4.1. Tropical Fruit Peel Materials
In silage-making practices, microorganisms are significant contributors to the fermentation process especially lactic acid bacteria, as they utilize the sugar content in the ensiling materials to support their growth and produce lactic acid in the silo [
27]. The optimal quantity of epiphytic LAB required for producing high-quality silage from tropical grass was determined to be 10
5 cfu/g FM [
28]. In the current study, the population of LAB in all fruit peel materials was ranged from 10
4 to 10
5 cfu/g FM (
Table 1), which might be adequate for fermenting and producing lactic acid. Whilst the population of coliform bacteria, aerobic bacteria, yeast, and mold were higher (10
7 to 10
9 cfu/g FM) and towered above epiphytic LAB population. These undesirable microbiotas can cause anaerobic spoilage or aerobic spoilage [
29]. Thus, to prevent fermentation failures, the addition of silage additives is necessary in the ensiling process. To ensure good quality silage, it is crucial to consider not only the LAB population but also the chemical composition of the ensiling material including the contents of DM and WSC. It is essential that these components are appropriate and present in adequate quantities. [
9]. In the present study, the DM content of fruit peels ranged from 17 to 22%, which lower the optimal range (30 to 35%) that reported by Wilkinson [
30] and McDonald et al. [
27], and the WSC content ranged from 4.20 to 4.61% of DM. The CP content of ensiling materials ranged from 2 to 5% of DM, while the NDF and ADF contents relatively high (55 to 68% and 43 to 55% of DM for NDF and ADF, respectively). The high NDF and ADF contents were not conducive to ensiling fermentation and animal digestion [
31].
4.2. Fermentation Characteristics of Tropical Fruit Peel after Ensiling
The most common parameters used to evaluate silage fermentation quality are pH, organic acids, alcohol, and ammonia-N contents, as well as the populations of microorganisms. The results show that all silages were well-preserved, with low pH values (3.5 to 4.2) and high lactic acid concentration (
Table 2). The DRP silages were higher (
p < 0.01) DM content, lactic acid, and ammonia-N than that of CCP and SPP silages, likely due to the DRP material having high DM and WSC contents. The high WSC content indicates that it could provide more ensiling substrate, such as sugar content, to produce lactic acid and rapidly decrease pH in the silo, this low pH could inhibit the undesirable fermentation and conserve more nutrient substrate [
27]. When compared to SPP and DRP silages, the CCP silages were the highest total alcohol concentration, this could be attributed to the abundance of yeast in the ensiling material (10
9 cfu/g FM), which utilized sugar as a substrate to produce alcohol during the fermentation process. Alcohol is one of the four main volatile organic compounds detected in silage [
32]. The SPP silages were the lowest pH and highest acetic acid concentration compared to CCP and DRP silages, which could be attributed to the high number of aerobic bacteria present in the SPP material (10
9 cfu/g FM). Aerobic bacteria, known as acetic acid bacteria are capable of growing at low pH and cultivate acetic acid by metabolizing ethanol [
33]. Our finding is consistent with Yang et al. [
5] who reported that the factors involved in assessing fermentation quality include the chemical composition and the physiological properties of epiphytic bacteria of fruit residue material.
The important silage fermentation product is lactic acid since it can rapidly reduce pH, inhibit the growth of harmful microorganisms, and preserve forage nutrients [
11]. Silages treated with TH14+AC and TH14+AC+molasses were lower pH and higher lactic acid content when compared with control treatments, this might be due to the synergetic effect between additives. Singh et al. [
34] reported the efficacy of exogenous fibrolytic enzyme and LAB inoculant was higher when used in combinations. Similar to our findings, Kaewpila et al. [
17] and Si et al. [
35] found that the lactic acid content increased in cassava pulp silage and mixed of alfalfa and
Leymus chinensis silage when treated with the combination of LAB and fibrolytic enzyme treatment.
The result found that acetic acid production increased when the additive was AC and Molasses+AC. The production of acetic acid could potentially improve the aerobic stability of silage [
27]. While Guan et al. [
36] reported that heterofermentative LAB produce acetic acid and their occupation of the microbial niche during terminal fermentation can enhance the aerobic stability of corn silage. For high-quality silage, Kaewpila et al. [
17] suggest that the acetic acid concentration should not exceed 10–20%. For this reason, the final acetic acid contents of silages may have been related to the presence of epiphytic LAB populations in ensiling materials.
