Experimental Biology and Medicine 230:444-454 (2005)
© 2005 Society for Experimental Biology and Medicine
MINIREVIEW
Trienzyme Extraction in Combination with Microbiologic Assay in Food Folate Analysis: An Updated Review
Taisun H. Hyun* and
Tsunenobu Tamura
,1
* Department of Food and Nutrition, Chungbuk National University, Gaeshin-dong, Cheongju, 361-763, Korea; and Department of Nutrition Sciences, University of Alabama at Birmingham, Birmingham, Alabama 35294
1
To whom requests for reprints should be addressed at
Department of Nutrition Sciences, 455 Webb, 1675 University Boulevard, University of Alabama at Birmingham, Birmingham, AL 35294–3360. E-mail: tamurat{at}uab.edu
 |
Abstract
|
|---|
For decades, the traditional food folate extraction method involved two steps including heat treatment, to release folate from its binding proteins, and folate conjugase treatment, to hydrolyze polyglutamyl folate to monoglutamyl folate. However, a trienzyme-extraction method of food folate was developed in the mid 1990s. This method involves the use of 
-amylase, protease, and folate conjugase and allows for a more complete extraction of folate trapped in carbohydrate or protein matrices in food than the traditional method. In the last several years, this extraction method became widely used. However, the method is not uniform among various investigators, and it may be difficult for a new investigator to select the most suitable method in his or her laboratory. Therefore, in the review presented here, we summarize a variety of trienzyme-extraction procedures that were used by various researchers and offer a recommended procedure for food folate extraction. It is our hope that the wide use of an appropriate procedure of the trienzyme-extraction method, in combination with a reasonable detection method, help in establishing accurate and reliable food-folate tables and that this, in turn, makes it possible to accurately assess folate intake in the general population.
Key Words: food folate • trienzyme extraction • folate conjugase • -amylase • protease
|
Introduction
|
|---|
Adequate folate nutritional status is essential for maintaining health and for the prevention of certain diseases (1, 2). In various parts of the world, it is often difficult to ascertain adequate folate intake through regular diet alone, although the folic acid (i.e., pteroylglutamic acid) fortification of staple foods is practiced in several countries (3–6). The method of food folate measurement has improved over the years; however, food folate tables are notoriously unreliable for estimating accurate dietary intake (7). To improve the method of food folate analysis, the trienzyme-extraction method (i.e., the use of preparations of -amylase, protease, and folylpoly 
-glutamate carboxypeptidase II [folate conjugase]) was developed about 15 years ago by researchers in Georgia, United States (8, 9). When a marked increase in folate content in a food composite was initially reported in 1997, and the increase was far beyond the imagination at that time (10), certain resentment was seen among researchers in the folate field. In the last several years, however, many investigators have begun using the method to measure food folate content. For example, as the findings of a recent international comparison of food folate analysis indicate, this extraction method is widely used (11). Furthermore, folate values in the USDA National Nutrient Database have been updated using microbiologic assay after trienzyme extraction (12).
We now realize that, compared with folate-conjugase treatment alone, food folate values do not always increase after the trienzyme extraction (13–15). The increase in folate values depends on the food item, which is possibly due to the difference in food matrices as well as forms of folate that can be labile against longer incubation periods than a single-enzyme treatment. In 1998, Tamura (7) published a review on the trienzyme-extraction method. However, this method has been constantly modified and improved to obtain the "highest values of folate" in foods. Therefore, we present this article to review the current use of trienzyme extraction of food folate and to propose a simplified, possibly better, method for food folate extraction.
|
Use of the Trienzyme-Extraction Method
|
|---|
In 1990, Martin et al. (8) first reported an increase of food folate values by the trienzyme-extraction method and established a procedure of the simultaneous incubation of a heated food homogenate with folate conjugase and -amylase followed by the incubation with protease. Using this method, De Souza and Eitenmiller (16) published folate contents in various food items. Tamura et al. (7, 10) proposed a slight modification of the original method by Martin et al. (8): the step of hydrolysis of polyglutamyl folates by folate conjugase should be done after all folates are released from the food matrix by -amylase and protease treatments. Subsequently, however, Pfeiffer et al. (17) and Rader et al. (18, 19) used the method established by Martin et al. (8). In the past several years, investigators have used a variety of procedures for the trienzyme extraction of food folate, as summarized in Table 1
(8, 10, 13–45). We reviewed the literature involving the trienzyme-extraction method for food folate analysis (published before April 2005) as exhaustively as possible, and we will examine each procedure of folate extraction from foods and the methods of folate detection in the following sections.
