased after the composting.The polysaccharides in the 1/5 and 1/10 groups were degraded rapidly, resulting in their short high temperature stages.Those in the 1/3 and 1/4 groups were decomposed slower, which explained their longer high temperature stages.The maximum decline in the intensity ratio of peak 1420 cm −1 to peak 1640 cm −1 (1420/1640) was calculated to be 27.1%, 26.3%, 39% and 34.4% for the 1/3, 1/4, 1/5 and 1/10 groups, respectively, indicating that the In summary, Camellia oleifera meal can significantly affect the decomposition degrees of the compounds in Camellia oleifera shell during composting but does not change their degradation order.The amount of Camellia oleifera meal affects the degradation rate and the quality of the final composting product of the shell.

Conclusions
The variation trend of absorption peaks in IR spectra of compost samples with different amounts of Camellia oleifera meal was basically consistent, but the temperature during composting increased more rapidly at the heating stage and the high temperature stage lasted longer as more meal added, reflected as increasing rate of change of height of the infrared characteristic peaks.Compared with temperature, IR spectra are less affected by the environment, and the height of the characteristic peak is directly related to the content of substances in composting.Therefore, IR spectra can be used as one of the indicators for judging compost maturity.
According to the infrared characteristic peaks, the amount of Camellia oleifera meal affected the progress of composting, but did not change the degradation order of the components in the shell.The microorganisms used the easily decomposable carbon source materials including proteins and sugars to grow and multiply first, and, after these nutrients were consumed to a certain extent, oxidatively cleaved the compounds, mainly hemicellulose, cellulose and lignin, and decomposed them.The decomposition products were then condensed into more stable humic acids.The degradation rate of Camellia oleifera shell is proportional to the amount of Camellia oleifera meal.The compounds in the shell were degraded more rapidly as higher amounts of meal added, resulting in less lignocellulose in the final compost.Therefore, Camellia oleifera meal can be used to promote the composting of Camellia oleifera shell, and improve the quality of composting products.
Figure 1.Temperature changes of Camellia oleifera shell/meal mixtures during composting.

Figure 3 .
Figure 3. IR spectra of compost samples on 0 day.

Figure 4 .
Figure 4. IR spectra of compost samples on 9 day.

Figure 5 .
Figure 5. IR spectra of compost samples on 12 day.

Figure 8 .
Figure 8. IR spectra of compost samples on 60 day.

Table 1 .
Properties of the raw materials for composting experiments (%).

Table 2 .
Design of composting experiments.

Table 3 .
Assignments of the infrared absorption peaks of the Camellia oleifera shell and meal samples and the representative compounds.

Table 4 .
Relative peak intensities of major IR absorption peaks of the compost stack during composting.groups in each group decreased, and those of aromatic carbons increased, yet with different reduction degrees at different times.In addition, the absorption peaks at 1030 cm −1 and 1420 cm −1 were significantly weakened at the heating stage, indicating that the polysaccharides were degraded rapidly at the high temperatures.All groups exhibited similar change trends and final values of 1030/1640.Those results, along with the temperature change trend of each group during the composting indicate that the protein degradation is more vigorous, and the composting is faster with the higher amounts of Camellia oleifera meal.The 1/3 group exhibited the highest 1420/1640 and that of the 1/10 group was the lowest, suggesting that Camellia oleifera meal could increase the degradation degrees of cellulose, hemicellulose and lignin.