Research Plant Physiologist
Agricultural Research Service, USDA
Agronomy Department, NDSU
Fargo, North Dakota
Sucrose loss during post-harvest storage of sugarbeet roots is a major concern of the sugarbeet industry. During post-harvest storage, sucrose losses can amount to 0.5 lb/ton/day. Sucrose is lost during storage through respiration, raffinose synthesis, storage pathogens, and inversion to glucose and fructose. Further sucrose loss during processing increases with storage due to an increase in raffinose and invert sugars which causes an increase in the melassigenic factor.
Respiration by the sugarbeet root accounts for 50 to 60% of the sucrose loss during post-harvest storage. Storage pathogens account for 10% of the loss in sucrose during storage in the Red River Valley of North Dakota and Minnesota. The objectives of this study were to compare sugarbeet cultivars and to determine the effects of mechanical damage during harvest on respiration and storability of sugarbeet roots. Commercial cultivars were grown in a silty clay soil and stored at 5 C and near 100% relative humidity for 150 day s.Respiration rates were measured using gas chromatographic techniques.
Cultivars differed in respiration rates each year and with no significant time in storage by cultivar interaction either year (Table 1). Respiration rates increased during storage. Cultivars differed in sucrose content and sucrose decreased with storage time.
Respiration rates of sugarbeet roots were significantly affected by the level of mechanical damage during harvest and piling of the roots for storage (Table 2). Invert sugars increased as the amount of damage to the sugarbeet roots increased during harvest and storage.
The results indicate that cultivars and mechanical damage during harvest can significantly affect respiration rates during post-harvest storage.
Studies were conducted to determine if there was genetic variation in internal CO2 levels in sugarbeet roots and to determine the effects of resident bacteria, storage temperature, decay, and root weight on internal CO2 levels. Significant differences in internal C02 levels were observed among different germplasm sources (Table 4). Internal CO2 levels of roots selected from American 3 Hybrid T were not correlated with the number of resident bacteria or weight of the roots after two storage periods (Table 3). Internal CO2 levels in roots with visible decay were significantly higher (272%) than CO2 levels in healthy roots. Storage temperature significantly affected internal CO2 levels in sugarbeet roots (Fig. 1). Our results show that there is genetic variation for internal CO2 levels in sugarbeet roots and that internal CO2 levels appear to be related to respiratory activity.
Changes in resident bacteria, pH, sucrose, and invert sugar levels in sugarbeet roots during storage
Internal tissues of various fruits, vegetables, and sugarbeet roots contain numerous bacteria, some of which are plant pathogens.
In the Red River Valley of North Dakota and Minnesota, sugarbeet (Beta vulgaris L.) roots are stored in open piles for up to 150 days after harvest. In commercial sugar beet storage piles, localized increases in temperature of the piles are often observed, which are called "hotspots". This temperature increase is usually associated with a blockage of natural air circulation through the pile, which is necessary for heat to dissipate from the roots.
Hotspots usually increase during storage, even under ambient air temperatures as low as -35 C. Visual examination of the sugarbeet roots located in a hot spot shows evidence of bacterial growth, and the air around the hotspot has a characteristic odor similar to silage, symptoms which suggest fermentation.
Our objectives were to compare resident bacterial populations, invert sugars, and pH changes with sucrose loss in sugarbeets stored aerobically and anaerobically at different temperatures.
Sugarbeet roots were selected from an area of a commercial storage pile which appeared normal after 130 days storage in 1974 to 1975. washed, and sorted into about 12-kg samples. Twelve root samples were placed into perforated bags; 12 were placed into 27.2 liter pails, and 2 samples were used to establish sucrose content at day 0. Bacterial populations of the sugarbeet roots were determined as described by Dr. Bugbee in the 1974 Sugarbeet Research and Extension Report.
All samples in bags and open plastic pails were stored at either 5 or 26 C for 2 days, after which O2 levels of the sugarbeet roots and of the surrounding air were determined by gas chromatography. The pails were then sealed and oxygen depletion was monitored for 4 to 5 days. Two different pails and two different bags were sampled at 7, 14, and 21 days after oxygen was depleted in the pails at 26 C, and at 14, 21, and 28 days after oxygen was depleted at 5 C. At each sampling date for roots stored at 26 C and day 28 for roots stored at 5 C, cores were removed from the same roots as day 0 to determine the change in pH, invert sugars, and resident bacteria. At each sampling date pulp was obtained with a multiple blade saw from all beets in a pail or bag and analyzed for sucrose content. After pulp was obtained, the roots were discarded.
The experiment was repeated during the spring of 1975 using freshly harvested roots from California. In the second experiment an additional treatment was included (anaerobic storage at 15 C).