|
Food Homogenization
|
|---|
The procedures of food folate assay generally begin with homogenization of a single food or food mixtures in a buffer with an appropriate pH containing ascorbic acid and/ or 2-mercaptoethanol. The pHs of these buffers ranged widely (Table 1
). It is uncertain whether the pH of the extraction buffer is important because -amylase and protease treatments, if perfect, should result in complete folate extraction.
A buffer (i.e., pH, 7.85; 2-[N-cyclohexylamino]-ethanesulfonic acid (CHES), 50 mM; and HEPES, 50 mM containing 2% ascorbic acid and 0.2 M 2-mercaptoethanol) that Wilson and Horne (46) developed has generally been considered best for folate extraction from biologic samples, including foods. However, when food composites were homogenized at a pH of 4.1 and treated with -amylase and protease at the same pH, Tamura et al. (10) obtained higher folate values than those after the procedures were done at higher pHs, including a pH of 7.85. Rader et al. (18) showed that there were no significant differences among folate values for four enriched-flour products obtained using various pHs (i.e., 4.3–7.8) for food homogenization and enzyme treatments. Aiso and Tamura (20) reported that the appropriate pH for -amylase and protease treatments varies depending on the food item. Recently, Pandrangi and LaBorde (38) reported a similar experiment using various pHs and incubation periods for -amylase and protease. To our knowledge, there have been no further studies to identify the most suitable pH for food homogenization and subsequent enzyme treatments. Because the trienzyme-extraction method should, theoretically, provide nearly complete folate extraction, we conclude that the pH of extraction buffers may no longer be an important issue, when microbiologic assay is used. However, for chromatographic analysis, pH is an important issue because the interconversion of folate derivatives can occur at different pHs.
|
Heat Treatment
|
|---|
For decades, a heat-treatment step after food homogenization has been used to release folates from folate-binding protein(s). However, one may wonder whether this extra step is necessary for food folate extraction because protease treatment, if perfect, should result in the complete destruction of proteins and the release of folates from the binding protein. In addition, by skipping this step, an extra chance of folate degradation may be eliminated.
We postulated that heat treatment is unnecessary and tested this hypothesis using 11 foods (Table 2). We found that the mean folate content (Lactobacillus rhamnosus [formerly known as Lactobacillus casei, ATCC 7469] activity) was about 14% higher when homogenates were not heated before enzyme treatments (47). Therefore, we conclude that heat treatment is unnecessary for releasing folates from the binding protein as long as protease treatment is performed.
|
Order of Enzyme Treatments
|
|---|
As shown in Table 1, the order of the treatments using the three enzymes varied among investigators. Originally, Martin et al. (8) simultaneously treated food homogenates with folate conjugase and -amylase, and then protease was used. However, Tamura et al. (10) proposed that folate-conjugase treatment should be carried out after all folates are released from the food matrix. Furthermore, Rader et al. (18) reported that when protease was added first, there were significant increases in total folates in certain foods compared with the procedure of -amylase treatment followed by protease treatment. Ndaw et al. (27) performed food folate extraction by simultaneous treatment with protease and -amylase before folate-conjugase treatment, and they concluded that the trienzyme extraction was not effective to obtain higher food folate values than folate-conjugase treatment alone. However, it is unclear whether the simultaneous use of protease and -amylase is valid because protease may destroy -amylase, as concluded by Martin et al. (8). Konings et al. (25) also showed no significant increases in folate values in certain foods after the trienzyme extraction compared with folate-conjugase treatment alone. According to the description of trienzyme treatment in a prior report by Konings (22), it appears that food homogenates were incubated with protease, -amylase, and folate conjugase all together, although clarification is needed for this issue. After the trienzyme extraction, the homogenates showed an increase in folate concentrations only in a few foods, compared with folate conjugase treatment alone (22, 25). Furthermore, when the values were compared with those obtained by separate treatments (13–15), the extent of the increases in folate values in these foods by the simultaneous treatment with three enzymes was relatively small (22, 25).
