Stage of maturity of cherries at harvest and OTA risk

Transcription

1 Stage of maturity of cherries at harvest and OTA risk Coffee flowers in response to the resumption of rain following a period of drought, then matures over seven to nine months from flowering to ripeness. Maturation is typically uneven especially where rains resume with a stop/go pattern or where there is no dry season. Often a regional harvesting period extends over 2.5 to 3.5 months. Fig. 1: Various stages of coffee ripening including green, ripe, over-ripe, and non-uniform ripening in arabica and robusta 1 More uniform ripening may occur under certain climatic conditions. Fig. 2: Examples of uniform coffee flowering, ripening and bearing Given the common situation of highly non-uniform ripening, decisions on the timing of the harvest and the method of harvesting largely determine the maturity profile of the harvest. 1 Photos courtesy of John Frank, FAO Consultant (upper left and right, lower left) and Jaques Snoek, CIRAD (upper centre) Page 1 of 6

2 Fig. 3: Cherries of non-uniform maturity, harvested by strip picking (Indonesia) (left), and freshly picked cherries in a basket (right) 2 The use of immature cherries in the production of coffee is associated with poor quality coffee. The beans from such these cherries are prone to blackening during the drying process and coffee made from immature cherries has undesirable taste characteristics. Furthermore, the wet processing of coffee requires uniformly ripe cherries as immature fruit cannot be pulped 3. Therefore, the presence of significant quantities of immature cherries in a harvest presents problems in relation to coffee quality and problems of a technological nature. When cherries are destined for wet processing, labour intensive selective picking is generally practised. Fig. 4: Selective picking 4 With dry or natural processing, stripping of the fruit, either by hand or mechanically, is more common. To avoid the quality problems posed by the use of green cherries, often some selective or semi-selective harvesting method is adopted. Cost and availability of labour is often an important factor, alongside expected price, in harvest method selection. 2 Photo (left) courtesy of John Frank, FAO Consultant. 3 In Brazil, a pulping machine has been devised that will pulp, with good efficiency, only the ripe cherries in a mixture of green and ripe cherries. The output of the pulping operation is the parchment and green cherry which is then separated: the resulting parchment can be fermented. The green cherry is dried separately to produce low quality coffee. 4 Photo (right) courtesy of John Frank, FAO Consultant. Page 2 of 6

3 Fig. 5: Semi-selective harvesting with tarpaulins (India) 5 With the emergence of the problem of OTA contamination of coffee, there has been an effort to understand whether maturity of the cherry is related to the risk of OTA contamination. Data collected within the FAO/CFC/ICO global project and elsewhere (Bucheli et al., see: Selected bibliography under Section 1) consistently show lower levels of OTA contamination in beans from green cherries. As outlined above, however, use of immature coffee cherries is limited by quality and technological considerations. Bucheli et al. (2002) report that surveys of nine farms in Thailand indicate that over-ripe cherries are more prone to OTA contamination. Data generated under the FAO/CFC/ICO global project in Uganda and Côte d Ivoire also suggest that over-ripe cherries might be more prone to OTA contamination, but the variability of the data is too great to allow conclusions to be drawn. Available data from the global project on OTA and mould contamination in different maturity classes are presented below. In some countries, most notably in certain dry regions of Brazil, tree-dried cherries, termed boia, constitute 40 to 70% of the total coffee harvest due to non-uniform ripening and the use of mechanical harvesting. Fig. 6: Mechanical harvesting with coquinho machine, Brazil 6 It is important to more thoroughly investigate mould and OTA contamination in tree-dried and over-ripe cherries so as to establish sound and realistic guidelines on good practice. Work is ongoing in Brazil to better understand this phenomenon. 5 Photo (right) courtesy of John Frank, FAO Consultant. 6 Photo courtesy of Ludwig Pfenning, UFLA Brazil. Page 3 of 6

4 Fig. 7: Tree-dried, over-ripe and ripe fruits displaying fungus 7 Fruit senescence in relation to mould and OTA contamination - theoretical considerations: Fungi can be found on and in fruit and bean tissue from an early stage. As the fruit matures, the frequency of fungi increases and the diversity of species falls. Of course some of these fungi may be pathogenic such as Colletotrichum or Botrytis but in the main the fungal community sustains a benign relationship with the plant and produce no symptoms. Even species that have been reported as pathogenic such as the ubiquitous Fusarium stilboides fall into this category. The community changes as the fruit and bean changes through maturation and senescence for which it is programmed. Unlike many well-known fruit such as apples or citrus, detaching the coffee cherry from the tree induces rapid senescence. This produces a big change in the selective conditions and suddenly fast-growing saprophytic species are favoured. If the fruit is allowed to senesce and dry on the tree the time-course and probably other conditions are very different. A thick superficial growth of Cladosporium can develop on tree dried fruit exposed to persistent showery weather. Coffee harvesting timing and technique are determined by social and technical considerations such as the intended processing method, cost of labour and the current state of the farmer s finances. With strip-picking, where all fruit is removed tree-by-tree, the amount of immature coffee decreases during the season and the amount of over-mature fruit increases. As the proportion of maturity classes differ, so the accompanying fungal communities that will be subject to the processing conditions differ. Fully ripe coffee fruit is dominated by yeast and a limited number of filamentous fungi such as F. stilboides, Cladosporium and the yeast-like organism Aureobasidium. Immature fruit have smaller microbial populations and at the other extreme, as arabica coffee becomes over-ripe on the tree, yeasts remain numerous and P. brevicompactum or other terverticillate species, the fungi mentioned above and occasionally mycotoxin-producing species representing the Aspergillus sections Nigri (black), Flavi (green) and Circumdati (ochre or beige) 7 Photos courtesy of Daniel Duris, CIRAD (left), John Frank, FAO Consultant (centre), and Ludwig Pfenning, UFLA Brazil (right). Page 4 of 6

