Short communication
Composition and digestibility of beet pulp with and
without molasses and dried using three methods
J.G. Fadel
*, E.J. DePeters, A. Arosemena
Department of Animal Science, University of California, Davis, CA 95616-8521, USA
Received 24 August 1999; received in revised form 6 March 2000; accepted 15 March 2000
Abstract
This study compared the effects of drying on beet pulp with and without added molasses by measuring the in sacco digestibility and composition including ®ber polysaccharides and minerals. Beet pulp was sampled before and after the addition of molasses from four different sugar processing plants within California. These samples were sub-sampled and then sun-, freeze-, or air-dried. The percentage molasses in the beet pulp with added molasses averaged about 27%. After glucose, arabinose was the second most prevalent sugar in the neutral non-starch polysaccharides. Molasses addition did not result in a signi®cant change in the crude protein, ether extract, TDN, ribose, or Mg content. Fiber components, other minerals, and neutral detergent ®ber (NDF) residues were signi®cantly different in beet pulp with versus without molasses. However, the NDF residues remaining at 24 h in sacco fermentation time differed by less than three percentage units. Method of drying affected NDF, acid detergent ®ber (ADF), crude protein in NDF and ADF, ribose, mannose, and pectin content of beet pulp but the differences were small and probably not biologically important. Although the NDF residues remaining in beet pulp were lowest for freeze-dried and highest for sun-dried at all fermentation times, they were only signi®cantly different at 6 h. Sun-dried beet pulp NDF residues remaining at 6 and 12 h of fermentation were signi®cantly higher than freeze-dried beet pulp. Drying had little affect on other components measured. Results from this experiment suggest the nutritional characteristics of sun-dried beet pulp are generally similar to wet beet pulp.#2000 Elsevier Science B.V. All rights reserved.
Keywords:Beet pulp; Drying; Molasses; Digestibility; Non-starch polysaccharides 85 (2000) 121±129
*Corresponding author. Tel.:1-530-752-1259; fax:1-530-752-0175.
E-mail address: jgfadel@ucdavis.edu (J.G. Fadel)
1. Introduction
Sugar beets (Beta vulgaris) account for about 50% of the sugar produced in the US (Christensen, 1989). Beet pulp is the primary by-product feedstuff remaining after sucrose is extracted from sugar beets (Fadel, 1999). Beet pulp is an important by-product feedstuff in California contributing to about 14% of the by-products surveyed in 1992 on a dry matter basis (Grasser et al., 1995) and having a market value of over 35 million US$. Variation in nutrient composition in beet pulp may be from the drying method used or the amount of molasses added back to the pulp. Differences in nutrient composition of beet pulp from different sources have been shown (Arosemena et al., 1995; DePeters et al., 1997), but the reasons for these differences were not elucidated. The ®ber fraction was shown to be the most variable component among sources. Beet pulp is a source of ®ber for lactating cows and understanding the factors affecting ®ber variation is important. Beet pulp is fed to ruminants in the wet and dry forms and is categorized as an energy by-product concentrate ingredient (Bath, 1981). Early work by Ronning and Bath (1962) demonstrated that beet pulp was similar in feeding value to barley for lactating dairy cattle supporting classi®cation of beet pulp as an energy concentrate. Beet pulp has a crude protein of 8.7% (range 7.1±9.6%), phosphorus of 0.06% (range 0.05±0.08%), ®ber (NDF) of 35.8% (range 32.9±41.1%), and calcium of 1.14% (range 0.77±1.36%) (Arosemena et al., 1995). The ®ber in beet pulp was reported to have the third highest ®ber rate of digestion of 9 by-product feedstuffs evaluated and hence rapidly digested in the rumen compared to this group of by-product feedstuffs (DePeters et al., 1997). The relatively fast rate of digestion contributes to beet pulp's high-energy content and high intake when fed to cattle. These characteristics account for the extensive use of wet and dry beet pulp in diets fed to lactating dairy cattle in California (Grasser et al., 1995).
Beet pulp can be fed wet or dry and with or without molasses. Also, several methods of drying are possible. Very little information is available that compares the nutritive differences among these different processes. This study compares the effects of drying on beet pulp with and without added molasses by measuring the composition, including ®ber polysaccharides, and in sacco digestibility.