It remains to be seen whether protease should not be used with -amylase and/or folate conjugase because protease may inactivate the other two enzymes if incubated together. Further studies are needed to clarify whether folate conjugase should be applied after treatments that release all folates from the food matrices with protease and -amylase. Nevertheless, to obtain the "the highest value" of food folate, the order of enzyme treatments should always be verified when the investigators intend to modify the previously established method.
|
Enzyme Sources
|
|---|
-Amylase and Protease Preparations.
For tri-enzyme extraction, Martin et al. (8) originally recommended the use of an -amylase preparation from Aspergillus oryzae and a protease preparation from Streptomyces griseus (Type XIV), and investigators followed this recommendation. The -amylase preparation obtained from Sigma Chemical Co. (St. Louis, MO) contains a large amount of endogenous folate; thus, it is necessary to eliminate it by charcoal treatment and filtration for the subsequent folate quantification (7). We used other preparations such as those purified from Bacillus licheniformis and other Bacillus species. However, these did not provide us with satisfactory enzyme digestion, although contents of endogenous folate in these preparations were much lower than those prepared from A. oryzae. Therefore, when we try to digest complex carbohydrates present in various foods, it may be necessary to use a crude -amylase preparation such as the one originally recommended by Martin et al. (8).
Folate Conjugase.
Naturally occurring folates exist in polyglutamyl forms (48). Prior to folate quantification by microbiologic assay or high-pressure liquid chromatography (HPLC) analysis (for the distribution of monoglutamyl folates), polyglutamyl folates must be hydrolyzed to monoglutamyl forms. Folate conjugases from several sources, including the serum of human or rat, chicken pancreas, and hog kidney, have been used for this purpose (49, 50). Since the characterization of rat-serum folate conjugase by Horne et al. (51), rat serum has been widely used in recent years because it is readily available, it is stable during long-term storage, and it is relatively easy to remove endogenous folate by charcoal treatment and filtration (7). Rat serum conjugase has its pH optimum between 6.2 and 7.4 (51). Chicken-pancreas folate conjugase, with a pH optimum between 7.0 and 8.5, has also been used because it is less susceptible to the folate-conjugase inhibitors that could be present in foods (7) and is stable for long-term storage. In addition, chicken-pancreas powder has been commercially available. However, because the final product of this folate conjugase reaction is diglutamyl folate, it is not suitable for the assessment of the distribution of monoglutamyl folates using the chromatographic analysis, and it is only suitable for obtaining total food folate values using the L. rhamnosus microbiologic assay. Hog-kidney folate conjugase has its pH optimum of about 4.7 and yields monoglutamates (49), which is ideal for both the microbiologic assay using either L. rhamnosus or Enterococcus hirae (formerly known as Streptococcus faecalis, ATCC 8047) and the HPLC analysis for monoglutamyl folates. The preparation of the enzyme must be initiated from the fresh hog kidney, although purification procedures do not involve laborious steps and the preparation is stable for months at –70°C.
Lim et al. (21) reported that these sources of folate conjugase had no effect on the folate content in human milk samples. In contrast, Ndaw et al. (27) demonstrated that hog-kidney folate conjugase was less effective in the hydrolysis of polyglutamyl folates than the other two folate conjugases. Iwatani et al. (33) compared the folate content of a few vegetables after hydrolysis with chicken pancreas and human plasma and showed that folate conjugase from chicken pancreas was more effective in hydrolyzing polyglutamyl folates than human plasma. However, it is unclear whether the actual comparisons of the enzyme activities were made in these studies to make such a distinction (27, 33). Engelhardt and Gregory (52) reported that hog-kidney folate conjugase was inhibited by extracts from a variety of foods. This inhibition, however, was reduced by increasing enzyme concentration or incubation time. Thus, complete hydrolysis may be achieved when the combination of the quantity of the enzyme and the incubation time is appropriate, regardless of the source of folate conjugases.
|
Duration of Enzyme Treatments
|
|---|
Martin et al. (8) recommended the procedure that included the 4-hr incubation of a food homogenate with folate conjugase and -amylase simultaneously, followed by an overnight incubation with protease (Table 1). However, Pfeiffer et al. (17) showed that such a long-term incubation with protease caused a 20% decrease in folate content in cereals compared with a short-term incubation, suggesting that labile folates may be destroyed after a long-term incubation. Therefore, it is advisable to have a shorter incubation time using a larger quantity of enzymes.
