A systematic review of the impacts of post-harvest handling on provitamin A, iron and zinc retention in seven biofortified crops

Approximately one in two women and children across the world continue to be affected by micronutrient deficiencies. Many populations, particularly those in low- to middle-income countries, are at risk, with women of reproductive age and children under five most affected. Diets often rely on staple crops that are energy dense but low in micronutrients, compounded by limited dietary diversity and affordability of nutrient-dense foods. Biofortification focuses on staple crops via conventional selective plant breeding, agronomic management and/or genetic engineering to increase concentrations of provitamin A, iron and zinc, with potential to improve intake and reduce deficiencies. Although biofortified crops have higher baseline micronutrient levels than non-biofortified counterparts, post-harvest handling (PHH), storage, processing, shelf life and cooking methods can influence micronutrient retention. Storage and cooking can cause vitamin losses via oxidation and heat, while milling can reduce minerals by removing husk and germ. Prior discussions exist on micronutrient retention in biofortified crops, but a systematic review had not been conducted before this study.

Protocol registered on PROSPERO (CRD42021254461) on 11 June 2021. The review targeted conventionally bred biofortified staple crops and associated food products subjected to post-harvest handling (PHH), including storage of fresh crops, processing (e.g., drying, milling, grinding, cooking, freezing) and post-processing storage/shelf life. Excluded were interventions using agronomic or genetic engineering biofortification methods and animal-based biofortified foods. Studies were eligible if they measured micronutrient content before and after processing; modelling or predictive studies were excluded. Primary outcome: micronutrient retention (apparent and true). Apparent retention was calculated when only concentration per unit weight before and after processing was reported; where possible, absolute concentrations were back-calculated. Reported losses (%) were converted to their reciprocal to harmonize as retention values. Retention interpretation used: >70% high; 50–70% moderate; <50% suboptimal. Search strategy: comprehensive searches in MEDLINE (PubMed), AGRICOLA, AgEcon, and CAB Abstracts, plus organizational websites (e.g., HarvestPlus, CGIAR and partners). Additional citations were identified via citation chasing, subject matter experts, and journal alerts. Citation management used EndNote X9. Screening and extraction: Records were screened at title/abstract and full-text levels using Covidence. Data extraction employed Microsoft Excel, FileMaker Pro, and PlotDigitizer for data captured from figures. Study flow (PRISMA): Identified 5,161 records (databases 3,767; organization websites 228; citation searching 1,151; subject matter experts 15). After removing 2,433 duplicates, 2,728 records were screened; 944 excluded. Sought 1,784 full texts; 4 not retrieved; 1,780 assessed for eligibility. Excluded 1,472 (agronomic biofortification 613; genetically engineered 176; theoretical 545; not micronutrient-biofortified 89; animal products 40; supplement comparator 5). Overall 308 reports eligible for at least one of four related topics; 67 reports included in this review on PHH-related micronutrient retention. Data synthesis: Summarized apparent and true retention across crops and processes; organized by processing method and storage conditions. Detailed tables (including individual carotenoids and total carotenoids) are provided in Supplementary materials and an online dashboard.

