Changes in protein nutritional quality in fresh and recombined ultra high temperature treated milk during storage

The objective of this study was to determine the changes in nutritional quality of protein in stored UHT milk. Commercial fresh and recombined ultra high temperature (FUHT and RUHT respectively) treated milk samples were used. Pasteurized milk (P-milk) samples were used for comparison. Milk samples were freeze-dried for the use in diet formulation. Pasteurized milk samples were freezedried within 24 h of production. FUHT and RUHT milk samples were freeze-dried after production (zero time), 3 and 6 months of storage at 37°C. Three nitrogen balance experiments were done at zero time, after 3 and 6 months of storage. The diet used in all experiments were similar in composition except for protein source. In each experiment egg white (low protein), casein (control), P-milk, and either 0, 3 or 6 months old FUHT and RUHT milk samples were used as the source of protein in the diets. In each experiment 30 (six for each diet) male Sprague-Dawley rats were used. Apparent digestibility (AD), true digestibility (TD), biological value (BV), and net protein utilization (NPU) were calculated for protein in casein and all milk samples. The nutritional quality of protein in FUHT milk before storage was similar to that of protein in pasteurized milk, but it decreased during storage for 3 months and then remained at the same quality up to 6 months of storage. BV and NPU of protein in FUHT milk after storage for 3 and 6 months were lower than that of protein in pasteurized milk by 4% (not significant) and 5% respectively. On the other hand, the nutritional quality of protein in RUHT milk before storage was lower than that of protein in pasteurized milk and remained at the same level after storage for 3 months: however, it decreased significantly after storage for 6 months. AD and TD of protein in RUHT milk stored for 6 months were lower than that of protein in pasteurized milk by 5% and BV and NPU were lower than that of protein in pasteurized milk by 9 and 12% respectively. The above reduction in nutritional quality of protein in UHT milk may be of significant value in feeding young children who might be dependent on this type of milk in their diet.     

Microbiological Evaluation of Ultra-High-Temperature (UHT)-Treated Milk Close to Expiry Date and Routine Home Practices for Preservation of Milk

Background:

As milk is an excellent medium for growth of microorganisms, milk gets contaminated very easily leading to its early spoilage and to milk-borne diseases.

Objectives:

To compare the quality of pasteurized milk with milk cooker-treated milk and to compare the quality of fresh ultra-high-temperature (UHT) milk with that of UHT milk close to expiry date.

Materials and Methods:

Samples of pasteurized milk, milk heated in milk cooker, fresh UHT milk, and UHT milk nearing expiry date were tested by methylene blue reduction test, coliform test, and colony count test. The results were analyzed statistically using Statistical Package for Social Sciences (SPSS) version 11.5.

Results:

All milk samples passed the methylene blue test and UHT-treated milk passed the coliform test also. Out of the 30 fresh pasteurized milk samples, seven (23.3%) passed the coliform test; but the same after milk cooker treatment, 16 (53.3%) passed the coliform test (P < 0.05). Fourteen (46.6%) out of 30 fresh pasteurized milk samples failed the colony count test, but the same after milk cooker treatment only eight (26.6%) failed the test (P = 0.583). Only seven out of the 60 UHT milk samples failed the colony count test and out of these six (20%) were close to expiry date (P < 0.05).

Conclusions:

Milk cooker-treated milk appears to be safer than the fresh pasteurized milk. Fresh UHT-treated milk is safer then UHT milk close to expiry date.     