Using a total of 65 foods, we compared values between the two incubation protocols: separate incubation (i.e., incubation with -amylase and folate conjugase separately for 2 hrs after protease treatment) and combined incubation (i.e., incubation with -amylase and folate conjugase together for 2 hrs after protease treatment). Folate values by combined incubation were comparable to those by separate incubation (47). These findings indicate that the final enzyme treatments (i.e., -amylase and folate conjugase) can be combined to shorten the entire incubation.
|
Centrifugation of Homogenates
|
|---|
Tamura (7) recommended centrifugation after treatments with -amylase and protease, followed by the incubation of the supernatant with folate conjugase before folate assay. However, the centrifugation step has not been clearly described in many articles. Shrestha et al. (24) and Iwatani et al. (33) followed the procedure that food homogenates were centrifuged before the treatments with enzymes, but they did not show a significant increase in folate values after the trienzyme extraction compared with those after folate-conjugase treatment alone.
We evaluated the effect of centrifugation before enzyme treatments. When homogenates of 110 foods were centrifuged and supernatants were subsequently treated with protease, -amylase, and folate conjugase, the changes in folate content compared with folate-conjugase treatment alone ranged from –100%–230%, with an average of only 8% (53). In contrast, when the centrifugation step before the enzyme treatments was eliminated for these food homogenates, the changes were 0%–900%, with a mean of 79% (15). Our data indicated that the initial centrifugation of food homogenate was not only unnecessary, but also provided much lower values because this initial step eliminates residues containing folates that could have been released from food matrices by the enzyme treatments. Thus, centrifugation before trienzyme treatment is unacceptable.
|
Sample Storage
|
|---|
Food homogenates, or those after the treatment with the three enzymes, may be stored at –70°C. Folates in these samples are stable for a few months, as long as ascorbate and/or 2-mercaptoethanol were added to the buffer for homogenization or enzyme treatment. The samples should be separated into a few aliquots before storage for later use to avoid repeated freeze-thaw procedures.
|
Folate Determination
|
|---|
As previously reviewed by Tamura (50), microbiologic assay is the most widely used procedure for the determination of total food folate content. Among the 35 studies shown in Table 1, 28 were performed using the microbiologic assay method using L. rhamnosus and 10 were done by the HPLC method. Because it responds to most metabolically active forms of folate, L. rhamnosus has been the preferred test microorganism (50).
Although microbiologic assay provides only a total folate value, the HPLC analysis makes it possible to obtain data on various forms of folates and possibly allows better prediction of their stability. A recent international, inter-laboratory comparison study indicated that 20 of 26 laboratories used microbiologic assay, 4 used an HPLC-spectrometry, 1 used liquid chromatography/mass spectroscopy (LC-MS), and 1 used a radiobinding assay for food folate analysis (11). Several HPLC methods to separate and detect the individual forms of folate in foods have been developed (9, 54). Pfeiffer et al. (17) first used both the HPLC method and the microbiologic assay independently after the trienzyme extraction for food folate analysis. They showed that total folate contents determined by the HPLC method for several foods were comparable to those obtained by microbiologic assay. Several groups of investigators, however, reported that folate values obtained using HPLC were lower than those obtained by microbiologic assay, although the mechanism of the discrepancy is unknown. For example, Konings (22) demonstrated that total folates in several foods analyzed by HPLC were 20%–35% lower than those obtained by the microbiologic-assay method. Ruggeri et al. (23) also reported that the results obtained by the HPLC method were 24%–52% lower in four food items and a mixed diet than those obtained using microbiologic assay. Doherty and Beecher (32) showed that the values determined by the HPLC procedure were slightly lower than those obtained by microbiologic assay.
