Bread Residues Conversion Into Lactic Acid by Alkaline Hydrothermal Treatments

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Bread is the most vital element in a human diet. It is consumed almost everywhere in the world. Since it is consumed in large amounts, thus a large amount of residue will also be produced from it. Reasons of residue can be leaving out crust of bread during consumption, ill storage management or poor quality bread. Bread is produced from fermentation (breaking down complex compounds into simpler ones) by yeast on sugar to carbon dioxide. Bread mainly contains starch and starch is a useful element as it can be used for energy storage or can be converted to useful chemicals like lactic acid, ethanol, etc.

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Lactic acid can be produced from starch by fermentation but the process is slow and complex. The better method of producing lactic acid is Hydrothermal Process (crystallizing substances from high-temperature aqueous solutions at high vapor pressures) using alkali like NaOH, KOH, Ca(OH)2, LiOH and K2CO3 as catalysts.

Initially, a piece of bread is reduced to pieces of less than 0,5 mm and then various tests were run over it to determine its composition.

Moisture content was determined by putting it up in an oven at 80 C, elemental analysis was done using an Elemental Analyser and starch content was found using spectrophotometry. It was found that the bread contains 5.6% moisture, 75% starch, and elemental analysis as “C: 43.14%, H: 7.24%, N: 2.36%, O: 47.26%”. After that, it was subjected to the hydrothermal process using an alkali as a catalyst. 2g of bread with 50 ml of alkaline was made and various experiments were done over it varying the alkali.

NaOH (98%), KOH (85%), Ca (OH)2 (95–100%), LiOH (98–100%) and K2CO3 (99%) solutions (0.5 M) were used as alkaline catalysts. Sodium, Calcium and Potassium Hydroxide presented the highest alkali yield (14.41 ± 0.31, 14.79 ± 0.45 and 14.15 ± 0.69 g/L).

The lowest lactic acid yield was obtained when the hydrothermal experiment was performed without any alkali (using water as catalyst) depicting that the hydrothermal process with an alkali performs better than the traditional hydrothermal process. After finding this out, experiments were performed by varying the concentration from 0 to 1 M for NaOH and KOH and 0 to 3.5 M for Ca(OH)2. It was found that KOH obtained a maximum yield of around 38 % at 0.4 M, NaOH obtained its maximum yield of around 35 % at 0.6 M and Ca(OH)2 obtained its maximum yield of around 72 % at 3.5 M. Therefore, Ca(OH)2 should be preferred for greater yield at large expense of alkali while KOH should be preferred when moderate alkali is to be used.

Hydrothermal treatment of starch not only produced lactic acid but also some useful byproducts. When KOH was used as an alkali, 4.93 ± 0.25 g/L formic acid and 2.06 ± 0.38 g/L acetic acid were produced while in the case of NaOH, 4.69 ± 0.34 g/L of formic acid and 1.99 ± 0.17 g/L of acetic acid was produced. Glucose, ethanol, acrylic acid, arabinose, xylitol, and xylose were also obtained but in very little quantities. For the case of Ca(OH)2, 1.21 ± 0.06 g/L of arabinose, 0.75 ± 0.13 g/L of xylitol, 4.00 ± 0.21 g/L of formic acid, 3.09 ± 0.18 g/L of acetic acid, 1.07 ± 0.05 g/L of acrylic acid and 1.16 ± 0.08 g/L of ethanol were obtained. The amount of major byproduct in Ca(OH)2 was still lower than the amount obtained in the case of NaOH and KOH. This leads to a conclusion that when catalyst concentration is higher more degradable byproducts will be formed.

Thus, the production of lactic acid from bread residue is a promising method with Ca(OH)2 as alkali as it produces 73 % yield and other useful byproducts like formic acid, acetic acid, etc.