Letters in Applied Microbiology 1998, 26, 175–178

The influence of yeast on thermophilic composting of foodwaste

M.H. Choi and Y.-H. ParkDepartment of Biotechnology, Ajou University, Suwon, Korea

1590/97: received 15 September 1997 and accepted 24 September 1997

M.H. CHOI AND Y.H. PARK. 1998. The microbial changes during thermophilic compostingof food waste were investigated at 50 °C using a laboratory-scale composter. The resultsshowed that an early increase in the growth of yeast was followed by the growth ofthermophilic bacteria, and that subsequently, the yeast declined rapidly. Thegrowth interactions of selected strains of yeast and bacteria were studied in culturemedia containing organic acids. The results demonstrated that the acidity of the media wasreduced during the growth of the yeast. Therefore, it is concluded that in the earlystages of composting, the growth of thermophilic bacteria is inhibited by organicacids, and that the growth of yeast enables subsequent bacterial growth by removingorganic acids from food waste.

INTRODUCTION MATERIALS AND METHODS

The disposal of large amounts of food waste generated by foodComposting

service establishments is one of the most serious problems incities. As food waste has a high water content and decomposes Composting was performed in a laboratory-scale composting

reactor. The compost was prepared by mixing food wastereadily, many unpleasant environmental consequences canarise during its storage, collection and transportation. from Ajou University Restaurant and sawdust in an 8 : 2

(w/w) ratio, to give a moisture content of approximately 65%Recently, several types of in-vessel composting systems havebeen developed for installation in food service establishments (w/w). During the composting process, the temperature was

maintained at 5022 °C and air was supplied at a flow rate ofto manage food waste as a recyclable resource. These com-posting systems primarily utilize a thermophilic method and 2·5 l min−1. The composting material was mixed by an impel-

ler at a speed of 5 rev min−1.the seeding of microbial agents.The microbial ecology of composting food waste should

be fully understood to evaluate the effect of seeding. The Isolation and counting of micro-organismsmicrobiology of composting has been reported in processes

The growth of yeast and thermophilic bacteria was measuredusing organic materials such as sewage sludge (Nakasaki etby a spread plate counting method. Composting material (1al. 1985 ; Fujio and Kune 1991) and paper residues (Stromg) was suspended in 50 ml of sterile saline and mixed for 151985a,b). There are only a few reports, however, on themin. The suspension was serially diluted in 0·1% (w/v)microbial changes in composting food waste which arepeptone water, and then spread onto agar plate media.responsible for the decomposition of its constituents (Nak-Thermophilic bacteria were counted on tryptic soy agarasaki and Akiyama 1988 ; Nakasaki et al. 1990, 1992).(Difco, Detroit, MI, USA) after 1–2 days of incubation atIn this study, the microbial changes that occur in com-50 °C. Yeast were counted on malt extract agar (Difco) afterposting of food waste, and the interactions between inoculated2–3 days of incubation at 30 °C.micro-organisms and those naturally present in the waste in

a laboratory scale composter, were investigated.Seeding

Strain Y60 was selected from 20 strains of yeast isolatedCorrespondence to : Dr Yun-Hee Park, Department of Biotechnology, AjouUniversity, San 5 Woncheon-dong, Paldal-gu, Suwon, 442-749, Korea. from composting food waste for its ability to grow at high

© 1998 The Society for Applied Microbiology

176 M.H. CHOI AND Y.-H. PARK

temperature, and was used for seeding the preparation. This which remained fairly constant (Fig. 1). In contrast, meso-philic bacterial counts decreased rapidly during the first daystrain was identified as Kluyveromyces marxianus using the

BIOLOG automated microbial identification system (Biolog from 108 cfu g−1 to less than 103 cfu g−1 (data not shown).Fungi and actinomycetes were scarce throughout the experi-Inc., Hayward, CA, USA). A yeast culture was grown in

yeast extract-malt extract (YM) broth (Difco) at 48 °C for 24 ments. These results show that in composting food waste,yeast and thermophilic bacteria are the micro-organismsh, and then pelleted by centrifugation for 20 min at 4000 g.

The cells were washed and resuspended in saline for seeding mainly responsible for decomposition. Common charac-teristic changes in the population of micro-organisms, seenat a level of 107 cells g−1 of composting material.in all experiments, are the early proliferation of yeast and thesequential growth of thermophilic bacteria, followed by rapiddecline of yeast. The reason for this loss of viability is

Culture of micro-organismsunknown, but may possibly be due to high temperatures.

Nutrient broth (Difco), adjusted to pH 4·7 with lactic acid,was used to compare the growth of selected strains of yeast

Effect of yeast seeding on the development ofand bacteria. Kluyveromyces marxianus Y60 and the thermo-

thermophilic bacteriaphilic bacteria, Bacillus sp. B77, also found in compostingfood waste, were both used for inoculation at levels of 105 When a starting material was seeded with a selected yeast

culture at a level of 107 cfu g−1 of compost, the thermophiliccolony forming units (cfu) ml−1, either separately or in amixed culture, at 48 °C. bacterial counts increased rapidly from the first day (Fig. 2).

