The digestibility of sugars in molasses: in vitro evaluation


Chard and cane molasses are produced all over the world, as a by-product of sugar extraction and are used in animal feed due to their chemical composition and characteristics. Several studies have reported that the addition of molasses reduces the selection activity of cows, stimulates the intake of dry matter due to its sweetening effect, increases the production of butyrate, has an influence on milk fat, rumen ammonia and digestibility. Overall, molasses could be added to diets as a replacement for starch sources, due to a different impact on ruminal fermentations and pH, due to its sugary composition. Molasses contains mono- di- and tri-saccharides, composed of hesoses, as well as pentosis in traces. So, one could assume that each of these compounds has a different fermentation, depending on the complexity of the molecule and its structure. The aim of this preliminary study was to evaluate the dynamics of digestion in vitro and the rates of the sugar fraction in molasses.


For in vitro fermentations, three chard molasses and three cane molasses were selected from a group of 32 samples collected around the world. For the determination of sugars, samples were clarified using a commercial kit based on Carrez reagents. After this procedure, glucose, fructose, sucrose, galactose, raffinose, arabinose and xylose were extracted and quantified using an enzymatic method. Then the content of ash, Ca, Mg, Na and K, organic acids and other components such as sulphates, phosphates, chlorides and nitrates was calculated. The in vitro fermentations were conducted following the procedure described by Palmonari et al. (2017) In short, two lactation Holstein cows were selected on the basis of a similar body weight, DIM, milk production and milk composition. The animals were milked twice a day and fed a diet of hay, alfalfa (45% aNDFom), grass hay (52% aNDFom), and corn wheat (62% starch). The fluid of the rumen was sampled via esophageal probe, pouring the first volume collected to avoid contamination of saliva or mucus, and immediately placed in a thermostatic bottle.

Sampling was carried out 0300h after feeding. The contents of the rumen were filtered under constant presence of oxygen and absence of carbon dioxide. Once filtered, an equal amount of each collected liquid was sampled and mixed with the other. The time points selected to assess the disappearance of sugar were 1, 2, 3, 4, 6, 8 and 24 hours.


The procedure adopted was able to extract residual sugars, even at longer fermentation times. After one hour of ruminal fermentation in vitro, the digestibility of sucrose was more than 90% (93.9 and 93.3% on average, for cane and chard molasses, respectively). A complete disappearance of this sugar seems to occur within 4 hours of incubation (at 99.7%, while it tended to decrease after 3 hours of fermentation. Glucose was fully fermented in 6 hours (99.0% on average) in the cane molasses. In addition, numerically higher variability in glucose degradation than sucrose was found among cane molasses. This fact may depend on the composition of the molasses, and less likely on the proportion of this sugar in the molasses. Its content was in fact similar between the samples, although the digestibility values differed for each time point up to 6 hours. A similar trend was observed for fructose, with numerically high variability in cane molasses, and complete digestion after 6 hours of fermentation (98.5% on average). Unlike cane, beet molasses has almost no glucose and fructose content. However, the digestibility values were close to those obtained in cane molasses. Glucose and fructose achieved complete fermentation after 6 hours of incubation (99.2 and 92.9%, respectively). In general, the slower digestion observed for these two sugars, compared to sucrose, could be due to the breakdown of more complex carbohydrates, such as sucrose, lactose (for glucose) or others. Since the residual amount is quantified, this process could lead to an underestimation of the dynamics of digestion. Raffinose was fully digested within 1 hour in chard molasses (97.8% on average), while it was slower in the barrel, for which a complete disappearance was obtained in 6 hours (97.5% on average). Galactose was rapidly digested in vitro in both groups, but the digestibility values were statistically different at 1 and 2 hours of fermentation (74.8 versus 86.8%, and 79.9 versus 87.5% at 1 and 2 hours, for cane and beetroot, respectively). Its complete disappearance was observed at 3 hours of fermentation in both groups (96.2 and 94.5% for cane and beet molasses, respectively). Another interesting point is related to some digestibility values that are numerically higher in the previous time points than in the following ones. As reported in Tables 2 and 3, this occurred in both cane and beet molasses for different sugars. This fact could be linked to the degradation of other compounds, which could release small amounts of simple sugars into the incubation medium. Starch, levans, hydrates and arabani are complex carbohydrates, so their fermentation could act as described. The values obtained in the rate of digestion were similar among cane and chard molasses, and above 50% hˉ¹ for most of the sugars evaluated. In particular, sucrose showed a rate of 54.5 and 57.9% hˉ¹ for both cane and beet molasses. Glucose and fructose showed similar rates, being 51.3 and 50.5% hˉ¹ in cane molasses, 53.0 and 53.8% hˉ¹ in beetroot. The rationing software uses an estimate of the digestion rate of ~20% hˉ¹, without any differentiation between the different sugars. Considering the values obtained in this preliminary study, a rate of ~55% hˉ¹ on average for the main sugars (sucrose, glucose and fructose), seems more appropriate. By applying an adequate rate for these carbohydrates, it would be possible to obtain a better estimate of the net energy of sugar sources, as well as their impact on microbial mass and rumen functions.


In conclusion, molasses should be considered as a specific component of a ration, and not an “invariable” food, taking into account that its most representative fraction, sugars, can vary in composition and digestibility. In addition, modern rationing models should consider a change in sugar digestion rates, since the current rate (~20% hˉ¹) is too slow compared to those observed in this study.

Taken from: “In vitro evaluation of sugar digestibility in molasses” Autor: Alberto Palmonari et al. Source: Italian Journal of Animal Science 2021, vol. 20, no. 1, 571–577 The bibliography is available from the Editorial Office.

The authors would like to thank EDF&Man for supporting the study.