In vitro evaluation of iron solubility and dialyzability of various iron fortificants and of iron-fortified milk products targeted for infants and toddlers

The objectives of the present study were: to compare the solubility and dialyzability of various iron fortificants (iron pyrophosphate, ferrous bis-glycinate, ferrous gluconate, ferrous lactate, ferrous sulfate) added, in the presence of ascorbic acid, to pasteurized milk samples produced under laboratory conditions; and to compare the solubility and dialyzability of iron in commercial pasteurized, UHT and condensed milk products available in the Greek market fortified with various vitamins and minerals including iron and targeted towards infants (6-12 months old) and toddlers. Iron solubility and dialyzability were determined using a simulated gastrointestinal digestive system. Ferrous dialyzable iron (molecular weight lower than 8000) was used as an index for prediction of iron bioavailability. Ferrous dialyzable iron in pasteurized milk samples fortified with iron pyrophosphate, ferrous lactate and ferrous bis-glycinate was higher (P < 0.05) than that in milk samples fortified with ferrous sulfate and ferrous gluconate. In commercial liquid pasteurized or UHT milk products, formation of ferrous dialyzable iron in products fortified with ferrous lactate was not different (P > 0.05) from those fortified with ferrous sulfate. Ferrous dialyzable iron in four condensed commercial milk products was higher (P < 0.05) than the corresponding values of the liquid UHT milk samples fortified with the same fortificant (ferrous sulfate). Ferrous dialyzable iron was higher (P < 0.05) in products targeted for infants compared with those targeted for toddlers. In conclusion, the type of iron source, milk processing and the overall product composition affect formation of ferrous dialyzable iron and may determine the success and effectiveness of iron fortification of milk.     

In vitro evaluation of iron solubility and dialyzability of various iron fortificants and of iron-fortified milk products targeted for infants and toddlers

The objectives of the present study were: to compare the solubility and dialyzability of various iron fortificants (iron pyrophosphate, ferrous bis-glycinate, ferrous gluconate, ferrous lactate, ferrous sulfate) added, in the presence of ascorbic acid, to pasteurized milk samples produced under laboratory conditions; and to compare the solubility and dialyzability of iron in commercial pasteurized, UHT and condensed milk products available in the Greek market fortified with various vitamins and minerals including iron and targeted towards infants (6–12 months old) and toddlers. Iron solubility and dialyzability were determined using a simulated gastrointestinal digestive system. Ferrous dialyzable iron (molecular weight lower than 8000) was used as an index for prediction of iron bioavailability. Ferrous dialyzable iron in pasteurized milk samples fortified with iron pyrophosphate, ferrous lactate and ferrous bis-glycinate was higher (P?<?0.05) than that in milk samples fortified with ferrous sulfate and ferrous gluconate. In commercial liquid pasteurized or UHT milk products, formation of ferrous dialyzable iron in products fortified with ferrous lactate was not different (P?>?0.05) from those fortified with ferrous sulfate. Ferrous dialyzable iron in four condensed commercial milk products was higher (P?<?0.05) than the corresponding values of the liquid UHT milk samples fortified with the same fortificant (ferrous sulfate). Ferrous dialyzable iron was higher (P?<?0.05) in products targeted for infants compared with those targeted for toddlers. In conclusion, the type of iron source, milk processing and the overall product composition affect formation of ferrous dialyzable iron and may determine the success and effectiveness of iron fortification of milk.     

Nutritional Adequacy of Commercial Infant Milk Formulas

The nutritional adequacy of six commonly consumed commercial infant milk formulas were evaluated chemically and biologically in growing rats. Infant formulas (per 100 kcal) provided protein (2.2-2.6 g), fat (4.9-5.4 g), available carbohydrates (10.3-11.4 g), Ca (62.5-80.8 mg), P (31.3-66.1 mg), Fe (0.7-1.8 mg), Zn (0.5-0.8 mg), and Cu (0.05-0.08 mg). The energy density varied between 503 and 526 kcal/100 g. Higher levels of linoleic and saturated fatty acids than in human milk were found. Methionine, tryptophan and lysine were the first, second, and third limiting amino acids in all infant formulas. The true protein digestibility (TD), net protein utilization (NPU), biological value (BV) and utilizable protein (UP) varied from 93-96%, 0.75-0.79, 80-82% and 8.7-10.5% respectively. The protein quality of infant formulas was lower than in cow's milk and human milk.     