Analysis

To measure the pH changes during composting, 9 ml ofdistilled water were added to a 1 g sample of the compostingmaterials. The slurry was mixed by shaking at 50 rev min−1

for 5 min and the pH of the slurry was measured with a pHmeter (Orion 520A, Orion, Beverly, MA, USA). Titratableacidity was determined according to the procedure of Stan-dard Methods for Water and Wastewater (American PublicHealth Association 1992). Composting material (2 g) wassuspended in 18 ml of distilled water and mixed by shakingat 100 rev min−1 for 10 min. The suspension was centrifugedat 4000 g for 15 min and the supernatant fluid was used to Fig. 1 Changes in thermophilic bacteria (ž), yeast (R) and pHdetermine the acidity. The suspension (10 ml) was titrated (�) during composting of food waste at 50 °Cwith 0·05 N standard sodium hydroxide to pH 8·3. Aciditywas calculated and expressed as weight percent of lactic acidfor the composting material. In culture broth experiments, 5ml of broth were used to determine pH and acidity. Theacidity was expressed as weight percent of lactic acid forthe broth. The dry weight of the composting material wasdetermined by drying samples weighing about 2 g at 105 °Cfor 24 h, or until a constant weight was reached.

RESULTS

Microbial succession in composting of food waste

Yeast grew most rapidly, reaching a maximum density of Fig. 2 Influence of yeast seeding on changes in thermophilic1·6×108 cfu g−1 of dry weight of compost, then rapidly bacteria (ž), yeast (R) and pH (�) during composting ofdeclined and disappeared. Thermophilic bacteria started to food waste at 50 °C. The food waste was seeded with

Kluyveromyces marxianus Y60 at a concentration of 107 cfu g−1grow 2 days later, reaching densities of 1·6×109 cfu g−1

© 1998 The Society for Applied Microbiology, Letters in Applied Microbiology 26, 175–178

EFFECT OF YEAST ON COMPOSTING OF FOOD WASTE 177

Yeast numbers declined after reaching their maximum level,while the population of thermophilic bacteria was maintainedat the maximum level of 5·5×109 cfu g−1 for several days.The results indicated the acceleration of bacterial devel-opment by seeding with yeast.

Some interesting observations were made in cases wherecomposting failure was reversed by seeding with yeast. Inone run (Fig. 3), there was a slight initial growth of yeast,but the micro-organisms failed to grow normally and thecounts of viable thermophilic bacteria stayed at initial levels Fig. 4 Changes in acidity (e) and pH (�) during the growthfor 8 d. After seeding with yeast at a level of 5×106 cfu of (a) Bacillus sp. B77 (ž), (b) Kluyveromyces marxianus Y60

(R), and (c) mixed culture of Bacillus sp. B77 and K. marxianusg−1 of compost, a proliferation of yeast was observed withY60 in nutrient broth (pH 4·7) at 48 °Cconcomitant reduction of acidity, followed by a drastic

increase in thermophilic bacteria numbers within 24 h. Theseresults confirm the theoretical model, that yeast stimulate thegrowth of thermophilic bacteria in composting food waste bythe elimination of organic acids.

clearly explain the stimulation of thermophilic bacteria bythe initial growth of yeast.

Growth interactions in acidified culture broth

In order to better understand the interactions that occurDISCUSSION

between yeast and thermophilic bacteria in composting foodwaste, a controlled study was conducted in a culture system The data presented in this study demonstrate substantial

populations and high activity of yeast in the initial stages ofof uniform composition with selected strains. In pure culture,Bacillus sp. B77 would not grow in a nutrient broth at pH 4·7 thermophilic composting of food waste. Previous studies have

found that mesophilic and thermophilic bacteria are the domi-and lost viability within 48 h (Fig. 4a), while Kluyveromycesmarxianus Y60 grew normally, rapidly reducing the acidity nant micro-organisms during thermophilic composting of

food waste and that the activity of actinomycetes or fungi isof the medium within 12 h (Fig. 4b). Comparison of yeastgrowth curves in pure and mixed cultures (Fig. 4b,c) demon- not significant (Nakasaki et al. 1990, 1992). As the presence

and activity of yeast during thermophilic composting has notstrated that the growth of yeast was unaffected by the pres-ence of bacteria. In contrast, in a mixed culture, Bacillus sp. been previously reported, the early rapid proliferation of yeast

in all our experiments was an unexpected observation. ThisB77 started to grow rapidly after the organic acid had beentotally removed by the yeast (Fig. 4c). In this culture, the proliferation was probably due to the composition of the food

waste, which contained rice and vegetable residues. Unlikegrowth of yeast resulted in a rapid reduction in acidity andconsequent growth of thermophilic bacteria. These results other composting materials with an initial pH near neutrality,

the food waste used in these experiments was characterizedby a low initial pH, between 4·5 and 5·0. The ability of yeastto grow at a lower pH than bacteria explains the initial rapidgrowth of the yeast. The only reference to microbial changein similar material appears to be that of Nakasaki et al. (1992)who reported the rapid increase of thermophilic bacteriaduring composting of garbage containing rice. However, inthat case, the pH of the starting materials was 8·0 or evenhigher, bacterial growth was dominant, and the populationof fungi or yeast was negligible.

In all the experiments, the composting process was initiatedwith actively growing yeast and subsequent growth ofthermophilic bacteria. Yeast seeding of the starting materialFig. 3 Effect of yeast seeding in cases of composting failure. Theshortened the lag-phase of thermophilic bacteria. Thesecomposting material was seeded with Kluyveromycesresults can be explained by postulating that thermophilicmarxianus Y60 at a concentration of 5×106 cfu g−1 after 8 days.bacteria started to develop only when the acidity was reduced(ž), thermophilic bacteria, (R) yeast, (�) pH, (e) acidity. The

arrow shows the inoculation of yeast by the growing yeast. This model was strongly supported by

© 1998 The Society for Applied Microbiology, Letters in Applied Microbiology 26, 175–178

178 M.H. CHOI AND Y.-H. PARK

the fact that the bacterial development was activated by REFERENCESseeded yeast in cases of composting failure. Consequently,

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906–913.activator in cases of composting failure.

© 1998 The Society for Applied Microbiology, Letters in Applied Microbiology 26, 175–178