Bioethanol and Fermentation Products from Carob
The fleshy part of the carob fruit contains up to half of its dry weight in sugar. This high and easily extractable sugar content makes carob an interesting raw material not only for food, but also for industrial fermentation. In the last three decades, numerous studies have been carried out to obtain different fermentation products from carob, especially bioethanol, citric acid, lactic acid and mannitol.
In this article, we discuss the sugar profile of carob, the methods used in bioethanol production and the results obtained, the evaluation of carob waste for second generation bioethanol, other fermentation products, and the economic and technical limitations in this field.
The Sugar Profile of Carob: The Foundation of Fermentation
Most products produced by fermentation rely on microorganisms converting sugars. Therefore, the value of a raw material in fermentation depends largely on its sugar content and how easily accessible these sugars are.
In the 2007 study of Biner et al., which examined carob grown in the Mediterranean and Aegean basins of Turkey, it was reported that the most abundant sugar in seedless pods was sucrose, while glucose and fructose were found in lower amounts. Total sugar was measured as 531 ± 93 g/kg in dry weight and 437 ± 77 g/kg in wild types in cultivated varieties.
A similar picture is seen in industrial by-products. Lima-Costa et al. stated that small shards from the carob processing industry contain 45-50 percent soluble sugar; Rodrigues et al. state that approximately 50 percent of the dry weight of carob waste consists of sucrose, glucose and fructose.
An important feature that distinguishes carob from many lignocellulosic raw materials is that its sugars are largely free. While raw materials such as wood and straw require pre-treatment and enzymatic hydrolysis to release sugars, sugars can be extracted with water in carob.
Key Steps of Bioethanol Production
The production of bioethanol from carob generally consists of the following steps:
| Preparation | Breaking the carob, separating the seeds |
| Sugar extraction | Removing sugars into the liquid phase with hot water |
| Fermentation | Conversion of sugar to ethanol by yeasts or bacteria |
| Distillation and purification | Separation of ethanol from fermentation medium |
Sánchez-Segado et al.'s process design for a hypothetical bioethanol plant follows this logic: aqueous sugar extraction from pods, followed by fermentation and distillation.
One of the important studies on extraction conditions belongs to Turhan et al. The researchers determined the optimum extraction conditions for carob fruit by response surface method; They obtained an extract containing 115.3 g/L of sugar at a temperature of 80 °C, 2 hours and a 1:4 fruit:water ratio.
Fermentation with Yeast: Research Findings
The most studied microorganism in the production of ethanol from carob extract is Saccharomyces cerevisiae, also known as bread and brewer's yeast. The following table summarizes some of the results reported in different studies:
| Operation | Method | Main reported result |
|---|---|---|
| Turhan et al. (2010) | Free S. cerevisiae, pH-controlled (5,5) | 42.6 g/L ethanol; 3.37 g/L/hr production rate |
| Yatmaz, Turhan and Karhan (2013) | Ca-alginate immobilized S. cerevisiae, stirred tank bioreactor | 40.10 g/L ethanol; 3.19 g/L/hr; sugar usage 90.66%; duration 24 hours |
| Roukas (1994) | Immobilized S. cerevisiae, agitated culture | Highest 65 g/L ethanol; 0.44 g/g yield |
| Lima-Costa et al. (2012) | High-sugar medium, batch and feed batch | 90–95% of sugar consumed; yield 0.47–0.50 g/g; 100–110 g/L ethanol |
| Rodrigues et al. (2015) | Carob waste, different initial sugar concentrations | Yield of 0.42 g/g at 250 g/L starter sugar |
The values in the table should not be directly compared with each other, as they were obtained under different conditions, at different scales and with different measurement methods. Nevertheless, the overall picture shows that carob sugars are highly consumed by yeasts and ethanol conversion close to theoretical yield is possible.
Some technical lessons from these studies are as follows:
| pH control | Turhan et al. achieved higher ethanol concentration and production rate than uncontrolled pH by keeping the pH at 5.5. |
| Nitrogen source | In the same study, economic nitrogen sources were investigated; Raposo et al. also screened different organic and inorganic nitrogen sources in ethanol production from carob waste. |
| Immobilization | In the study of Yatmaz et al., it was shown that immobilized cells can be reused for five cycles. |
| Sugar concentration | Rodrigues et al. reported that the initial sugar concentration significantly changes yeast metabolism; at high concentrations, sugar is almost completely directed to ethanol. |
Bacterial Fermentation and Solid-State Approaches
In addition to yeasts, the bacterium Zymomonas mobilis has also been tested in ethanol production from carob. Vaheed et al. optimized ethanol production from carob extract with Z. mobilis by statistical experimental design. Saharkhiz et al., on the other hand, pointed out that sugar extraction is a costly step and examined "solid-submerged" fermentation that does not require extraction and obtained 0.42 g of ethanol per gram of starting sugar with Z. mobilis under 30 °C and pH 5.3 conditions.