Diet and disadvantage: observations on infant feeding from an inner city

Background: There are few detailed nutritional studies analysing dietary intakes and weaning practices of inner city infants aged 0–12 months. Pasteurized cow's milk (PCM) is not recommended as a main drink until after 1 year of age, although early usage is still common. Premature introduction of pasteurized cow's milk is associated with increased risk of iron deficiency anaemia. Methods: We therefore prospectively examined the dietary effect of early introduction of pasteurized cow's milk on the nutritional intake of 100 infants (mean age at recruitment 7.8 months), whose mothers had already elected to introduce cow's milk before the recommended age of 12 months in a deprived inner city area. In addition, a retrospective questionnaire on feeding practices and food choices was administered and information on parental education and employment was collected, together with a 3-day dietary diary of weighed intakes at recruitment. Results: The results indicate that weaning practices are handed down from family and friends and are intuitive rather than informed. Over 80% of the infants had intakes of iron, zinc and vitamin D below the reference nutrient intake (RNI) and a further 41% had low vitamin C intakes. Prior to introduction of PCM, there was also misuse of infant formula. Twenty per cent added an extra scoop of powder to the feeds, 10% added milk powder to the bottle before addition of water and 30% used microwave ovens to heat the infant bottle. Conclusion: Feeding practices in a deprived inner city area differed substantially from guidelines and infants were at risk of developing nutrient deficiencies as well as poor feeding practices.     

Vitamins,fatty acids,and antioxidant capacity stability during storage of freeze-dried human milk

Although freezing is the most common method used to preserve human milk, nutritional and immunological components may be lost during storage. Freeze-drying could increase the shelf life of human milk, while preserving its original characteristics. Seventy-two samples of freeze-dried human milk were stored for different periods of time, up to a maximum of 3?months, at 4?°C or 40?°C. Vitamin C, tocopherols, antioxidant capacity, and fatty acids composition were analyzed. A new HILIC–UHPLC method improving vitamin C determination was also validated. Ascorbic acid and total vitamin C concentrations significantly decreased at both temperatures, while antioxidant capacity only decreased at 40?°C. Fatty acids composition and both γ-tocopherol and δ-tocopherol contents remained unaltered. The stability after storage of freeze-dried milk was higher than that reported for frozen or fresh milk indicating that freeze-drying is a promising option to improve the preservation of human milk in banks.     

Nutritional response of growing rats to faba beans (Vicia faba L., minor) and faba bean fractions.

The effects of raw faba bean (Vicia faba L., minor) meal (VFM) and its fractions on the growth and nitrogen utilization of rats have been determined in two experiments. Two commercial varieties of VFM were tested, local VFM (409-439 g/kg diet) and Troy VFM (439 g/kg diet). The bean fractions tested were V. faba lectin-depleted protein (VFDP), V. faba lectin (VFL) and V. faba cotyledon residue (VFCR). All diets were supplemented with amino acids to target requirements. Body-weight, body N and lipid contents of rats fed on VFM were reduced significantly in comparison with control rats fed on lactalbumin. This was due, in part, to the lower digestibility of the protein, lipid and dry matter (DM) of VFM diets. As a result, net protein utilization (NPU) and biological value (BV) of faba bean proteins were less than expected. Urine and urea-N outputs of the VFM-fed rats were also elevated in both experiments. Increasing the energy content of local VFM diets led to significantly higher dry body-weight, body N and lipid contents, with the result that the NPU and BV values of the protein also increased. However, the NPU values for VFM-fed rats were still significantly lower than those for the controls in both experiments. In contrast, true N, lipid and DM digestibilities in rats given local VFM were not significantly affected by the difference in the energy content of the diets. The replacement of two-thirds of the lactalbumin in the diet with VFDP (65 g/kg) reduced dry body-weight, N and lipid contents, NPU and BV compared with the control rats, even though N, lipid and DM digestibilities were not significantly different. The nutritional performance of rats fed on lactalbumin-based diets containing 7 g VFL/kg was similar to that of the controls. Similarly, the inclusion of the cotyledon residue (237 g VFCR/kg diet) had no appreciable effect on any of the variables studied. As VFL and VFCR had no antinutritional effects in these rats, it appears that the low nutritional value of VFM for rats (NPU 0.66) results not only from the low digestibility of the bean proteins, but also from disturbances in N metabolism.     