At the beginning of the experiment, atmospheric O2 levels were normal (20.7%) in the open plastic pails and perforated plastic bags. The roots had similar O2 levels in the plastic pails and plastic bags at the same temperature. However, the roots had an O2 level of 19.3% at 5 C, but a level of only 13.5% at 26 C. This indicates that O2 is used faster at the higher temperature.
Chamber oxygen was depleted in less than 24 hours at 26 C after the pails were sealed (Fig. 2 and Fig. 3A). At 5 C, oxygen was depleted more slowly. Oxygen uptake rate at 26 C decreased as oxygen levels decreased; however, O2 uptake rates at 5 C were not affected until oxygen levels in the pails dropped below5% (Fig. 2 and Fig. 3). Rate of O2 uptake and chamber O2 depletion at 15 C was intermediate to the rates at 26 and 5 C (Fig. 3).
Number of bacteria capable of hydrolyzing sucrose in vitro before aerobic and anaerobic storage of the roots was less than 10% of total bacteria present.After O2 was depleted at 26 C, the total number of bacteria increased rapidly (Fig. 4), and after anaerobic storage for 7 days at 26 C, most bacteria present in both commercially stored and fresh roots was capable of hydrolyzing sucrose in vitro (Table 5 and Table 6). Under aerobic storage at 26 C, sucrose-hydrolyzing bacteria increased between 14 and 21 days in beets stored for 130 days (Table 6). However, no change in hydrolyzing bacteria was detected in fresh roots aerobically stored, but the number of non-hydrolyzing bacteria increased (Table 6).
Invert sugar levels in the roots stored under anaerobic conditions increased at 26 C ((Fig. 4). Roots stored under aerobic conditions changed little in invert sugar levels, except those stored 21 days at 26 C (Fig. 4A). These roots began to show decay from storage fungi and their bacteria levels increased (Table 5).
Under anaerobic conditions, the pH of the core juice from commercially stored and fresh sugarbeets declined significantly as the number of bacteria increased in the juice (Fig. 4). The decrease in bacterial counts may have resulted from a decrease in available substrate or from growth inhibition from the low pH.
Sucrose content of pulp from the roots decreased as the number of bacteria increased (Fig. 4 and Fig. 5) under anaerobic storage. Apparently, during anaerobic storage, sucrose was hydrolyzed by the bacteria. Some acid hydrolysis may also have occurred, especially after the pH decreased below 4.5. Higher temperatures and a pH of 4.5 are used in the commercial production of invert sugar from sucrose.
Our data suggest that O2 depletion will activate internal bacterial populations and that the bacteria present in sugarbeet roots could completely deplete sucrose in 21 days at 26 C under anaerobic storage conditions. This indicates that air movement in commercial sugarbeet storage piles is important early in the storage, since fermentation can begin within 24 hours after oxygen is depleted. Although the sugarbeet roots in this investigation did not appear excessively deteriorated after 21 days of anaerobic storage at 26 C, they were almost completely lacking in sucrose. This may explain why factory sucrose extraction de,creases when beets from a hotspot are processed.
Sugarbeets are normally flailed and scalped before harvesting. The purpose of scalping is to remove a portion of the crown material which is known to be higher in impurities and lower in sucrose when compared to the main body of the sugarbeet root. The crown is the area above the lowest leaf scar. However, there are data which indicate that more sugar per acre can be recovered from beets which are only flailed before harvesting. This increase can come about for several reasons: (a) sugar can be extracted from the crown material, (b) respiration losses would be reduced by not cutting the crown, and (c) the amount of rot would be reduced by not exposing the most susceptible tissue, which is the center portion of the crown.
If sugarbeets were only flailed at harvest, an increase in tonnage and a reduction in sucrose content would be expected. However, no data is available to indicate the change in either tonnage or sugar content. A survey at one location in the Red River Valley in the 1974-75 processing campaign indicated that only 6% of the sugar beets were topped at the lowest leaf scar, 71% were partially topped, and 23% were only flailed. This data indicates that a considerable amount of crown tissue is being processed at the present time.
A survey of commercial sugarbeet growers was conducted during the week of September 29 to October 3, 1975, in the Red River Valley of North Dakota and Minnesota to determine the change in tonnage and root quality if sugarbeets were only flailed.
Sugarbeet samples were obtained from growers selected at random in each factory district in the Red River Valley. Four ten-beet samples were harvested from each grower and/or location in a field. In each field tested, samples were obtained from adjacent rows where the grower had temporarily stopped scalping. From the row which had not been flailed or scalped, two ten-beet samples were manually harvested. The leaves were removed at the base of the petiole with a knife. The crowns were then removed at the lowest leaf scar on one sample and weighed. The 10 roots were then placed into a 'tare bag' for further analysis. The other 10-beet sample was also placed into a 'tare bag'. Two 10-beet samples were manually harvested from the adjacent row where the grower had flailed and scalped the sugarbeets. The remaining crown tissue on one sample was removed and weighed. Roots of both samples were placed into 'tare bags'. Length of row harvested for each 10-beet sample was determined. Sixty-eight locations were sampled. Flailed samples were obtained from an additional 6 fields in the East Grand Forks, Minnesota, area. Grower-scalped samples were not obtained on these 6 fields because the factory was not receiving beets at the time of the survey.