2. Materials and methods
2.1. Experimental design, source, and drying
Beet pulp samples were collected from four different processing plants in California. Samples were collected either with or without molasses and then sun-, freeze-, or air-dried. Wet beet pulp was collected on site just prior to and just after the molasses addition step. Samples were transported to UC Davis, CA on ice and frozen atÿ208C until dried. Samples were thawed in closed bags at room temperature for about 48 h, thoroughly mixed, separated into approximately three equal portions, and spread as a thin layer of about 2 cm onto trays. The samples were then sun-, freeze-, or air-dried during the summer in Davis, CA. The freeze-dried beet pulp samples were assumed to have similar physical and chemical characteristics as wet beet pulp. Samples were sun-dried by
placing the samples in direct sunlight for one day. Sun-dried samples were mixed once during the day. Samples were freeze-dried on stainless steel trays and frozen (ÿ508C). A temperature probe was inserted into the sample on each tray to monitor internal temperature and to ensure dryness. Dryness was complete, and the samples removed when the sample temperature reached308C. Six trays of frozen beet pulp were place into a Virtis (The VirTis Company, Inc., Gardiner, NY) model 50-SRC freeze dryer and lyopholized to dryness, using a shelf temperature of308C, while maintaining a vacuum of 50 mTorr and a condenser temperature ofÿ508C. Samples were air-dried by placing the trays in a forced air oven for 48 h. The air in the oven was not heated, and circulating outside air through the oven, dried the samples. All dried samples were ground through a 1 mm screen using a Wiley mill (Arthur H. Thomas, Philadelphia, PA) and stored for later analysis.
2.2. Chemical analyses and ®ber digestion
Samples were analyzed for DM, CP (N6.25), ether extract, and ash (AOAC, 1990); NDF (Van Soest et al., 1991); ADF and acid detergent lignin (ADL) (Goering and Van Soest, 1970); neutral non-starch polysaccharides (NSP) and Kason lignin (Theander and Westerland, 1986) and pectin as uronic acids (Blumenkrantz and Asboe-Hansen, 1973). The NDF and ADF residues were ashed and reported on an ash-free basis (Crocker et al., 1998). Calcium, Mg, K, and Na were analyzed by atomic absorption spectrophotometry (Model 3030B, Perkin-Elmer, Norwalk, CT) using standard procedures (AOAC, 1990). Phosphorus was analyzed with a technicon autoanalyser using method N-4C (Kraml, 1966). All results are presented on a dry matter basis. The CP fractions associated with NDF (NDFCP) and ADF (ADFCP) were determined by Kjeldahl analysis of the NDF and ADF fractions.
In sacco ®ber digestibility was determined in triplicate by weighing about 1 g samples into 5 cm10 cm nylon multi®lament bags (ANKOM, Fairport, NY) that had a pore size of about 50mm. The bags were heat sealed on three sides by the manufacture (ANKOM, Fairport, NY) and heat sealed on the fourth side (Lorvic Corp., St. Louis, MO) after the sample was weighed into the bag. The bags were suspended in the rumen in a loose mesh, weighted nylon sack and incubated for 6, 12, and 24 h. The cow used for incubations and the processing of the bags to measure neutral detergent ®ber are described elsewhere (DePeters et al., 1997). The diet contained on an as-fed basis 10% cracked corn, 14% beet pulp, 5% molasses, 15% chopped alfalfa hay, 54.5% chopped oat hay, 1% fat (yellow grease), and 0.5% trace-mineral mix.
2.3. Calculations and statistical analyses
were replicated. However, only one interaction term was chosen a priori to be included in the model because the authors felt this interaction was the most important. The other interaction terms not included in the model were considered error and this error was used to test all main effects and the interaction effect. Standard deviations were included for all locations for each molasses and drying combination. These standard deviations were included to demonstrate the variability within a location and treatment sub-group. They should be higher than the standard errors presented because the standard deviations have four observations and the standard errors are from the analyses of variance which includes all 24 observations.
3. Results and discussion
Chemical analyses and NDF disappearance of beet pulp with and without added molasses were different for all ®ber components and fermentation times (Table 1). The crude protein fractions in NDF (NDFCP) and ADF (ADFCP) were signi®cantly different with the addition of molasses but the differences were small and probably are not biologically important. Crude protein, ether extract, and calculated TDN were not signi®cantly different between beet pulp with or without molasses. These ®ndings were expected because molasses and beet pulp without molasses had similar CP, ether extract, and TDN values while different values are evident for the ®ber fractions (NRC, 1989). Although the percentage of NDF remaining for beet pulp with and without molasses following 6 h of fermentation was greatly different (27 versus 42), the percentage of NDF remaining after 24 h of fermentation was signi®cantly different but biologically similar (8 versus 10). Rates were not estimated because of the small number of fermentation times. However, if these data were calculated as a percentage of the NDF of the unfermented sample, then any differences among the proportions of NDF remaining at the different fermentation times were less. Hence, the proportions of NDF remaining over time were similar in beet pulp with or without molasses. The percentages of molasses added to the beet pulp can be estimated by using the NDF values from these data and assuming molasses contains a negligible amount of NDF. The percentage of molasses added was estimated to average 27% on a DM basis with a range from 15 to 34% which is greater than the range of 18±22% reported (Kelly, 1983).