In intercomparison studies, Finglas et al. (55) concluded that microbiologic assay is more appropriate than the HPLC method because it is difficult to measure individual folates in certain foods, especially when folate content was low in food samples. The introduction of mass spectrometry in combination with HPLC may enhance the specificity of folate detection (34, 39). Although the procedures of food folate extraction require further scrutiny, the method may be most promising for its specificity.
|
Comparison of Folate Values Among Laboratories
|
|---|
To improve the accuracy of food folate data, researchers tested the agreement of folate values among laboratories (11, 55, 56). Using an HPLC technique after folate-conjugase treatment alone, Vahteristo et al. (56) examined the agreement of various forms of folate in a few food items among three laboratories and found that there was little agreement, with the exception of 5-methyltetrahydrofolate. Five intercomparison studies from 1990–1997 showed that the most consistent values obtained using the HPLC method were for 5-methyltetrahydrofolate, and it was suggested that more work is needed for an HPLC analysis of folate forms other than 5-methyltetrahydrofolate (55).
Puwastien et al. (11) reported that coefficients of variation of three food items among seven laboratories using trienzyme extraction with microbiologic assay (L. rhamnosus activity) ranged from 24%–34%, whereas average coefficients of variation of triplicate assays within the laboratory was 3%–6%. The wide variation between laboratories was mainly due to the different methods that were used for folate extraction and detection. Therefore, it is important to standardize the methods of folate extraction and detection, and the use of reliable reference materials should be encouraged to monitor the interassay or intraassay variations to ensure the quality of folate analysis.
|
Recommended Procedures
|
|---|
A recommended experimental procedure for food folate determination is shown in Figure 1. An appropriate amount of food, depending on folate content, is homogenized in a buffer with 57 mM ascorbic acid and 0.2 M mercaptoethanol. The homogenates can be divided into several aliquots and frozen at –70°C until further processing. The initial step after homogenization had been heating the homogenates to release folate from the binding protein; however, as previously discussed, this step can be eliminated once a protease preparation is used for folate extraction.
Protease and -amylase are dissolved in distilled, deionized water or a buffer at concentrations of 40 mg/ml, which is equivalent to 1572 U–2240 U of -amylase/ml and 208 U–220 U of protease/ml, depending on the batch of enzyme preparations. Based on our experience, the minimum of 3 U of -amylase and 0.4 U of protease against 1 mg of the original food are generally sufficient for satisfactory folate extraction. The preparation of -amylase contains a large amount of endogenous folate. However, it is possible to increase the quantity of -amylase without creating a high folate blank for folate assay because endogenous folate in -amylase can be removed by charcoal treatment and filtration through a 0.22-µm microfilter. Although only a minute amount of endogenous folate has been detected in the preparation of protease, it is always better to check the amount of endogenous folate for each batch of the preparation, especially when a large quantity of enzyme preparation is used. As a source of folate conjugase, rat serum appears to be most suitable due to its availability and easy preparation. Folate conjugase from chicken pancreas can be used for microbiologic assay, but it is not suitable for the HPLC method, as previously noted.
The incubation of food homogenates with each enzyme for 2–3 hrs instead of more than 12 hrs (overnight) is considered appropriate. We recommend the incubation of food homogenates with folate conjugase after the treatments with -amylase and protease, which allows the release of polyglutamyl folates from food matrices. We also recommend the treatment of homogenates with -amylase and folate conjugase simultaneously after the protease treatment. After trienzyme treatment, if necessary, the samples should be stored at –70°C in a few aliquots before folate determination.