The 'tare bags' were transported to the tare laboratory of American Crystal Sugar Company at East Grand Forks, Minnesota, for further analysis. At the tare laboratory the roots were washed, weighed, and sawed to obtain pulp for determination of sucrose, nitrate grade, and conductivity grade. Percent crown tissue was calculated using the weight of the crown material removed and the root weight.
Two samples of sugarbeet roots were obtained from grower trucks at six piling stations in the Valley. Samples obtained at Crookston factory, Midway, Hillsboro factory, Drayton factory, and Hamilton consisted of 10 roots selected at random from a loaded truck. The samples from the Moorhead factory were obtained by using the sample bucket on the piler to catch two samples per truck load. One sample from each load was used to determine the weight of the crown material that was delivered to the factory by the grower. Both samples were used to measure sucrose, nitrate grade, and conductivity grade at the tare laboratory.
Average length of row to harvest 10 sugarbeet roots varied from 11.6 to 12.1 ft (Table 7), which would give an average population of 19,721 to 20,434 plants per acre. Sugarbeet roots harvested from the grower-scalped row were lower in sucrose compared to sugarbeets harvested from the row with intact leaves which was used to simulate flailing. The reduced sucrose levels were probably a result of respiration caused by the scalping and, also, the grower-scalped sugarbeet roots had had the leaves removed for an undetermined time period which could have reduced the sucrose level due to elimination of photosynthesis, whereas the flailed sugarbeets were capable of Photosynthesis up to harvest.
Removal of the entire crown (sample 2, Table 7) resulted in a 1.2% increase in sucrose, a 7.1% reduction in nitrade grade, a 5% reduction in conductivity grade, and an 18.6% reduction in yield compared to sugarbeet roots with intact crown (sample 1, Table 7). Removal of the crown material remaining on sugarbeet roots after scalping by the grower (sample 4, Table 7) resulted in a 1.9% increase in sucrose, a 3.4% reduction in nitrate, a 7.3% reduction in conductivity, and a 15.5% reduction in yield compared to grower-scalped sugarbeet roots (sample 3, Table 7).
Sugarbeet crown material accounted for 19.4% of the total yield for the flailed roots and 15.5% of the total yield for the grower-scalped roots (Table 7). This would indicate that a considerable amount of the crown material is harvested and delivered to the factory.
An inverse relationship exists between sucrose and nitrate grade (Fig. 6).Flailed and grower- scalped sugarbeet roots exhibited similar trends in sucrose reduction with nitrate grade.
A positive relationship between soil nitrate levels and percent crown tissue has been established. A similar relationship was observed between nitrate grade and percent crown (Fig. 7). This relationship indicates that the amount of crown material produced can be regulated by nitrogen management. Nitrogen management can result in an increase in sucrose content and a reduction in crown material.
Sucrose content showed a decline as percent crown material increased in both flailed and grower-scalped sugarbeet roots (Fig. 8 and Fig. 9). A highly significant negative correlation was observed between sucrose and crown material. This relationship can partially be explained by the differential between sucrose level of root, vs. crown material, since the difference becomes larger as nitrogen increases. Nitrogen causes a reduction in sucrose and an increase in the amount of crown produced.
The data reported above were obtained from manually harvested roots where the tap root and lateral roots remained primarily intact. However, sugarbeet roots harvested mechanically rarely have lateral roots and the main tap root may be broken or cut by the lifter wheels. Therefore, a change in percentage crown material would be expected when comparing manually harvested to mechanically harvested roots.
Crown material accounted for 20.5% of the tonnage delivered to the piler and/or factory station-by the growers (Table 8). Removal of all the remaining crown material resulted in a 1.2% increase in sucrose, a 5.3% reduction in nitrate grade, and a 2.2% reduction in conductivity grade (Table 8) averaged over allocations.
The data indicate that the factories are processing at least 15.5% of all crown material produced, which accounts for 20.5% of +he total tonnage processed. The data indicated that the grower removed 20% of the crown material produced. Assuming a 13.5 T/A yield, the grower could expect an additional 0.7 T/A from crown material if he flailed rather than scalped the beets. This would increase the amount of crown material being processed by the factory to 24.6% of the total tonnage processed. The total tonnage processed by the factory from 50,000 acres would be increased from 675,000 to 710,000 tons. The additional tonnage would increase the slicing campaign 7 days for a 5,000 ton per day factory.
1975 Sugarbeet Research and Extension Reports. Volume 6, pages 44 - 58.
