Sustainability of Low Starch Concentrations in Sugarcane through

Chapter 14

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Sustainability of Low Starch Concentrations in Sugarcane through Short-Term Optimized Amylase and Long-Term Breeding Strategies Marvellous Zhou,1,5 Collins Kimbeng,*,1,6 Serge Edme,2 Anna Hale,3 Ryan Viator,3 and Gillian Eggleston4 1School

of Plant, Environmental and Soil Sciences, LSU AgCenter, Baton Rouge, LA 70803, USA 2Sugarcane Field Station, ARS, USDA, Canal Point, FL 33438, USA 3Sugarcane Research Laboratory, ARS, USDA, Houma, LA 70360, USA 4Commodity Utilization Unit, ARS, USDA, New Orleans, LA 70124, USA 5South African Sugarcane Research Institute, P. Bag X02, Mt. Edgecombe, KwaZulu-Natal 4300, South Africa 6Sugar Research Station, LSU AgCenter, St. Gabriel, LA 70776, USA *[email protected]

Starch negatively affects the quantity and quality of raw sugar produced. Starch reduces crystallization and centrifugation rates, occludes into sucrose crystals, and impedes refinery decolorization processes. The problem of starch in sugarcane juice has been exacerbated by the widespread adoption of green cane harvesting and also, perhaps by the necessity to incorporate useful traits from wild Saccharum germplasm into cultivated sugarcane. Use of α-amylase to hydrolyze starch during processing should be viewed as a short-term solution as the enzyme is relatively expensive and not always efficient. Availability of sugarcane varieties low in starch content should present a more sustainable, long-term solution. This chapter highlights problems caused by starch during the processing of sugarcane, as well as presents data suggesting that it would be possible to deploy sugarcane varieties low in starch content.

© 2010 American Chemical Society In Sustainability of the Sugar and SugarEthanol Industries; Eggleston, G.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


Introduction Starch is an impurity in sugarcane juice that can impede the extraction of sugar during processing as well as affect the quantity and quality of raw and refined sugars (1, 2). Starch can reduce crystallization and centrifugation rates, occlude into the sucrose crystals, increase the production of molasses, reduce filterability and affination of raw sugars, and impede refinery decolorization processes. These processing problems are currently being mitigated in the factory by using α-amylase to hydrolyze starch, which is a short-term solution because the enzyme is relatively expensive and not always efficient. The problem of starch in sugarcane juice has been exacerbated by the widespread adoption of green cane harvesting and also perhaps by the necessity to use wild Saccharum germplasm for incorporation of useful traits into cultivated sugarcane. Availability of sugarcane varieties low in starch content would be a more preventative, economical, and efficient solution. Therefore, research focused on breeding for low starch in sugarcane could provide the envisaged long-term solution. Sugarcane breeding programs consider several traits during selection. Adding an additional selection trait such as starch would encumber the selection process and may result in the reduction of selection gains. A good strategy would involve using parents with low starch during crossing and selecting for yield and quality from the progeny that are expected to produce low starch. In this chapter, processing issues related to starch in sugarcane juice are reviewed. This is followed by a review of data from multiple, recent studies that were designed to survey relative starch content in a wide collection of varieties and wild Saccharum species used in sugarcane breeding programs. Finally, we propose a more sustainable, longer-term strategy to lower starch content in sugarcane varieties, without putting a burden on breeding programs with the introduction of a new selection trait.

Starch in the Sugarcane Plant Starch (α-1→4-glucan) is a sugarcane juice impurity that adversely affects factory and refinery processes and subsequently the quantity and quality of sugar products (1, 2). Unfortunately, the delivery of sugarcane starch to U.S. and other countries’ factories has risen markedly in recent times because of the increased adoption of green (unburnt) cane harvesting (3) and the introduction of newer varieties with higher contents of starch (4). Starch exists as semi-crystalline granules (1 to 10 µm) in sugarcane tissue and extracted juice. These granules are smaller than those from corn (5 to 25 µm) and potato (15 to 100 µm) (5). Sugarcane starch granules contain two glucose polysaccharides: ~19% amylose and 81% amylopectin (6). Amylose is linear with the glucose molecules α-D-(1→4) linked (Figure 1). Amylopectin, in addition to the α-D-(1→4) linked glucose found in amylose, has α-D-(1→6) linked branch points (Figure 1). Amylose forms a blue color in the presence of iodine (5), while amylopectin forms a red-violet color. Starch is produced in the sugarcane plant as a storage polysaccharide (carbohydrate reserve) and utilized during periods of rapid growth, e.g., during 230 In Sustainability of the Sugar and SugarEthanol Industries; Eggleston, G.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


