C. van Rooijen

The Maillard reaction and pet food processing: effects on nutritive value and pet health

The Maillard reaction, which can occur during heat processing of pet foods or ingredients, is known to reduce the bioavailability of essential amino acids such as lysine due to the formation of early and advanced Maillard reaction products (MRP) that are unavailable for utilisation by the body. Determination of the difference between total and reactive lysine by chemical methods provides an indication of the amount of early MRP present in foods, feeds and ingredients. Previous research reported that the difference between total and reactive lysine in pet foods can be up to 61.8%, and foods for growing dogs may be at risk of supplying less lysine than the animal may require. The endogenous analogues of advanced MRP, advanced glycation endproducts, have been associated with age-related diseases in humans, such as diabetes and impaired renal function. It is unknown to what extent advanced MRP are present in pet foods, and if dietary MRP can be associated with the development of diseases such as diabetes and impaired renal function in pet animals. Avoidance of ingredients with high levels of MRP and processing conditions known to favour the Maillard reaction may be useful strategies to prevent the formation of MRP in manufactured pet food. Future work should further focus on understanding the effects of ingredient choice and processing conditions on the formation of early and advanced MRP, and possible effects on animal health.

Quantitation of Maillard reaction products in commercially available pet foods.

During processing of pet food, the Maillard reaction occurs, which reduces the bioavailability of essential amino acids such as lysine and results in the formation of advanced Maillard reaction products (MRPs). The aim of this study was to quantitate MRPs (fructoselysine (FL), carboxymethyllysine (CML), hydroxymethylfurfural (HMF)) and the cross-link lysinoalanine (LAL) in commercial pet foods. Sixty-seven extruded, canned, and pelleted dog and cat foods for growth and maintenance were analyzed using UPLC-MS. Canned pet foods contained on average the most FL, CML, and HMF (4534, 37, and 1417 mg/kg dry matter, respectively) followed by pelleted and extruded foods. Average daily intake (mg/kg body weight(0.75)) of HMF is 122 times higher for dogs and 38 times higher for cats than average intake for adult humans. As commercial pet foods are most often the only source of food for dogs and cats, future research focus should be on the bioavailability and long-term health implications of MRP consumption by dogs and cats.

Effects of acid extrusion on the degradability of maize distillers dried grain with solubles in pigs.

Commonly used feed processing technologies are not sufficient to affect recalcitrant nonstarch polysaccharides (NSP) such as arabinoxylans present in maize distillers dried grain with solubles (DDGS). Instead, hydrothermal treatments combined with acid catalysts might be more effective to modify these NSP. The objective of this experiment was to investigate the effects of hydrothermal maleic acid treatment (acid extrusion) on the degradability of maize DDGS in growing pigs. It was hypothesized that acid extrusion modifies DDGS cell wall architecture and thereby increases fermentability of NSP. Two diets, containing either 40% (wt/wt) unprocessed or acid-extruded DDGS, were restrictedly fed to groups of gilts (n=11, with 4 pigs per group; initial mean BW: 20.8±0.2 kg) for 18 d and performance and digestibility were analyzed. Acid extrusion tended to decrease apparent ileal digestibility (AID) of CP (approximately 3 percentage units [% units]); P=0.063) and starch (approximately 1% unit; P=0.096). Apparent digestibility of CP and starch measured at the mid colon (2% units, P=0.030, for CP and 0.3% units, P<0.01, for starch) and apparent total tract digestibility (ATTD; 3% units, P<0.01, for CP and 0.2% units, P=0.024, for starch) were lower for the acid-extruded diet compared with the control diet. Hindgut disappearance was, however, not different between diets, indicating that reduced CP and starch digestibility were mainly due to decreased AID. Acid extrusion tended to increase AID of NSP (6% units; P=0.092) and increased digestibility of NSP measured at the mid colon (6% units; P<0.01), whereas hindgut disappearance and ATTD of NSP did not differ between diets. Greater NSP digestibility was mainly due to greater digestibility of arabinosyl, xylosyl, and glucosyl residues, indicating that both arabinoxylan and cellulose degradability were affected by acid extrusion. In conclusion, these results show that acid extrusion did not improve degradation of DDGS for growing pigs. Although acid extrusion seemed to facilitate more rapid degradation of NSP and shifted fermentation to more proximal gastrointestinal segments, total extent of NSP degradation was not affected. More than 35% of the NSP from DDGS remained undegraded, independent of technological processing. Enzyme technologies that specifically target ester-linked acetyl, feroloyl, or coumaroyl groups were identified to be of interest for future research.