Drying had less effect on composition and NDF disappearance than adding molasses (Table 1), but the drying effect was signi®cant for several components. Freeze-dried beet pulp had, generally, the lowest ®ber composition when compared to sun- or air-dried. Drying had a signi®cant affect on NDF fermented at 6 and 12 h but no affect at 24 h of fermentation. This suggests that sun-dried beet pulp may contain some non-®ber carbohydrates as part of the NDF fraction that are not fermented at 6 or 12 h of fermentation but are at 24 h. Drying also caused signi®cant differences among ADF, ADFCP, and NDFCP values but these differences were small and probably not biologically important.
Lignin contents were not signi®cantly different with the addition of molasses or method of drying and are low compared to other feedstuffs with similar NDF levels. The low lignin values may explain why the ®ber components are so digestible.
All NSP were signi®cantly higher in beet pulp without molasses, except for ribose (Table 2). The total NSP values from Table 2 were slightly lower than the estimated values from NDF minus ADL presented in Table 1. Protein included in the NDF as NDFCP can partly explain the higher values in NDF. However, the loss of alfalfa pectin in NDF analyses would suggest a lower NDF because most of the alfalfa pectin is found in the cell solubles and only a small portion of alfalfa pectin is associated with the NDF fraction (Ben-Ghedalia and Miron, 1984). The partitioning of pectin in beet pulp is not known but may be different than alfalfa pectin. The pectin content of beet pulp without molasses is relatively high and averaged 22%. Pectins in the cell soluble frac-tion are probably fermented rapidly in the rumen, but pectins found in the ®ber fracfrac-tion are less digestible (Ben-Ghedalia and Miron, 1984). More research is needed to characterize the digestibility of pectin in beet pulp. Most non-starch polysaccharides in Table 2 are assumed to be associated with the hemicellulose ®ber fraction except for glucose which is assumed to be primarily from hydrolysis of cellulose. Arabinose was found to be the predominant sugar in the hemicellulose fraction. Most mature forages contain xylose as the primary sugar of hemicellulose, and xylose affects digestibility because increasing levels of xylose parallel a decrease in ®ber digestibility Table 1
Ash, ®ber, ®ber remaining, lignin, crude protein (N6.25), crude protein in ®ber, ether extract (EE), and TDN as a percentage of beet pulp dry matter with or without molasses averaged across four locations and dried by three methods
Mola Loca MethodaAsh NDF NDF_6b NDF_12b NDF_24bADF ADF
lignin
CP NDFCPcADFCPcEE TDNd
No 4 Sun 3.35 62.66 46.43 25.26 10.93 28.68 2.26 7.54 4.82 0.83 0.42 66.26 0.399e 2.473 2.978 2.613 1.347 1.107 0.229 0.313 0.469 0.051 0.126 1.173
No 4 Freeze 3.55 57.94 37.52 16.73 9.70 28.14 2.08 7.42 4.32 0.78 0.51 67.66 0.368 3.319 4.252 2.708 0.808 0.994 0.253 0.211 0.527 0.078 0.054 0.995
No 4 Air 3.19 60.21 41.91 17.85 9.93 29.57 2.49 7.58 4.64 0.83 0.45 66.59 0.370 2.688 4.105 2.077 0.853 0.747 0.266 0.270 0.469 0.078 0.135 0.819
Yes 4 Sun 7.43 45.37 30.16 9.69 7.73 21.83 1.76 8.17 3.59 0.63 0.41 67.52 2.068 2.458 3.803 0.807 0.793 1.452 0.316 1.134 0.290 0.083 0.181 1.683
Yes 4 Freeze 7.58 42.11 23.99 8.74 7.64 20.34 2.09 7.75 3.17 0.55 0.48 67.61 2.343 3.928 5.329 0.724 0.954 1.047 0.605 1.698 0.537 0.029 0.128 1.896
Yes 4 Air 6.99 44.41 27.27 9.18 7.93 21.96 1.95 8.26 3.45 0.66 0.87 68.38 1.942 4.152 4.321 1.050 0.615 1.327 0.433 1.187 0.376 0.050 0.830 2.821
Average without Mol 3.36 60.27 41.95 19.95 10.19 28.80 2.28 7.51 4.59 0.81 0.46 66.8 Average with Mol 7.33 43.96 27.14 9.20 7.77 21.38 1.94 8.06 3.41 0.61 0.59 67.8