The protein quality of instant blends for infants and children based on milk or cereals and milk

The protein quality of industrially produced and home prepared milk or cereal-milk based foods for infants and children were evaluated by means of biological assays. The net protein utilization (NPU) values of human milk substitutes based on cow's milk varied between 75 and 78 and the NPU of a soy-based product was found to be 70. Cereal-milk based precooked gruel powders had NPU values of 69 to 77. Furthermore, a comparison between industrially processed products and their corresponding home prepared alternatives was made. Although the protein nutritional products, i.e., including the vitamine and mineral content, is more standardized due to supplementation. The NPU assays of some cereal-milk based porridge powders revealed a wide variations (28 to 69); the lowest NPU values reflect the influence of the processing technique. This illustrates the necessity of performing biological evaluations of processed foods.     

Effect of fiber-rich brown seaweeds on protein bioavailability of casein in growing rats

The effect of three subtropical fiber-rich brown seaweeds, Sargassum hemiphyllum (Turn.) C. Ag., Sargassum henslowianum C. Ag. and Sargassum patens Ag., on protein bioavailability of casein were evaluated by growing rats. The effect of the fiber-rich seaweeds was determined by comparing the net protein ratio (NPR), true protein digestibility (TD), nitrogen balance (NB), biological value (BV), net protein utilization (NPU), and fecal and urinary nitrogen loss of seaweed-based diet groups with those of the cellulose-based control group. There were no significant differences on NPR and urinary N loss in all diet groups. However, the values of TD (range 85.6-89.6%), BV (range 96.9-97.5%), NPU (range 83.5-86.8%) and NB (range 145-150 mg/rat/day) in all seaweed-based diet groups were significantly (p<0.05, one-way ANOVA, Tukey-HSD) lower than those of the control group. On the contrary, the fecal weight (range 5.59-6.30 g/rat/day) and fecal nitrogen (N) loss (range 31.1-41.2 mg/rat/day) of rats fed seaweed-based diets were significantly (p<0.05, one-way ANOVA, Tukey-HSD) higher than that of the control one. However, the differences on the body weight gain and weight of internal organs including liver, kidney and spleen between the seaweed-based diets and control groups were insignificant except that the weight of caecum of rats fed seaweed-based diets was significantly (p<0.05, one-way ANOVA, Tukey-HSD) higher than that of control group. The possible mechanism of fiber-rich brown seaweeds in affecting the protein bioavailability of casein and their related nutritional implications were discussed.     

Taurine in milk and yoghurt marketed in Italy

Taurine, a free amino acid, was studied as natural compound of different typologies of milk: pasteurized, ultra-high temperature (UHT), microfiltered whole and semi-skimmed cow's milk; pasteurized and UHT goat's whole milk and raw buffalo's whole milk. Moreover, taurine contents in yoghurt from cow and goat's milk were evaluated. The data obtained in this research showed that no significant variations of taurine occurred in cow's milk subjected to different technological processes and between whole and semi-skimmed milk. The amount of taurine was less (p < 0.05) in cow's milk (0.60 mg/100 g) than in goat and buffalo's milk (6.55 and 7.32 mg/100 g, respectively). No significant differences in taurine occurred between goat and buffalo's samples. The amounts of taurine in yoghurt reflected, substantially, the content of this molecule in the milk of the relevant animal species. These results are noteworthy because data available in the literature on this molecule in commercial dairy products are old or few.     