5 and Eurotium can become relatively numerous. In robusta coffee Aspergillus niger is the most common fungus to develop along with yeasts and some of the same species listed above can be found. As coffee cherries dry after being detached from the tree, the species that become numerous are those that are capable of exploiting saprophytic conditions where water becomes less available. Many of the important fungi in coffee seem able to exist in the fruit and / or bean without producing much growth until conditions alter inducing a change in their performance. Many of the OTAproducing species, in particular, occur at or below the detection limit of enumeration methods but are often easily counted after several days drying as they grow out. Bacteria are less well studied than fungi and there may be extreme seasonal/regional variation according to prevailing climatic conditions. In wet seasons Pseudomonas and species of the Enterobacteriaceae predominate but in dry seasons gram-positive species of Bacillus and Cellulomonas are important. Lactic acid bacteria are uncommon (C.F. Silva, et al., 2000). Few bacteria grow below A w of 0.95 but apparently survive drying, unlike the yeasts isolated from coffee. The large numbers involved suggest these organisms have a role to play while the fruit and bean remain moist as during fermentation. If coffee is processed through a fermentation step, yeast infect the bean during fermentation displacing filamentous fungi but perish during drying often leaving coffee containing almost no viable fungi. This pattern is not always observed and it is likely that if filamentous fungi reach some threshold biomass in the bean before fermentation they will survive and grow out well during drying. From the perspective of OTA contamination during coffee maturation and drying, relief from competition from hydrophilic species and the advent of moisture conditions approaching optimal for the mesophilic OTA-producing species applies to both cases. The activity of the fruit / bean and the coffee-associated species mentioned above distinguish maturation on the tree from senescence induced prematurely by harvest. Also, drying on the tree requires more time than on a well-managed terrace. There is much data showing frequencies of A. ochraceus and concentration of OTA do not always increase during drying despite their presence when the coffee is committed to drying. Corresponding studies of tree drying, which must account for both wet and dry climatic conditions, are pending. The system is complex and other factors such as the presence / absence of liquid water (irrespective of A w ), presence absence of competing / complementary species or the concept of threshold biomass requirements may be expected to produce different outcomes from ostensibly identical trials. 8 8 See: Silva, C.F., Schwan, R.F., Dias, E.S., & Wheals, A.E Microbial diversity during maturation and natural processing of coffee cherries of Coffea arabica in Brazil. Int Journal of Food Microbiology. 60: Page 5 of 6

6 Table 1: Frequency of ochre group contamination in relation to maturity of cherries (data from India, 2001) Category of Cherry Frequency of ochre group contamination in the coffee bean 9 Green 0 Just ripe 0 Sun-scorched / tree dried % Floats % Bulk 4-6.3% Table 2: Mould infection of coffee cherries (data from India, 2003) Stage of % Bean infection in final beans Drying days to reach Maturity Total bean infection Ochre Niger desired moisture level Greens Ripe Over-ripe Bulk Tree dried Table 3: OTA contamination in beans produced from cherries of different maturity classes (data from Côte d Ivoire, 2002) OTA content (ppb)* Stage of Maturity Farm 1 Farm 2 Bean D 0** Bean D final*** Bean D 0** Bean D final*** Green ,8 4,5 Ripe ,0 126,2 Over-ripe ,5 81,3 Tree dried 52,3 73,9 * Due to logistical problems at the trial sites, fresh cherries were held in bags for 3 days before commencing drying **Contamination of bean from fresh cherry (before drying) *** Contamination of bean at the end of sun drying Table 4: OTA contamination in beans produced from cherries of different maturity classes (data from Uganda, 2002) Maturity OTA content (ppb) Husk (trial 1) Beans (trial 1) Beans (trial 2) Green cherry 2,1 0,5 Yellow cherry 3,1 0,1 0,2 Red cherry 26,7 0,6 3,2 Over-ripe cherry 59,3 2,1 Cherry dried on 3,4 1,5 0,1 tree 9 10 kg cherries were harvested from the Chettali experimental farm and sorted into the five categories listed in Table 1. The cherries were sun dried to 12% moisture and the internal infection of the bean analysed according to the method outlined in the FAO/CFC/ICO project mycological handbook. Page 6 of 6