- Overall: 67 studies on micronutrient retention in conventionally bred biofortified maize, orange sweet potato (OSP), cassava, pearl millet, rice, beans and wheat. Provitamin A (PVA) crops generally maintained high PVA relative to non-biofortified counterparts. Iron and zinc retention varied by processing method; whole-grain consumption (e.g., whole wheat flour; minimally milled brown rice) supports maximal mineral intake. - Maize (19 PVA studies; 1 zinc study): - Storage: Shelled kernels/on-ears stored for 6 months retained ~40% beta-carotene equivalents (BCE), with most loss in the first 15 days; preconditioning at 4 °C before −20 °C improved retention to >100% in one study. Vacuum sealing may aid short-term storage of minimally processed ears. - Processing: Unfermented cooking and grinding generally did not reduce PVA/BCE; variety had strong effects. Retention >100% was observed depending on method, likely due to isomerization and matrix breakdown. Boiling or drying yielded high (>100%) zinc retention in zinc-biofortified maize. - Storage of processed/milled maize: Aluminum packaging and use of oxygen scavengers minimized carotenoid degradation; BCE in cooked products from kernels stored 90 days remained high. - Orange sweet potato (28 studies): - Metrics reported as beta-carotene (BC) and all-trans beta-carotene (ATBC). Fresh storage for 15 days reduced BC by ~10% or more, variety-dependent. Drying retained at least 60% of ATBC/BC across methods, with up to 99% ATBC retention after solar drying (Ejumula variety). Packaging that limits oxygen and moisture ingress and deep freezing at −80 °C favored higher carotenoid retention for cooked OSP and OSP flour. - Cassava (10 PVA studies; 2 reported BCE): - Focus on commonly consumed end-products (e.g., boiled cassava, porridges). Data indicate variability by process and variety; detailed intermediate processing steps and retention provided in Supplementary materials and dashboard. - Pearl millet (iron and zinc): - Potential iron contamination from cookware/ingredients must be considered. Iron retention was high (≈88% to ≥100%) after parboiling, oven drying, milling, steeping/fermenting; ≤1 month storage did not reduce iron retention. For zinc, parboiling and oven drying were advantageous; soaking at grain:water 1:5 for 12 h maximized retention. Malting/germination decreased zinc retention for whole/decorticated grains, while germination of raw flour maintained high retention. Zinc retention after processing was near 100%, and ≤1 month storage maintained it. - Beans (3 studies; iron and zinc): - Iron was well retained across boiling, refrying, and flour processing, often approaching or exceeding 100%; extrusion may outperform malting/roasting for iron retention. Zinc was also highly retained; malting/roasting may be slightly preferable to extrusion, though both maintained high zinc. - Rice (2 studies; iron and zinc): - Variety and growing location affected retention. Polishing at 5–10% degree of milling consistently reduced iron by about 50%; consuming brown (dehulled, unpolished) rice maximizes iron. For zinc, polishing reduced content by ~20–40%; brown rice maximizes zinc as well. - Wheat (zinc): - Milling at 95% extraction retained more zinc than 80% extraction in biofortified wheat. - Comparisons: - Versus non-biofortified crops: Even with processing/storage losses, biofortified crops generally retained higher micronutrient contents than conventional counterparts (e.g., biofortified maize porridges retained high PVA; pearl millet processed with recommended methods retained ~53 µg/g iron and ~40 µg/g zinc vs ~20 and ~19 µg/g in conventional millet; beans and rice similarly showed advantages under specific processing conditions; brown rice favored for higher minerals; higher extraction wheat flour retained more zinc). - Versus fortified foods: Fortified flours show wide vitamin A retention (≈30–95%) depending on storage/packaging; fortified rice generally had 75–100% retention for several micronutrients (vitamin A lower with some methods). In one comparison, PVA-biofortified maize porridge had 78–99% beta-carotene retention, yielding ~936–1,200 µg RAE/100 g, versus ~104 µg RAE/100 g from vitamin A–fortified white maize flour porridge (≈40% retention), noting genotype and processing strongly influence outcomes. - Practical guidance emerging from evidence: For households, practices that maximized retention included boiling/roasting maize with a lid or in husk, drying OSP unpeeled, boiling whole cassava, parboiling or oven drying pearl millet, boiling beans (with or without refrying), and dehulling rice without polishing. Packaging that limits oxygen and moisture and cool storage/freezing improved carotenoid stability in PVA crops. - Tools: An interactive Micronutrient Retention Dashboard compiles minimum/maximum retention by processing method across crops.

The review addresses how post-harvest handling (storage, processing, and shelf life) affects micronutrient retention in conventionally bred biofortified staples. Findings show PVA-rich crops generally maintain high carotenoid levels through common processing, whereas iron and zinc retention is more sensitive to processing choices, reinforcing the importance of whole-grain consumption and minimal polishing for cereals. Variety and growing environment substantially influence retention within the same processing method, highlighting the need to tailor recommendations by genotype and context. Comparisons with non-biofortified crops demonstrate that biofortified crops typically deliver superior micronutrient content even after processing, supporting their role in combating deficiencies. Contrasts with fortified foods underscore different stability challenges and that fortification and biofortification are complementary strategies with distinct considerations for storage and processing stability. The results inform practical recommendations for households and stakeholders (e.g., optimal cooking methods, packaging, and storage), and identify research gaps in mineral-biofortified crops, storage/shelf-life parameters in PVA crops, and consistent evaluation of bioaccessibility/bioavailability.

PVA-biofortified crops generally maintain high provitamin A content relative to non-biofortified counterparts. For iron and zinc, retention is more variable and depends on processing; consuming whole-grain products (e.g., whole wheat flour, minimally milled brown rice) supports maximal mineral intake. There are gaps in evidence on storage and shelf life for PVA crops (optimal temperature, humidity, and duration), and future studies should routinely include bioaccessibility and bioavailability assessments alongside retention. Clear preparation guidance for processors and consumers can help maximize micronutrient retention and the nutritional impact of biofortified foods.

Potential contamination (especially iron and zinc) from cookware, water, or other ingredients was not consistently reported and could inflate mineral retention estimates. Laboratory and methodological differences across studies introduce variability in measured concentrations. The review primarily reports retention without uniformly measuring bioaccessibility or bioavailability, which limits direct inference about nutritional impact; future work should integrate these measures.