Nutritional Adequacy of Commercial Infant Milk Formulas

The nutritional adequacy of six commonly consumed commercial infant milk formulas were evaluated chemically and biologically in growing rats. Infant formulas (per 100 kcal) provided protein (2.2–2.6 g), fat (4.9–5.4 g), available carbohydrates (10.3–11.4 g), Ca (62.5–80.8 mg), P (31.3–66.1 mg), Fe (0.7–1.8 mg), Zn (0.5–0.8 mg), and Cu (0.05–0.08 mg). The energy density varied between 503 and 526 kcal/100 g. Higher levels of linoleic and saturated fatty acids than in human milk were found. Methionine, tryptophan and lysine were the first, second, and third limiting amino acids in all infant formulas. The true protein digestibility (TD), net protein utilization (NPU), biological value (BV) and utilizable protein (UP) varied from 93–96%, 0.75–0.79, 80–82% and 8.7–10.5% respectively. The protein quality of infant formulas was lower than in cow's milk and human milk.     

Effect of pectin structure on protein utilization by growing rats

The effect of pectins' molecular weight (MW) and degree of esterification (DE) on apparent digestibility (D), net protein utilization (NPU) and biological value (BV) of casein were tested. The pectins prepared differed in MW and DE respectively as follows: P-1 (180,000, 73%); P-2 (300,000, 28%); P-3 (60,000, 74%) and P-4 (75,000, 40%) and were fed as 10% of a purified diet containing 10% casein to growing male rats for 10 days. All preparations decreased D, with P-2 and P-4 showing milder effects than P-1 or P-3. Increased NPU was observed with pectins of low NW and/or DE, being significant only with P-4. Diets containing P-2, P-3 or P-4 had higher BV for casein than the pectin-free diet. Feeding these pectins in a protein-free diet did not cause a significant change in the rats' body N content. Thus, the increase in the BV of casein was attributed to improvements in the pattern of amino acids absorbed by rats fed pectins with low MW and/or low DE.     

Folic acid and 5-methyltetrahydrofolate in fortified milk are bioaccessible as determined in a dynamic in vitro gastrointestinal model

Dairy products are a potential matrix for folate fortification to enhance folate consumption in the Western world. Milk folate-binding proteins (FBP) are especially interesting because they seem to be involved in folate bioavailability. In this study, folate bioaccessibility was investigated using a dynamic computer-controlled gastrointestinal model [TNO gastrointestinal model (TIM)]. We used both ultrahigh temperature (UHT)-processed milk and pasteurized milk, differing in endogenous FBP concentrations and fortified with folic acid or 5-methyltetrahydrofolate (5-CH(3)-H(4)folate). To study FBP stability during gastrointestinal passage and the effect of additional FBP on folate bioaccessibility, FBP-fortified UHT and pasteurized milk products were also tested. Folate bioaccessibility and FBP stability were measured by taking samples along the compartments of the gastrointestinal model and measuring their folate and FBP concentrations. Folate bioaccessibility from folic acid-fortified milk products without additional FBP was 58-61%. This was lower (P < 0.05) than that of the 5-CH(3)-H(4)folate-fortified milk products (71%). Addition of FBP reduced (P < 0.05) folate bioaccessibility from folic acid-fortified milk (44-51%) but not from 5-CH(3)-H(4)folate-fortified milk products (72%). The residual FBP levels in the folic acid- and 5-CH(3)-H(4)folate-fortified milk products after gastrointestinal passage were 13-16% and 0-1%, respectively, of the starting amounts subjected to TIM. In conclusion, milk seems to be a suitable carrier for folate, because both folic acid and 5-CH(3)-H(4)folate are easily released from the matrix and available for absorption. However, our results suggest that folic acid remains partly bound to FBP during passage through the small intestine, which reduces the bioaccessibility of folic acid from milk in this model.     

The source of Yersinia spp. in pasteurized milk: an investigation at a dairy

Pasteurized bottled milk supplied by a single dairy was frequently found to be contaminated with Yersinia spp. Investigations were carried out at the dairy in an effort to pinpoint the source of these organisms. Viable counts obtained from milk bottle rinses indicated that bottle washing was often unsatisfactory, and on one occasion Y. frederiksenii was isolated from the pooled rinse water of six bottles. Samples of milk were taken on arrival at the dairy and at various stages following pasteurization. Heat resistance tests carried out on strains of yersinia isolated from pasteurized milk indicated that they would not survive the pasteurization process. However two strains of yersinia were isolated from a sample of milk taken immediately after pasteurization but before bottling. The thermograph indicated that the time/temperature conditions applied during pasteurization were adequate. The presence of yersinia strains in the milk at this stage therefore suggests that undetectable levels of raw milk were being allowed to contaminate the pasteurized milk. The absence of yersinia in cartoned samples produced on the same day as contaminated bottled samples indicated that environmental contamination of the bottle filler valve also may have occurred at times. Results of this investigation indicate that increased vigilance is required to ensure proper operation of pasteurizers and bottle washers.     