P-values less than 0.001 0.001 0.001 0.001 0.001 0.001 0.020 0.128 0.001 0.001 0.366 0.17 S.E. 0.338 0.784 1.033 0.492 0.245 0.280 0.926 0.239 0.110 0.015 0.098 0.48
Average sun drying 5.39 54.02 38.29 17.48 9.33 25.26 2.01 7.85 4.20 0.73 0.42 66.9 Average freeze drying 5.57 50.03 30.76 12.74 8.67 24.24 2.09 7.58 3.75 0.66 0.50 67.6 Average air drying 5.09 52.31 34.59 13.51 8.93 25.76 2.22 7.92 4.05 0.74 0.66 67.5
P-values less than 0.724 0.032 0.003 0.001 0.321 0.020 0.452 0.690 0.081 0.012 0.362 0.65 S.E. 0.414 0.960 1.266 0.602 0.301 0.343 0.113 0.292 0.135 0.018 0.120 0.59
a
MolMolasses; Locnumber of locations; Method: Sunsun-dried, Freezefreeze-dried, Airair-dried. b
NDF_6, NDF_12, and NDF_24: the NDF remaining after 6, 12, and 24 h fermentation. c
CP in NDF (NDFCP) and ADF (ADFCP) as a percentage of sample. d
Calculated according to Eq. (14) of Weiss et al. (1992). e
(Hat®eld, 1993). The estimation of cellulose from Table 1 (ADFÿADL) is greater than the glucose estimate in Table 2, but the difference could be non-cellulose contamination in ADF (Jung, 1997).
Method of drying did not affect most ®ber components (Table 2). This observation is in contrast to the marked effects of drying that were found in Table 1. For the analyses in Table 2, a strong acid hydrolysis step is required to hydrolyze the polymers before they are prepared for gas chromatograph or colorimetric analysis (Theander and Westerland, 1986). In contrast, the NDF procedure is milder and requires an extraction of non-®ber material in a neutral detergent solution (Van Soest et al., 1991). Any effect from drying could be masked when hydrolyzing the sample with a strong acid because polymers that would remain intact in neutral detergent solution may be hydrolyzed with a strong acid.
Mineral content of the beet pulp with and without molasses was signi®cantly different for Ca, P, K, and Na but not Mg (Table 3). These ®ndings are supported from calculations using reported (NRC, 1989) K content of molasses (6.07%) and beet pulp without molasses (0.19%). Calculating a weighted average assuming 27% molasses yields a K content of 1.76% (6.070.270.190.63) in beet pulp with molasses, which agrees with Table 2
Neutral non-starch polysaccharides, pectin, Klason lignin, and total ®ber as a percentage of beet pulp dry matter with or without molasses averaged across four locations and dried by three methods
Mola Loca Methoda Rhab Fucb Ribb Arab Xylb Manb Galb Glub NSPc Pectin Klason
lignin Totald
No 4 Sun 2.09 0.16 0.11 18.64 2.96 1.52 6.36 23.42 55.25 22.67 4.17 82.09 0.103e 0.007 0.109 0.983 0.269 0.148 0.291 1.487 3.053 0.568 0.499 3.709
No 4 Freeze 2.00 0.15 0.03 18.51 3.02 1.51 6.26 22.85 54.33 21.02 3.83 79.17 0.094 0.012 0.036 0.642 0.298 0.051 0.109 0.554 1.346 0.793 0.387 1.439
No 4 Air 2.08 0.15 0.00 18.56 2.92 1.52 6.32 23.22 54.76 21.42 4.07 80.25 0.023 0.003 0.000 0.928 0.212 0.057 0.098 0.558 1.379 1.117 0.641 2.870
Yes 4 Sun 1.59 0.11 0.04 13.99 2.25 1.42 4.79 17.76 41.95 16.04 3.32 61.31 0.074 0.007 0.083 1.313 0.200 0.076 0.280 1.099 2.667 1.069 0.508 4.016
Yes 4 Freeze 1.45 0.10 0.03 12.90 2.11 1.22 4.38 16.19 38.50 14.93 3.22 56.65 0.142 0.010 0.065 1.609 0.336 0.079 0.521 1.659 4.151 1.555 0.230 5.435
Yes 4 Air 1.39 0.10 0.00 12.52 2.03 1.32 4.32 16.22 37.88 15.71 3.47 57.06 0.090 0.029 0.000 1.032 0.190 0.096 0.185 0.959 2.183 1.646 0.338 3.929
Average without Mol 2.06 0.15 0.04 18.57 2.97 1.52 6.31 23.16 54.78 21.70 4.03 80.50 Average with Mol 1.48 0.10 0.02 13.14 2.13 1.32 4.49 16.72 39.44 15.56 3.34 58.34
