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Response surface methodology was used to optimize the processing conditions of iron‐fortified water chestnuts by vacuum impregnation (VI) technology. The results showed that the optimum conditions were achieved at a vacuum level of 51 kPa, vacuum time of 20 min, and restoration time of 9 min. Under optimal conditions, the vacuum impregnated water chestnuts provided 2.2 and 14.1 times higher iron content compared with the atmospheric pressure samples and fresh water chestnuts, respectively. Iron‐fortified water chestnuts were dried by vacuum drying, freeze‐drying, and hot air drying (AD). Five thin‐layer drying models were fitted during the hot AD of water chestnuts. The results indicated that the hot AD rates from vacuum impregnate samples were higher than that of atmospheric pressure samples and fresh water chestnuts samples. The Page model was suitable for the characterization of drying characteristics of water chestnut. The results of the scanning electron microscope showed that water chestnuts still maintained good microstructures after drying. PRACTICAL APPLICATIONS: Iron is required for hemoglobin production in the cell precursors. Iron deficiency results in impaired erythropoiesis and iron‐deficient anemia. The iron deficiency is one of the major public nutrition problems that developing countries are facing. In this study, VI technology was used to replace conventional atmospheric pressure impregnation for iron enrichment of fruits and vegetables, which effectively improved the enhancement efficiency. This study can provide a reference for the development of functional food.

Macadamia nut shell (MNS) is a type of waste lignocellulose obtained from macadamia nut production processing. Large MNS wastes caused serious resource waste and environmental pollution. So, preparation of hydrochars from MNS via hydrothermal carbonization (HTC) is of great significance. HTC of MNS was conducted to study the effect of process parameters, including HTC temperature (180–260°C) and residence time (60–180 min) on the properties of hydrochars. Results showed that the increase in HTC temperature and residence time decreased the mass yield of hydrochars and increased the high heating value of hydrochars. Furthermore, the C content of hydrochars increased, whereas the H and O contents decreased. Mass yield of hydrochar is 46.59%, energy yield is 64.55% and the higher heating value is 26.02 MJ kg −1 at a temperature of 260°C and time of 120 min. The surface structure of hydrochars was rougher compared with that of MNS as observed via scanning electron microscopy. The chemical and combustion behaviour of MNS and hydrochars was analysed by Fourier transform infrared spectroscopy, and thermogravimetric analysis indicated that decarboxylation and dehydration reactions were the predominant pathways during the HTC process. Results showed that HTC can facilitate the transformation of MNS into solid fuel.