The idea of reducing the extraction step also comes to the fore in the evaluation of carob waste. Bahry et al. described a process for the production of second-generation ethanol from solid wastes left over from the preparation of carob syrup in the Lebanese food industry; compared submerged fermentation with solid-state fermentation. In the study, it was reported that the operation of solid-state fermentation is simpler and more cost-effective, with a yield of 0.458 g of ethanol per gram of sugar consumed.
Carob Waste and Second Generation Bioethanol
"First generation" bioethanol is produced directly from sugary or starchy feedstocks, which can also be used as food, while "second generation" bioethanol is produced from agricultural and industrial waste. Carob occupies an interesting position in this distinction.
The main economic value of carob comes largely from the gum obtained from the kernel; pulp is processed into products such as flour, molasses and feed. Small kibble produced during processing and pulp from molasses or syrup production can still contain significant amounts of sugar. Lima-Costa et al. identify the waste materials of the carob processing industry as a potential source for second-generation bioethanol.
This approach also makes sense for the circular economy in the carob industry: the conversion of the sugary by-products left after the production of food products into energy or chemical raw materials allows more value to be extracted from the same raw material.
Fermentation Products Other Than Bioethanol
Carob sugars have been tried as a nutrient medium not only for ethanol but also for different biotechnological products.
Citric acid: In Roukas' 1998 study, the production of citric acid by surface fermentation from carob extract with Aspergillus niger mold was investigated; 200 g/L initial sugar and citric acid concentration up to 85.5 g/L were obtained under pH 6.5 and 30 °C conditions.
Lactic acid: Azaizeh et al. examined the production of lactic acid from carob, banana and sugar cane biomasses; reported that the addition of yeast extract in carob increased productivity. Bahry et al. also optimized lactic acid production from carob waste of the Lebanese food industry with Lactobacillus rhamnosus trapped in alginate beads.
Mannitol: In the study of Carvalheiro et al., eight lactic acid bacteria that produce mannitol were produced in fortified carob syrup; all strains were reported to produce mannitol with yields of over 0.70 g per gram of fructose.
| Product | Microorganism (example) | Possible use case |
|---|---|---|
| Ethanol | Saccharomyces cerevisiae, Zymomonas mobilis | Fuel, solvent, chemical industry |
| Citric acid | Aspergillus niger | Food acidifier, pharmaceuticals and cleaning products |
| Lactic acid | Bacillus coagulans, Lactobacillus rhamnosus | Food additive, bioplastic (PLA) raw material |
| Mannitol | Lactic acid bacteria (Leuconostoc, etc.) | Low-calorie sweetener, pharmaceutical industry |
Economic and Technical Limitations
The promising results obtained on a laboratory and pilot scale do not mean that carob can be used as a bioethanol feedstock on an industrial scale. There are several reasons for this:
| Raw material cost | Sánchez-Segado et al.'s economic analysis for the hypothetical plant showed that the plant's profitability is highly dependent on the price of the locust bean pod, and that the plant can only be profitable below a certain raw material price. Increasing capacity has also been reported to affect costs. |
| Competing uses | Carob pulp is used in the food (flour, molasses, syrup) and feed sectors; these uses generally offer higher added value than energy production. |
| Seasonality and supply | Carob is harvested once a year; for a constantly operating plant, it is necessary to store the raw material. |
| Extraction cost | As Saharkhiz et al. emphasize, sugar extraction is a costly step; therefore, alternatives such as solid-state and solid-submerged fermentation are being explored. |
For these reasons, bioethanol production from carob gains meaning within a biorefinery approach in which sugary by-products that emerge after food processing are evaluated, not as a primary product.
Frequently Asked Questions
Why is carob considered a suitable raw material for bioethanol? Carob pulp contains up to about half of its dry weight of sugars, which can be easily extracted with water because they are largely free.
Is bioethanol produced from carob on a commercial scale? The sources examined in this article are mainly laboratory, pilot-scale and hypothetical plant studies. Economic analyses show that profitability is strongly dependent on the price of raw materials.
What does second generation bioethanol mean? It is bioethanol produced from agricultural and industrial wastes instead of raw materials used directly as food. Carob processing wastes and syrup production pulps are evaluated in this context.
What can be produced from carob other than ethanol? In research, products such as citric acid, lactic acid and mannitol were obtained from carob sugars by fermentation.
Carob Raw Material with Carob Türkiye
In fermentation-based applications, as in food applications, the cleanliness, moisture and consistency of the sugar content of the raw material directly affect the result. Using standard raw materials in research and development projects is also important for the reproducibility of the results.
At Carob Türkiye, we supply carob fruit, crumb, flour/powder, kernel and extract. For related products Carob Kibble (Granular) and Carob Extract pages for our entire product range Our Products You can review our page. For your wholesale and export requests Contact Now You can contact us on our page.
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