P-values less than 0.001 0.001 0.474 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 S.E. 0.029 0.004 0.019 0.271 0.065 0.024 0.083 0.293 0.702 0.296 0.121 0.983
Average sun drying 1.84 0.14 0.07 16.32 2.61 1.47 5.57 20.59 48.60 19.35 3.75 71.70 Average freeze drying 1.72 0.12 0.03 15.71 2.57 1.36 5.32 19.52 46.41 17.97 3.53 67.91 Average air drying 1.74 0.12 0.00 15.54 2.47 1.42 5.32 19.72 46.32 18.56 3.77 68.65
P-values less than 0.067 0.238 0.111 0.253 0.498 0.068 0.161 0.115 0.140 0.051 0.462 0.094 S.E. 0.035 0.005 0.024 0.332 0.080 0.029 0.102 0.359 0.860 0.362 0.149 0.120
a
Molmolasses; Locnumber of locations; Method: Sunsun-dried, Freezefreeze-dried, Airair-dried. b
Rharhamnose; Fucfucose; Ribribose; Araarabinose; Xylxylose; Manmannose; Galgalactose; Gluglucose. c
NSPFucRibArabXylManGalGlu. d
TotalNSPKlason ligninpectin. e
S.D.
the 1.85% found in this experiment. Drying had little biological effect on mineral content although signi®cant differences were found for P and Mg.
4. Conclusions
The addition of molasses to beet pulp resulted in signi®cant compositional change in most components except for CP, ether extract, TDN, ribose, and Mg. The decrease in the ®ber components was due to the dilution of ®ber by the addition of molasses. Using this premise, the amount of molasses added was calculated to be about 27% (w/w) of the beet pulp with molasses. The rapid digestibility of the beet pulp ®ber could be due to the high arabinose content of hemicellulose rather than xylose. Xylose is typically the dominant hemicellulose sugar in mature forages. The low lignin content of beet pulp could also affect the rate of digestion. Beet pulp with molasses had about four times more K than beet pulp without molasses and this re¯ects the high K content of molasses. The NDF remaining at earlier times of in sacco fermentation was higher for sun-dried compared to air- or freeze-dried beet pulp but not different at 24 h. The NDF of sun-dried beet pulp may contain non-®ber carbohydrates that are fermented at 24 h. The method of drying Table 3
Calcium (Ca), phosphorus (P), magnesium (Mg), potassium (K), and sodium (Na) as a percentage of beet pulp dry matter with or without molasses averaged across four locations and dried by three methods
Mola Loca Methoda Ca P Mg K Na
No 4 Sun 0.47 0.05 0.31 0.46 0.46
0.184b 0.008 0.034 0.097 0.208
No 4 Freeze 0.42 0.06 0.25 0.47 0.28
0.175 0.005 0.029 0.101 0.050
No 4 Air 0.48 0.04 0.33 0.42 0.42
0.184 0.007 0.025 0.134 0.051
Yes 4 Sun 0.86 0.06 0.31 1.96 0.84
0.432 0.027 0.031 0.144 0.585
Yes 4 Freeze 0.76 0.07 0.27 1.77 0.67
0.345 0.013 0.047 0.294 0.493
Yes 4 Air 0.66 0.05 0.32 1.83 0.84
0.212 0.017 0.047 0.226 0.578
Average without Mol 0.45 0.05 0.29 0.45 0.39
Average with Mol 0.76 0.06 0.30 1.85 0.79
P-values less than 0.009 0.032 0.671 0.001 0.010
S.E. 0.073 0.003 0.011 0.044 0.096
Average sun drying 0.67 0.06 0.31 1.21 0.65
Average freeze drying 0.59 0.07 0.26 1.12 0.48
Average air drying 0.57 0.05 0.33 1.12 0.63
P-values less than 0.728 0.035 0.008 0.452 0.548
S.E. 0.089 0.004 0.013 0.054 0.118
signi®cantly affected several other components but the magnitudes of the differences were small and probably of little biological importance. If one assumes freeze-dried beet pulp has similar characteristics as wet beet pulp, then this research shows very little difference in chemical composition or NDF fermentation characteristics between sun-dried and wet beet pulp.
Acknowledgements
Research was supported by the California Agricultural Experiment Station and the California Milk Advisory Board. The authors would like to thank the companies that supplied the beet pulp samples.
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