It should be noted that the conditions of trienzyme treatments differ among foods (20, 38). Thus, it becomes potentially time consuming and labor intensive to measure folate content in each individual food. Our current recommendation is a more efficient and practical method of food folate analysis than previously published (7). We hope that the method recommended here enhances the wide use of the trienzyme-extraction method and the reliability and accuracy of food folate tables.
|
References
|
|---|
- Refsum H, Ueland PM, Nygard O, Vollset SE. Homocysteine and cardiovascular disease. Annu Rev Med 49:31–62, 1998.[Medline]
- van der Put NMJ, van Straaten HWM, Trijbels FJM, Blom HJ. Folate, homocysteine and neural tube defects: an overview. Exp Biol Med 226:243–270, 2001.[Abstract/Free Full Text]
- Food and Drug Administration. Food standards: amendment of standards of identity for enriched grain products to require addition of folic acid. Fed Regist pp. 8781–8797, 1996.
- Ray JG, Vermeulen MJ, Boss SC, Cole DE. Declining rate of folate insufficiency among adults following increased folic acid food fortification in Canada. Can J Public Health 93:249–253, 2002.[Medline]
- Hertrampf E, Cortés F. Folic acid fortification of wheat flour: Chile. Nutr Rev 62:S44–S48, 2004.[Medline]
- Chen LT, Rivera MA. The Costa Rican experience: reduction of neural tube defects following food fortification programs. Nutr Rev 62:S40–S43, 2004.[Medline]
- Tamura T. Determination of food folate. J Nutr Biochem 9:285–293, 1998.
- Martin JI, Landen WO, Soliman A-GM, Eitenmiller RR. Application of a tri-enzyme extraction for total folate determination in foods. J Assoc Off Anal Chem 73:805–808, 1990.[Medline]
- Eitenmiller RR, Landen WO Jr. Vitamin Analysis for the Health and Food Sciences. Boca Raton: CRC Press, pp411–466, 1999.
- Tamura T, Mizuno Y, Johnston KE, Jacob RA. Food folate assay with protease, -amylase and folate conjugase treatments. J Agric Food Chem 45:135–139, 1997.
- Puwastien P, Pinprapai N, Judprasong K, Tamura T. International inter-laboratory analyses of food folate. J Food Comp Anal 18:387–397, 2005.
- United States Department of Agriculture. USDA National Nutrient Database for Standard Reference, Release 17: Composition of foods, raw, processed, prepared. Available at: www.nal.usda.gov/fnic/food-comp. Accessed April 25, 2005.
- Johnston KE, DiRienzo DB, Tamura T. Folate content of dairy products measured by microbiological assay with trienzyme treatment. J Food Sci 67:817–820, 2002.
- Johnston KE, Lofgren PA, Tamura T. Folate concentrations of fast foods measured by trienzyme extraction method. Food Res Int 35:565–569, 2002.
- Yon M, Hyun TH. Folate content of foods commonly consumed in Korea measured after trienzyme extraction. Nutr Res 23:735–746, 2003.
- De Souza S, Eitenmiller R. Effects of different enzyme treatments on extraction of total folate from various foods prior to microbiological assay and radioassay. J Micronutr Anal 7:37–57, 1990.
- Pfeiffer CM, Rogers LM, Gregory JF III. Determination of folate in cereal-grain food products using trienzyme extraction and combined affinity and reversed-phase liquid chromatography. J Agric Food Chem 45:407–413, 1997.
- Rader JI, Weaver CM, Angyal G. Use of a microbiological assay with tri-enzyme extraction for measurement of pre-fortification levels of folates in enriched cereal-grain products. Food Chem 62:451–465, 1998.
- Rader JI, Weaver CM, Angyal G. Total folate in enriched cereal-grain products in the United States following fortification. Food Chem 70:275–289, 2000.
- Aiso K, Tamura T. Trienzyme treatment for food folate analysis: optimal pH and incubation time for -amylase and protease treatments. J Nutr Sci Vitaminol 44:361–370, 1998.
- Lim H-S, Mackey AD, Tamura T, Wong SC, Picciano MF. Measurable human milk folate is increased by treatment with -amylase and protease in addition to folate conjugase. Food Chem 63:401–407, 1998.
- Konings EJM. A validated liquid chromatographic method for determining folates in vegetables, milk powder, liver, and flour. J AOAC Int 82:119–127, 1999.[Medline]
- Ruggeri S, Vahteristo LT, Aguzzi A, Finglas P, Carnovale E. Determination of folate vitamers in food and in Italian reference diet by high-performance liquid chromatography. J Chromatogr A 855:237–245, 1999.[Medline]
- Shrestha AK, Arcot J, Paterson J. Folate assay of foods by traditional and tri-enzyme treatments using cryoprotected Lactobacillus casei. Food Chem 71:545–552, 2000.