Nutritional evaluation of protein and starch of mature Canavalia gladiata seeds

Evaluation of true digestibility (TD), biological value (BV) and net protein utilisation (NPU) of diets made with raw and processed seed flour from mature Canavalia gladiata seeds were carried out with male Sprague-Dawley rats. The weight gain of the rats fed with diets containing raw whole seed and raw cotyledon alone were significantly lower (P = 0.05) than that of the group fed with the reference diet. Processing the cotyledons (dry-autoclaved or roasted) significantly (P < or = 0.05) increased the weight gain of the rats when compared to the weight gain of rats fed diets prepared with raw seed flour. NPU of raw (whole seed 13.8; cotyledon 27.6) was significantly lower (P < or = 0.05) than the reference diet (79.5). The NPU of processed samples was also significantly lower (P < or = 0.05) when (dry-autoclaved 25.1; roasted 25.1) compared to the reference diet fed group (79.5). The BV of the processed samples (dry-autoclaved 31.1; roasted 37.7) was significantly lower (P < or = 0.05) than that of raw (53.6) cotyledon. In contrast TD increased (P < or = 0.05) with processing (dry-autoclaved 80.9; roasted 65.9) when compared to raw cotyledon (51.4). In vitro protein digestibility for the raw whole seed and cotyledon was 71.7 and 70.1% respectively. In vitro starch digestibility of raw and processed cotyledon flour samples indicated dry-autoclaved sample to have the highest digestibility.     

The effect of varying crude protein level and the proportions of fibre in diets containing a mixture of meat-and-bone meal and wheat bran as the only protein source on nitrogen balance indices and energy digestibility in the rat

1. The experiment was designed to investigate the effect of protein level in diets on apparent protein digestibility (AD), true protein digestibility (TD), biological value (BV), net protein utilization (NPU) and energy digestibility (DE) in rats. The design involved fourteen dietary treatments in which a combination of wheat bran and meat-and-bone meal (1:1, w/w dry matter (DM) basis) was the only nitrogen source. Crude protein (N x 6.25; CP) level ranged from 60 to 329 g/kg DM in approximately even increments. All diets contained a constant level of minerals and vitamins. The balance of the diet consisted of a N-free mixture. 2. AD increased from 0.602 at 60 g CP/kg DM to 0.702 at 120 g CP/kg DM, with little change at the higher levels of dietary protein. The fitting of a curve indicated an asymptote of 0.721. 3. TD did not vary significantly between 60 and 225 g CP/kg DM (mean 0.781). At the higher protein levels there was a slight but significant reduction in the mean value (0.758). 4. BV increased from 0.475 at 60 g CP/kg DM to 0.498 at 80 g CP/kg DM where it plateaued, and then from 160 g CP/kg DM it decreased at an increasing rate to 0.216 at 329 g CP/kg DM. 5. The response in NPU closely reflected that obtained with BV, rising from 0.370 to 0.384, then declining to 0.162 at the highest CP concentration. 6. DE decreased linearly from 0.851 at 60 g CP/kg DM to 0.617 at 329 g CP/kg DM. 7. The results confirmed that AD values should only be compared at a defined protein level. TD was essentially independent of dietary CP level, except for a small reduction above 250 g CP/kg, whereas BV and NPU were stable only within the range 80-140 g CP/kg DM. DE was negatively related to dietary CP and fibre level.     

