Elsevier

Bioresource Technology

Volume 97, Issue 8, May 2006, Pages 1030-1035
Bioresource Technology

Bioconversion of waste office paper to gluconic acid in a turbine blade reactor by the filamentous fungus Aspergillus niger

https://doi.org/10.1016/j.biortech.2005.04.040Get rights and content

Abstract

Gluconic acid production was investigated using an enzymatic hydrolysate of waste office automation paper in a culture of Aspergillus niger. In repeated batch cultures using flasks, saccharified solution medium (SM) did not show any inhibitory effects on gluconic acid production compared to glucose medium (GM). The average gluconic acid yields were 92% (SM) and 80% (GM). In repeated batch cultures using SM in a turbine blade reactor (TBR), the gluconic acid yields were 60% (SM) and 67% (GM) with 80–100 g/l of gluconic acid. When pure oxygen was supplied the production rate increased to four times higher than when supplying air. Remarkable differences in the morphology of A. niger and dry cell weight between SM and GM were observed. The difference in morphology may have caused a reduction of oxygen transfer, resulting in a decrease in gluconic acid production rate in SM.

Introduction

Since the Industrial Revolution, people have benefited from petroleum resources. As a consequence, serious environmental problems have occurred such as air pollution, global warming and deforestation. Finding alternatives to petroleum products is thus an increasingly important objective for research. Recently, instead of using petroleum products, products utilizing biomass as raw materials have been developed for many markets. One of these, cellulolytic biomass, is known as a carbon neutral material because it does not increase the amount of carbon dioxide in the air.

Waste paper is one of the cellulolytic biomasses targeted to be recycled because it is a cause of environmental problems in Japan. Today in Japan about 30 million tons of paper is produced and consumed each year. The increased visibility of recycling has caused an increase of public awareness so that 66.1% of annual paper production is collected and 60.2% is reused (Terasawa, 2005). Although this recycling ratio is relatively high, it has not yet reached a satisfactory level. Excess paper that is not used by the Japanese market is exported to other countries. However, the price of paper has been rapidly decreasing and in this decade alone paper prices have been reduced by half. Moreover, when paper materials are recycled, they are usually turned into lower grade paper products; for example, conversions from office paper to magazine paper and from cardboard to sanitary products. With further recycling of paper, fiber length in the paper becomes shorter. Since the shortening of paper fibers decreases the quality of paper, the maximum ratio of paper-to-paper recycling is said to be 65%. In addition, paper manufacturers are concerned about waste fibers which are not fit for recycling. This byproduct is disposed of by incineration or landfill without being reused. However, due to the shortage of new suitable disposal sites, environmental awareness, and the awareness of the greenhouse effect, these methods of disposal will be impossible in the near future. Finding alternative ways to recycle paper is an urgent necessity. One way of using waste paper is to decompose it to reducing sugars and to convert the sugars to value-added bioproducts; one such use of reducing sugars is fermentation into ethanol (Scott et al., 1994; Wayman et al., 1993).

Bioconversion of waste paper to l-(+)-lactic acid by the filamentous fungus Rhizopus oryzae has been studied previously by some of the present authors. Although the lactic acid yield from waste office paper hydrolysate was similar to that of a glucose medium, the production rate was inhibited by either xylose derived from hemicellulose or unknown compounds originating from paper pulp (Park et al., 2004).

This paper suggests an alternative way of using waste paper by converting it to gluconic acid. Gluconic acid has been used as a food additive, in sterilization solution or bleach in food manufacturing factories, and as salt in chemical components for medication. Using waste paper hydrolysate, the gluconic acid yield and production rate were compared to those obtained with a glucose medium in a flask and bioreactor using the filamentous fungus Aspergillus niger.

Section snippets

Enzyme hydrolysis

Waste office automation (OA) paper was used as the experimental material because of its availability. OA paper refers to the papers used in plain paper copiers (PPC), and waste OA paper is also defined as printed PPC paper after printing in a printer or copy machine. Four hundred grams of waste OA paper were cut into small pieces by a standard office shredder to rectangles 2 mm wide and 1.5 cm long, and were used for enzymatic hydrolysis without any further pretreatments (Park et al., 2001). They

Gluconic acid production using SM in flask

When the initial glucose concentration was adjusted to 50 g/l in SM, 46.0 g/l of gluconic acid was produced, while 40.4 g/l of gluconic acid production was obtained from 50 g/l of glucose in GM (Fig. 1). The profiles of glucose consumption in SM and GM overlapped (Fig. 1(a)). The yields of gluconic acid based on glucose consumption were 92% in the SM and 80% in the GM.

Reducing sugars, such as xylose and cellobiose contained in the culture broth of A. niger, were measured as shown in Fig. 1(b). The

Discussion and conclusions

Much research on gluconic acid production has been done in cultures of A. niger using glucose as a carbon source, as shown in Table 1. Vassilev et al. (1993) immobilized A. niger on polyurethane foam and obtained 143 g/l of gluconic acid from 150 g/l of reducing sugar from corn hydrolysate. Sankpal and Kulkarni (2002) immobilized A. niger on cellulose microfibers and obtained 6.56 g/l d with 158 g/l of gluconic acid in a continuous recirculation reactor. They also immobilized A. niger on cellulose

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