Urinary excretion of dietary maillard reaction products in healthy adult female cats

During processing of foods, the Maillard reaction occurs, resulting in the formation of advanced Maillard reaction products (MRP). Varying amounts of MRP have been found in commercially processed pet foods. Dietary MRP can be absorbed and contribute to the endogenous pool of MRP and possibly the etiology of age-related diseases. The aim of the present study was to determine urinary excretion of dietary MRP in cats fed commercial moist and dry foods. A pilot study with 10 cats, conducted to determine the adaptation time required for stable urinary excretion of MRP when changing to a diet with contrasting MRP content, showed an adaptation time of 1 d for all components. In the main study, 6 commercially processed dry and 6 moist diets were fed to 12 adult female cats in 2 parallel randomized, 36-d Latin square designs. The 24-h urine was collected quantitatively using modified litter boxes, and fructoselysine (FL), carboxymethyllysine (CML), and lysinoalanine (LAL) were analyzed using ultra high performance liquid chromatography (UHPLC) - mass spectrometer. Daily urinary excretion of FL and CML showed a positive relationship with daily intake in the dry ( = 0.03 and < 0.01, respectively) and moist ( < 0.01) foods. For LAL, no significant relationship was observed. Urinary recovery (% ingested) showed a negative relationship with daily intake for FL, CML, and LAL in the dry foods ( < 0.01, < 0.01, and = 0.08, respectively) and for CML and LAL in the moist foods ( < 0.01). The observed increase in urinary excretion with increasing dietary intake indicates that dietary MRP were absorbed from the gastrointestinal tract of cats and excreted in the urine. The adaptation time with change in diet indicates a likely effective excretion of MRP. Minimum apparent absorption of FL, CML, and LAL was found to range between 8% and 23%, 25% and 73%, and 6% and 19%, respectively. The observed decrease in urinary recovery suggests a limiting factor in digestion, absorption, metabolism, or urinary excretion. This study shows that dietary MRP in commercial diets are absorbed and excreted via the kidneys in cats.

Reactive lysine content in commercially available pet foods

The Maillard reaction can occur during processing of pet foods. During this reaction, the ε-amino group of lysine reacts with reducing sugars to become unavailable for metabolism. The aim of the present study was to determine the reactive lysine (RL; the remaining available lysine) to total lysine (TL) ratio of commercial pet foods and to evaluate whether RL levels meet minimal lysine requirements (MLR). Sixty-seven extruded, canned and pelleted commercially available dog and cat foods for growth and maintenance were analysed for proximate nutrient composition, TL and RL. RL was expressed on a metabolisable energy basis and compared with the MLR for maintenance and growth. In dog foods, average RL:TL ratios were 0·87 (se 0·02) for extruded, 0·97 (se 0·02) for canned and 0·85 (se 0·01) for pelleted foods, with the lowest ratio of 0·77 in an extruded diet for growing dogs. In extruded and canned cat foods, the average ratio was 0·91 (se 0·02) and 0·90 (se 0·03), respectively, with the lowest ratio being 0·67 in an extruded diet for growing cats. Variation in the RL:TL ratio between and within processing type indicate that ingredients rather than processing might be the key factor influencing RL content in pet foods. Eight dry foods for growing dogs had RL contents between 96 and 138 % of MLR, indicating that RL has to be between 62 and 104 % digestible to meet the MLR. Considering the variability in RL digestibility, these foods could be at risk of not meeting the MLR for growing dogs. Ingredients and pet foods should be characterised with respect to the RL content and digestibility, to avoid limitations in the lysine supply to growing dogs.