CANE DETERIORATION AS AFFECTED BY BILLNET SIZE , DELAY IN MILLING AND OTHER FACTORS

Trials were conducted to determine comparative changes with time in weight, juice quality and recoverable sugar of whole stalk and chopped sugarcane of several varieties, and to study the effect of billet size on rate of deterioration. In addition, an estimate was made of size distribution of billets from a chopper harvester. It was concluded that under hot, humid conditions chopped cane deteriorates much faster than whole stalk cane, rate of decline in purity and recoverable sugar tending to increase with decrease in stalk length, while reducing sugars showed a corresponding increase. The chopper harvester, set to cut 30 cm billets, delivered a high proportion (25 %) of short billets (< 20 cm) many of them immature and partly damaged, and these would tend to accelerate deterioration under field conditions. Introduction The advent into the South African sugar industry of chopper harvesters for sugarcane has focussed attention on problems related to post harvest deterioration of chopped cane in other parts of the world, particularly Australia. A number of Australian and other workers (Eganl, Egan and Rehbein3, Irvine and Legendre6, Kirbyg, and Vallance and Younglo) have reported on the increased rate of deterioration of chopped cane by comparison with whole stalk cane, and the importance of keeping periods of storage before its processing to a minimum. Changes in juice quality, the effects of burning, weather and varieties and more recently the effect of billet size on cane deterioration have received attention (Gentry and Gascho5, Irvine and Legendre6, Ivin7 and Ivin and ~ e v a n ~ ) . In this paper results are reported of three trials with chopped cane, two of them conducted in collaboration with the Tongaat Sugar Company which operates a chopper harvester. Procedure TRIAL 1 The object of this trial was to study deterioration in several varieties of whole stalk and chopped cane, both burnt and unburnt at time of harvest. Five varieties (N50/211, NCo 382, NCo 376, N51/168 and N53/216) of ratoon cane aged 18 months were used. The unburnt cane was taken from the net plots of a variety trial, and two days later the cane set aside to allow for end effects and the guard rows were burnt and harvested. Samples comprising 280 randomly selected stalks of unburnt or burnt cane of each varietv were taken on day 0. Each of these samples was then made into 40 bundles, eaih consisting of seven stalks. Each batch of 40 bundles was immediately weighed and then treated as follows: (a) 16 bundles were retained as whole stalk samples, 4 bundles being analysed for changes in juice quality on days 0, 1, 2 and 5 after harvest. (b) 12 bundles weIe chopped into 40 cm lengths and rebundled, and 4 bundles were analysed for changes in juice quality on days 1, 2 and 5 after harvest. (c) 12 bundles were chopped into 20 cm lengths and treated as in (b) above. All bundles except those for analysis on day 0 were returned to the field. Selected bundles were s~ubsequently reweighed on the day of sampling in order to determine changes in weight with time. All determinations were carried out on stripped whole stalks which had been topped by hand. All analyses reported were carried out on first expressed juice and were based on the java ratio. During the period of the trial on unburnt cane (12/11/7317/11/73), a total of 8,7 mm of rain was recorded, all of which fell on day 1. During the burnt cane trial (14/11/7319/11/73) 6,4 mm of rain fell on day 5. Temperature and humidity were low throughout, humiture at 2 pm reaching 90 on one day only. In trial I, differences in rates of deterioration between whole and chopped stalk were generally far smaller than had been expected from overseas results. This was probably because of the ielatively cool conditions and low humidity prevailing at the time, but possibly also because the chopped cane was left outside to dry (see Figure 1) rather than being closely packed with neighbouring billets as would be the case in the field bin. In this latter condition chemical and biochemical changes within the stalk would probably have been accelerated. To seek confirmation of this and study further the effect of billet size on rate of cane deterioration a second trial was conducted using chopper harvested cane undem hotter, more humid conditions. FIGURE 1 Whole stalk and chopped cane of several varieties (Trial 1). 24 rectangular metal baskets 54 litres (2 cu ft) in volume were placed in a cane bin and filled with chopped, burnt cane direct from the harvester. The baskets were weighed on day 0 and all but those for immediate analysis were stsllcked outside undel cover (see Figure 2). Selected baskets were subsequently reweighed on the day of sampling in order to determine changes of weight with time. On each day of sampling 4 baskets were examined as follows: With an average billet size of 30 cm, the cane in each basket was separated into billets of 30 cm or longer and less than 30 Proceedings of The South African Sugar Technologists' Association -June 1976 13 FIGURE 2 Chopper harvested cane in baskets. cm. The long and short billets were weighed and counted before analysis, and extraneous material (leaf, trash, etc) was discarded. (b) 24 plastic dustbins were placed around a loaded cane bin and chopper harvested cane was dropped into them by hand. In this instance trash and immature stalk were discarded at the time when the dustbins were filled so that extraneous matter was minimal. The bins were weighed on day 0 and all but those for immediate analysis were kept outside with the tops covered in plastic sheeting to create conditions of high humidity conducive to rapid deterioration. Selected bins were subsequently reweighed on the day of sampling but no measurable changes in weight were recorded as the plastic sheeting had prevented moisture loss. On each day of sampling 4 bins were examined and the contents divided into long and short billets as for the baskets. For both the baskets and the bins, the long and short billets were analysed separately for changes in juice quality, and recoverable sugar, on days 0, 1, 2, 3, 4 and 5 following harvest, as were bundles of whole stalk cane taken from the same rows as the chopper harvested cane on day 0. Jn addition first expressed juice from both the long and the short billets and the whole stalk cane was analysed for changes in pH, purity and reducing sugars with time. During the period of the trial (2211176-27/1/76) the weather was hot and the humidity generally high, the humiture at 2 pm on days 0-4 averaging 94. Plant cane of variety NCo 376 was used and an extremely even and hot burn was achieved. TRIAL 3 The results of trial 2 emphasised the need for further information regarding the distribution of immature stalk (i.e. green tops etc.) between long and short cane billets, and the effect, if any, of such stalk on rate of deterioration of long and short billets. Sixteen dustbins were placed in a cane bin and filled with chopped cane direct from the harvester. The bins were weighed TABLE 1 A Losses due to deterioration of five varieties of unburnt cane harvested as whole and chopped stalk (Means of four bundles adjusted for change in weight)* Trial 1A November 1973 (mean temp. 19,I0C) * Figures adjusted for weight loss or gain from that on day 0. Day 0 . I . 2 . 5 . 0 . 1 . 2 . 5 . 0 . 1 . 2 . 5 . 0 . 1 . 2 . 5 . 0 . 1 . 2 . 5 . 0 . 1 . 2 . 5 . Ws