Generally, propionic acid and butyric acid is usually low or undetectable in well-fermented silages, high concentrations of propionic acid (>0.3–0.5%) are more commonly found in clostridial fermentations, and the presence of butyric acid indicates metabolic activity from clostridial organisms, which leads to large DM losses and poor recovery of energy [
6]. In this study, propionic acid was highest in the control compared to other treatments, while butyric acid and ammonia-N were not significantly different among treatments. Possibly, the presence of propionic acid in control silage may be due to propionic acid bacteria converting glucose and lactic acid to propionic and acetic acids. However, the concentration of these acids and ammonia-N of tropical fruit peel silages were all within the acceptable ranges as suggested by [
37]. Consequently, the use of a silage additive such as LAB inoculant, cellulase, and their combination could improve fermentation quality and inhibit the growth of harmful microorganisms.
In our study, it was observed that silage treated with AC alone or in combination with TH14 and molasses showed the highest total alcohol content compared to the other treatments. This could be attributed to the fact that AC aided in breaking down the fiber into sugar, which served as a substrate for yeast to produce alcohol. The high alcohol (ethanol) contents in the silages result from the metabolism of yeasts and heterofermentative LAB that convert the available WSC into ethanol and CO
2 [
27,
38]. Kung et al. [
6] stated that the high concentrations of ethanol in silages (>3–4% on DM) are often associated with high numbers of yeasts, and such silages usually spoil readily when exposed to air, and high amounts of ethanol are also associated with high losses of DM.
4.3. Chemical Composition and Microbial Populations of Tropical Fruit Peel after Ensiling
In the present study, the SPP and DRP silages were higher CP content and lower NDF, ADF, and ADL contents compared to CCP silages (
Table 3), this could be due to the chemical composition property of ensiling materials, which has a higher CP and lower fiber contents than CCP, indicating that SPP and DRP are appropriate for use as an alternative feed resource for ruminants. Kaewpila et al. [
14] stated that the chemical compositions of crop silage were specifically affected by the silage additives. In this study, TH14+AC, Molasses+TH14, Molasses+AC, and Molasses+TH14+AC treatment increased the CP content more than the control and other treatments. This result may be clarified by the ability of the additives to rapidly decrease the pH value of silage, inhibit the activity of harmful microorganisms reduce the CP degradation and ultimately reduce the loss of nutrients [
9]. The silages treated with molasses+AC, and molasses+TH14+AC not only increased CP content but also decreased NDF and ADF content. This finding is consistent with Kaewpila et al. [
14] who mentioned the reduction of NDF and ADF contents with AC is associated with enzymatic saccharification, releasing fermentable sugars for lactic acid fermentation.
Our study revealed that all tropical fruits peel silages ensiled for 30 days still had abundant LAB and aerobic bacteria, while coliform bacteria, yeast, and mold were below the detectable level (
Table 4). The CCP silages showed higher LAB and aerobic bacteria populations than SPP and DRP silages (
p < 0.01). However, silage treated with additives showed lower aerobic bacteria numbers than control treatments. These findings are attributed to the additive having the potential to improve the ensiling process [
39].
4.4. In Vitro Rumen Fermentation
The in vitro fermentation is important for estimating energy partition potential of ruminant feedstuffs [
17,
40]. The physical properties of ensiling materials play a significant role in determining the digestibility efficiency of silages. In the present in vitro experiment, after 24 and 48 h of incubation, the SPP and DRP silages were significant higher IVDMD, IVOMD, and total VFAs content, while pH and ammonia-N were lower than CCP silages. (
Table 5 and
Table 6). For this reason, it could be attributable to the CCP material having high lignin content, which is difficult to digest. These results agree with those of Hartati et al. [
41], who reported that if the lignin content in the feed is high, the digestibility coefficient of the feed is low. After 24 and 48 h of incubation, silage treated with molasses was highest IVDMD and IVOMD compared to other treatments. Meanwhile, total VFAs contents was higher when silage treated with molasses and AC after incubation at 24 and 48 h, respectively. In the agreement of these results, Yildiz et al. [
42] reported that silage prepared from
Brassica rapa at the end of the flowering stage treated with molasses leads to increased IVDMD and IVOMD. Similar to our results, Dong et al. [
43] and Gül [
15] also reported increased IVDMD and IVOMD due to the supplementation of molasses. On the other hand, Xie et al. [
9] and Wang et al. [
12] reported that the IVDMD and total VFAs of alfalfa and paper mulberry silage treated with molasses did not differ with control silage. The addition of molasses enhanced digestibility may be explained by the higher residual contents of WSC, which can be utilized by rumen microorganisms for degradation during in vitro incubation [
43].