@article{qiao_gumpertz_van kempen_2005, title={Stability of a pancreatic enzyme cocktail during in vitro protein digestibility assays}, volume={29}, ISSN={["0145-8884"]}, DOI={10.1111/j.1745-4514.2005.00003.x}, abstractNote={To maximize the efficiency of utilization of a pancreatic enzyme cocktail and estimate the contamination for in vitro protein digestibility assays, the specific activity losses of trypsin and chymotrypsin and the digestion of the enzyme proteins were studied. In the absence of protein substrate, increase of enzyme concentration augmented the half-lives of trypsin and chymotrypsin and decreased the digestion of enzymatic proteins. In contrast, in the presence of substrate, increase of enzyme concentration decreased trypsin's half-life. Increase of pH augmented the digestion of enzymatic proteins. The results indicated the optimum time for utilization of the enzymes depended on pH, enzyme concentration and presence of substrate. At the time when digestion of the proteins ceased, the average size of the hydroly sates was calculated between 3.1 and 5.4 amino acid residues, suggesting most proteins in the enzyme cocktail would be detected as digestible proteins.}, number={2}, journal={JOURNAL OF FOOD BIOCHEMISTRY}, author={Qiao, YR and Gumpertz, M and Van Kempen, T}, year={2005}, month={Apr}, pages={205–220} }
 @article{van kempen_peak_yanrui_2004, title={In vitro digestibility could meet quality control needs}, volume={76}, ISBN={0014-9624}, number={8}, journal={Feedstuffs}, author={Van Kempen, T. and Peak, S. and YanRui, Qiao}, year={2004}, pages={11} }
 @article{qiao_lin_odle_whittaker_van kempen_2004, title={Refining in vitro digestibility assays: Fractionation of digestible and indigestible peptides}, volume={82}, ISSN={0021-8812 1525-3163}, url={http://dx.doi.org/10.2527/2004.8261669x}, DOI={10.2527/2004.8261669x}, abstractNote={Typically, in vitro methods used for estimating the amount of ileal digestible AA do not exhaustively digest samples, and arbitrary methods for separating digestible from indigestible protein are used. This may lead to over- or underestimation of digestibility coefficients. A method that exhaustively digests proteins using pepsin and pancreatin was developed, and the first objective of this research was to confirm that exhaustive digestion was indeed appropriate and to determine the fractionation method for separating digestible from indigestible proteins. For this, three homoarginine-labeled animal proteins were prepared. Samples were subsequently digested in vivo and in vitro to determine which fraction should be considered indigestible, and in vitro followed by in vivo to determine whether the extent of digestion in vivo was improved by predigestion. In vivo, soluble but unabsorbed peptides were smaller than 1 kDa, suggesting that the size of soluble peptides is not what prevents their absorption. Thus, all in vitro-soluble proteins should be considered digestible. In vitro, 88 +/- 3% of the soluble peptides were smaller than 1 kDa, with the remainder between 1 and 5 kDa, suggesting that in vitro digestion is less complete. Predigested samples were digested in vivo to the same size distribution as the nonpredigested samples. The second objective was to test whether in vitro digestibility assays based on these principles equaled in vivo digestibility. For this, digestibility data for 25 animal proteins were compared. Results showed a lack of correlation between lysine digestibility coefficients; however, across samples, the extent of digestion did not differ for lysine (P = 0.71), threonine (P = 0.26), methionine (P = 0.18), or valine (P = 0.66), whereas in vitro digestibility coefficients were lower for (the less water-soluble) histidine (P = 0.05), isoleucine (P < 0.01), leucine (P < 0.01), and phenylalanine (P = 0.05). In conclusion, in vitro digestibility assays should exhaustively digest proteins to mimic in vivo digestibility. All in vitro-soluble peptides could be considered digestible, because in vivo, no large soluble peptides were observed whose size prevented them from being absorbed. However, an in vitro assay based on these principles lacked precision for highly water-soluble AA, and underestimated digestibility for other AA. Better solubilization of the digesta and more replicates may improve the in vitro assay further.}, number={6}, journal={Journal of Animal Science}, publisher={Oxford University Press (OUP)}, author={Qiao, Y. and Lin, X. and Odle, J. and Whittaker, A. and van Kempen, T. A. T. G.}, year={2004}, month={Jun}, pages={1669–1677} }
 @article{qiao_kempen_2004, title={Technical note: Comparison of Raman, mid, and near infrared spectroscopy for predicting the amino acid content in animal meals}, volume={82}, DOI={10.2527/2004.8292596x}, abstractNote={The objective of this study was to compare three infrared spectroscopy techniques for routine evaluation of AA in animal meals. Animal meals (n = 54) with known AA contents were scanned with a near (NIRS), mid (FTIR), and Raman infrared spectrometer. For NIRS and Raman, samples were scanned "as is", whereas for FTIR, samples had to be finely ground before scanning to obtain reasonable spectra. Both FTIR and Raman data suffered from noise; for Raman, this prevented the development of calibrations. Using derivatized spectral data and a standardized outlier removal procedure, calibrations for nutritionally relevant AA could be developed that were equivalent for both NIRS and FTIR. The variation across AA tested explained (r2) by these calibrations was 70% for NIRS and 68 + 3% for FTIR. Removing spectral data between 4,000 and 2,000 cm(-1) from the FTIR data improved calibrations (P = 0.09) and explained an average of 77% of the variation with prediction errors lower than obtained with NIRS (P < 0.01). However, FTIR calibrations based on the entire or the shortened spectrum contained fewer samples than did NIRS calibrations (41 and 39 vs. 48, respectively; P < 0.01) because more samples were removed as outliers. In conclusion, Raman did not yield acceptable spectra for animal meals. For FTIR, sample preparation was more time-consuming because the samples required grinding before analysis. Using the entire mid-infrared range, FTIR calibrations were comparable to NIRS calibrations. Calibrations for FTIR were improved by eliminating wave numbers that exhibited more noise, resulting in prediction errors better than those for NIRS. Thus, FTIR has the potential to yield better calibrations for AA in animal meals than NIRS, but it requires greater care in sample preparation and scanning.}, number={9}, journal={Journal of Animal Science}, author={Qiao, Y. and Kempen, Theo}, year={2004}, pages={2596–2600} }
 @article{qiao_gumpertz_van kempen_2002, title={Stability of pepsin (EC 3.4.23.1) during in vitro protein digestibility assay}, volume={26}, ISSN={["0145-8884"]}, DOI={10.1111/j.1745-4514.2002.tb00759.x}, abstractNote={To maximize the efficiency of utilization of pepsin and estimate the contamination of pepsin for in vitro protein digestibility assays, the specific activity decay and peptide bond hydrolysis of pepsin incubated at different pH and concentration were studied with the bovine hemoglobin method and the o-phthaldialdehyde method, respectively. It was found that increase of pH and concentration of pepsin increased pepsin's half-life for both specific activity decay and peptide bond hydrolysis. The half-life for specific activity decay was not extended by the presence of a substrate protein. The results indicated the time needed to maximize pepsin utilization depended on pH and the concentration of pepsin. At the time when all specific activity of pepsin was lost, the average size of pepsin autolysates was between 6.9 and 12.1 amino acid residues, suggesting most peptic protein would be fractionated as digestible protein.}, number={4}, journal={JOURNAL OF FOOD BIOCHEMISTRY}, author={Qiao, YR and Gumpertz, M and Van Kempen, T}, year={2002}, month={Sep}, pages={355–375} }