Searches for an embargo of the separatist republics doesn't yield much except a very recent one by the U.S., of all countries, except that one explicitly allows the export of food and other humanitarian items, and even some technology and IT services.
On the other hand, it seems that it's a fact that there's been at least some form of [railroad] blockade of those republics since December 26. OSCE's monitoring mission reports:
n Kyiv, on 26 December the SMM observed a press conference organized by former Donbas and Aidar battalions, where the speakers announced that they would conduct a trade blockade on areas not under government control as people allegedly being detained in those areas had not been released yet. On the evening of that day on Independence Square the SMM saw a yellow tent with flags of Donbas and Aidar battalions and a national flag and a banner marking the tent as the headquarters of the blockade of trade.
And later on 12 February 2017:
The SMM continued monitoring blockades of railway tracks (see SMM Daily Report 11 February 2017). On 11 February, the SMM saw ten people in camouflage-type clothing, four of whom were armed , blocking a rail crossing about 2km north of government-controlled Petrivka (43km north of Donetsk), as well as two military-type tents, national flags of Ukraine, and black-and-red flags. The SMM saw six of the people (all unarmed) reinforcing the blockade point. A train consisting of about 30 empty wagons was stationary south of the crossing. The SMM noted that road vehicle traffic was not impeded.
On 12 February, the SMM observed that the blockade of a railway track in government-controlled Hirske (63km west of Luhansk) remained unchanged (see SMM Daily Report 11 February 2017). About ten unarmed persons in camouflage fatigues at the blockade site told the SMM they had been members of volunteer battalions.
At that time, the blockade was carried out by the volunteers. However, on March 15, after a period of controversy, Ukraine decided to make the blockade official. DW reports on it in Russian, and there's a broad overview of the blockade (pdf, English) by Hrant Kostanyan and Artem Remizov from CEPS (Centre for European Policy Studies, a think tank).
Some of the links they use as sources, mainly Ukrainian government edicts, are not available now (at least to me), but it seems that there was indeed a blockade of the DNR and LNR by the Ukrainian government, and it seems like it included the transport of food as well.
On the other hand, there's no evidence of an embargo on food supplies to DNR and LNR that I could find, so countries would've been free to trade with them if they wanted to. It seems that Russia was already buying coal and supplying power, so they must've been able to sell food as well.