Protein quality and iron availability of intermediate moisture beef stored at 38 degrees C

The longissimus dorsi of a bull and steer were cut into cubes 1.5 cm3 and processed to a water activity (aw) 0.85 by canning in a solution of 9.5 p. 100 sodium chloride, 0.5 p. 100 potassium sorbate and a pre-determined amount of glycerol and water for sixteen hours with continuous tumbling on an end over shaker. After partial drying the intermediate moisture (i.m.) meat pieces were stored at 38 degrees C for periods up to 24 weeks and then freeze-dried before milling and incorporation into test diets fed to rats. Protein quality of fresh cooked beef and i.m. meat stored at 38 degrees C was measured in terms of net protein utilisation (NPU). There was no significant difference in NPU between cooked beef and freshly processed i.m. beef. There were no changes in NPU of i.m. meat from bull up to 9 weeks of storage. After 3 weeks of storage of the meat from the steer however, the NPU fell to 53.0, a level characteristic of cereal protein. This fall in NPU was associated with a decrease in the levels of all essential amino acids (in the protein hydrolysate). Valine and threonine being first and second limiting amino acids. Further storage of i.m. beef after 3 weeks produced a slower rate of decrease in NPU, the value at 24 weeks being 32.1 (61 p. 100 fall). Available lysine decreased by only 15 p. 100 after twenty-four weeks, this measurement under-estimating the fall in protein quality. The decrease in solubility of the meat in SDS/beta-mercaptoethanol on storage was of similar magnitude to that of NPU. Iron availability of i.m. meat, measured by haemoglobin regeneration in rats, showed improved iron availability compared to freshly cooked beef, even though marked changes had occurred in the meat heamatin complexes.     

The effect of methods of sterilization on the nutritive value of protein in a commercial rat diet.

1. The effect on protein quality of treating a commercial rat diet by autoclaving at various temperatures for different periods of time, or by irradiation with 2-5 or 10 Mrd, was studied. True digestibility (TD) and biological value (BV) were measured and the available and total amino acids in the diets were estimated using microbiological and chemical methods. 2. Autoclaving at 121 degrees for 60 min reduced BV, TD and net protein utilization (NPU) more than autoclaving at 134 degrees for 3 min. Availability of amino acids was reduced by both treatments but to a greater extent by autoclaving at 121 degrees for 60 min. Total amino acids were essentially unaffected. Irradiation had no effect on BV, TD, NPU or total amino acids, and the availability of amino acids was also unaffected, with the exception of lysine which was slightly reduced. 3. When the diet was autoclaved at 115 or 121 degrees for 15, 30 or 60 min, or at 134 degrees for 3 min the availability of the amino acids was reduced with increasing time and temperature of treatment. Treatment at 134 degrees for 3 min had an effect on available amino acids similar to treatment at 121 degrees for 15 or 30 min. 4. Ethylene oxide fumigation of the diet caused reduced availability of histidine, methionine and tryptophan but had negligble effect on arginine, leucine and lysine. 5. It is concluded that from a practical point of view irradiation causes least damage to proteins in rodent diets. If such diets are to be autoclaved they should be supplemented with complete protein to counteract amino acid destruction.     

山羊奶粉蛋白质营养价值的评价

本文以成分分析、大鼠实验评价国产山羊奶粉蛋白质的营养价值,并与牛奶粉蛋白质和大豆蛋白进行比较。山羊奶粉蛋白质的主要限制氨基酸是含硫氨基酸,氨基酸分(AS)和化学分(CS)分别为0.91和0.52,其氨基酸模式与牛奶粉蛋白质相近,稍优于酪蛋白,远优于大豆蛋白;蛋白质功效比值(PER)、净蛋白质比值(NPR)、表观消化率(AD)、真消化率(TD)、生物价(BV)和蛋白质净利用率(NPU)分别为3.01、4.62、90.49％、96.75％、86.02和83.22％,除AD稍高外,其他指标均与牛奶粉蛋白质无差别,除PER外各项指标均高于酪蛋白,所有指标均远高于大豆蛋白质。实验结果证实山羊奶粉蛋白质具有和牛奶粉蛋白质一样优良的营养价值。