Figure 1. Chemical structure of amylose and amylopectin starch polysaccharides (12). the sprouting of roots and buds, seedling germination, and emergence (5, 7). Starch granules are present in stalks, leaves (both green and brown; (8)), and roots of the sugarcane plant (9), but are most abundant in the green leaves and growing point region (Table I). There is strong varietal effect on starch content in the total juice (8, 10) and the distribution of starch among different plant tissues (11) (Table I). Starch decreases with sugarcane maturity. In sugarcane stalks, starch granules are deposited mainly at the nodes and disappear during rapid growth. Growing conditions such as soil type, nutrients, agronomic practices, water supply, and temperature have been reported to affect the levels of starch found in sugarcane stalks (5). Although starch levels in stalks are relatively low compared with other tissues (11), when calculated on a % tissue wet weight basis, it is observed that stalks actually deliver a considerable amount

231 In Sustainability of the Sugar and SugarEthanol Industries; Eggleston, G.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.

of starch to the factory, just because of their much higher weight and volume (11). Therefore, starch delivery to the factory by stalks should not be underestimated.


Sugar Processing Problems Associated with Starch In recent years, there have been warnings by some U.S. raw sugar refineries that they may impose a penalty on high-starch levels in raw sugar if starch control is not improved (1, 2). Processing costs increase, not only in terms of additional processing aids, but also from increased viscosity of massecuites, reduction of crystallization and centrifugation rates, occlusion of starch into the sucrose crystal, increased molasses production (13), reduced filterability and affination of raw sugars, and impediment of refinery decolorization processes (1, 2). Mud filtration is particularly impeded when a carbonatation refinery processes raw sugar containing >250 ppm/Brix starch. For these reasons, U.S. factories are being encouraged to deliver raw sugar containing 0.90; P 0.70 were found between replications, locations or crop years in these studies. In one study, 76 clones including 6 varieties and 70 unselected clones of F1 and BC1 origin, derived from crosses between varieties and S. spontaneum (see Tables IV and V), were evaluated in 3 replicates over 2 crop years. The starch content for each clone was averaged over replicates and crop years and the lowest and highest 10% of clones were plotted for each replicate within a crop year (Figure 6). Starch content varied across replicates within crop years, but amid all these environmental variation, 241 In Sustainability of the Sugar and SugarEthanol Industries; Eggleston, G.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.

Table V. Variance components and broad sense heritability estimates for starch content in different sugarcane populations Population parameters

SESpop

Larta

Advanced clones

120 clones derived from a S. officinarum x S. spontaneum cross evaluated across three replications at one location.

70 clones of F1 and BC1 origin derived from crosses between cultivars and S. spontaneum evaluated across three replications over two years at a single location.

19 varieties evaluated across two replications at three locations with each location harvested on a different date

σg2

7754

127786

9422.26

σyv2

N.A.

31372

N.A.

σlv2

N.A.

Population and trial description


Population

σe2 Formula Heritability

8441.73

7629.72

85805

225.23

σg2/(σg2+σe/r2)

σg2/(σg2+σyv/y2+σe/ ry2)

σg2/(σg2+σlv/l2+σe/ rl2)

75.3

80.9

76.8

clones in the lowest 10% group produced consistently less starch than those in the highest 10%.

Environmental Temperature Effects on Sugarcane Starch Temperature is an environmental factor most likely to influence starch accumulation in sugarcane. In a separate study, the mean starch value from a population of 300 clones derived from selfing the variety LCP 85-384 (23) was averaged across two replications and ranked from lowest to highest. The mean 10% of clones with the lowest and highest starch content were plotted for each replication (Figure 7). Replication 1 was sampled on November 13, 2006 (before a freeze) and replication 2 was sampled on December 27, 2006, after 5 days of freezing temperature (Table VI). The clones accumulated significantly (P

Sustainability of Low Starch Concentrations in Sugarcane through

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