- Konings EJM, Roomans HHS, Dorant E, Goldbohm RA, Saris WHM, van den Brandt PA. Folate intake of the Dutch population according to newly established liquid chromatography data for foods. Am J Clin Nutr 73:765–776, 2001.[Abstract/Free Full Text]
- Whittaker P, Tufaro PR, Rader JI. Iron and folate in fortified cereals. J Am Coll Nutr 20:247–254, 2001.[Abstract/Free Full Text]
- Ndaw S, Bergaentzlé M, Aoudé-Werner D, Lahély S, Hasselmann C. Determination of folates in foods by high-performance liquid chromatography with fluorescence detection after precolumn conversion to 5-methyltetrahydrofolates. J Chromatogr A 928:77–90, 2001.[Medline]
- Arcot J, Wong S, Shrestha AK. Comparison of folate losses in soybean during the preparation of tempeh and soymilk. J Sci Food Agric 82:1365–1368, 2002.
- Pentieva K, Kidd JA, McKillop DJ, Strain JJ, Scott JM, McNulty H. Folate analysis of composite meals. Proc Nutr Soc 61:92A, 2002.
- McKillop DJ, Pentieva K, Daly D, McPartlin JM, Hughes J, Strain JJ, Scott JM, McNulty H. The effect of different cooking methods on folate retention in various foods that are amongst the major contributors to folate intake in the UK diet. Br J Nutr 88:681–688, 2002.[Medline]
- Johansson M, Witthöft CM, Bruce A, Jägerstad M. Study of wheat breakfast rolls fortified with folic acid. The effect on folate status in women during a 3-month intervention. Eur J Nutr 41:279–286, 2002.[Medline]
- Doherty RF, Beecher GR. A method for the analysis of natural and synthetic folate in foods. J Agric Food Chem 51:354–361, 2003.[Medline]
- Iwatani Y, Arcot J, Shrestha AK. Determination of folate contents in some Australian vegetables. J Food Comp Anal 16:37–48, 2003.
- Freisleben A, Schieberle P, Rychlik M. Specific and sensitive quantification of folate vitamers in foods by stable isotope dilution assays using high-performance liquid chromatography-tandem mass spectrometry. Anal Bioanal Chem 376:149–156, 2003.[Medline]
- McKillop DJ, Pentieva KD, Scott JM, Strain JJ, McCreedy R, Alexander J, Patterson K, Hughes J, McNulty H. Protocol for the production of concentrated extracts of food folate for use in human bioavailability studies. J Agric Food Chem 51:4382–4388, 2003.[Medline]
- Han J-Y, Tyler RT. Determination of folate concentrations in pulses by microbiological method employing trienzyme extraction. J Agric Food Chem 51:5315–5318, 2003.[Medline]
- Pentieva K, McNulty H, Reichert R, Ward M, Strain JJ, McKillop DJ, McPartlin JM, Connolly E, Molloy A, Krämer K, Scott JM. The short-term bioavailabilities of [6S]-5-methyltetrahydrofolate and folic acid are equivalent in men. J Nutr 134:580–585, 2004.[Abstract/Free Full Text]
- Pandrangi S, LaBorde LF. Optimization of microbiological assay of folic acid and determination of folate content in spinach. Int J Food Sci Tech 39:525–532, 2004.
- Rychlik M. Revised folate content of foods determined by stable isotope dilution assays. J Food Comp Anal 17:475–483, 2004.
- Johnston KE, Tamura T. Folate content in commercial white and whole wheat sandwich breads. J Agric Food Chem 52:6338–6340, 2004.[Medline]
- Hannon-Fletcher MP, Armstrong NC, Scott JM, Pentieva K, Bradbury I, Ward M, Strain JJ, Dunn AA, Molloy AM, Kerr MA, McNulty H. Determining bioavailability of food folates in a controlled intervention study. Am J Clin Nur 80:911–918, 2004.
- Picciano MF, West SG, Ruch AL, Kris-Etherton PM, Zhao G, Johnston KE, Maddox DH, Fishell VK, Dirienzo DB, Tamura T. Effect of cow milk on food folate bioavailability in young women. Am J Clin Nutr 80:1565–1569, 2004.[Abstract/Free Full Text]
- Ginting E, Arcot J. High-performance liquid chromatographic determination of naturally occurring folates during tempe preparation. J Agric Food Chem 52:7752–7758, 2004.[Medline]
- DeVries JW, Rader JI, Keagy PM, Hudson CA. Microbiological assay-trienzyme procedure for total folates in cereals and cereal foods: collaborative study. J AOAC Int 88:5–15, 2005.[Medline]
- Han YH, Yon M, Hyun TH. Folate intake estimated with an updated database and its association to blood folate and homocysteine in Korean college students. Eur J Clin Nutr 59:246–254, 2005.[Medline]
- Wilson SD, Horne DW. High-performance liquid chromatographic determination of the distribution of naturally occurring folic acid derivatives in rat liver. Anal Biochem 142:529–535, 1984.[Medline]
- Hyun TH, Ji H. Modified trienzyme extraction of food: no heat treatment and simultaneous incubation of -amylase and conjugase after protease treatment. FASEB J 19:A50, 2005.
- Krumdieck CL, Tamura T, Eto I. Synthesis and analysis of the pteroylpolyglutamates. Vitam Horm 40:45–104, 1983.[Medline]
- Bird OD, McGlohon VM, Vaitkus JW. Naturally occurring folates in the blood and liver of the rat. Anal Biochem 12:18–35, 1965.[Medline]
- Tamura T. Microbiological assay of folates. In: Picciano MF, Stokstad ELR, Gregory JF III, Eds. Folic Acid Metabolism in Health and Disease. New York: Wiley-Liss, pp121–137, 1990.
- Horne DW, Krumdieck CL, Wagner C. Properties of folic acid -glutamyl hydrolase (conjugase) in rat bile and plasma. J Nutr 111:442–449, 1981.
- Engelhardt R, Gregory JF III. Adequacy of enzymatic deconjugation in quantification of folate in foods. J Agric Food Chem 38:154–158, 1990.
- Yon M. Folate analysis of foods commonly consumed in Korea. Master’s thesis, Chungbuk National University, Korea, 2001.
- Ball GFM. Bioavailability and Analysis of Vitamins in Foods. London: Chapman & Hall, pp439–496, 1998.
- Finglas PM, Wigertz K, Vahteristo L, Witthöft C, Southon S, de Froidmont-Görtz I. Standardisation of HPLC techniques for the determination of naturally-occurring folates in food. Food Chem 64:245–255, 1999.
- Vahteristo L, Finglas PM, Witthöft C, Wigertz K, Seale R, de Froidmont-Görtz I. Third EU MAT intercomparison study on food folate analysis using HPLC procedures. Food Chem 57:109–111, 1996.
This article has been cited by other articles:
|
|
|
|
 
R. S. Gibson, Y. Abebe, S. Stabler, R. H. Allen, J. E. Westcott, B. J. Stoecker, N. F. Krebs, and K. M. Hambidge
Zinc, Gravida, Infection, and Iron, but Not Vitamin B-12 or Folate Status, Predict Hemoglobin during Pregnancy in Southern Ethiopia
J. Nutr.,
March 1, 2008;
138(3):
581 - 586.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
R. Poo-Prieto, D. B. Haytowitz, J. M. Holden, G. Rogers, S. F. Choumenkovitch, P. F. Jacques, and J. Selhub
Use of the Affinity/HPLC Method for Quantitative Estimation of Folic Acid in Enriched Cereal-Grain Products
J. Nutr.,
December 1, 2006;
136(12):
3079 - 3083.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
T. Tamura and M. F. Picciano
Folate and human reproduction
Am. J. Clinical Nutrition,
May 1, 2006;
83(5):
993 - 1016.
[Abstract]
[Full Text]
[PDF]
|
|
|
Top
Abstract
Introduction
